Beginning welders and even those that are more experienced commonly struggle with the problem of weld distortion, (warping of the base plate caused by heat from the welding arc). Distortion is troublesome for a number of reasons, but one of the most critical is the potential creation of a weld that is not structurally sound. This article will help to define what weld distortion is and then provide a practical understanding of the causes of distortion, effects of shrinkage in various types of welded assemblies and how to control it, and finally look at methods for distortion control.
Fig. 3-1 Changes in the properties of steel with increases in temperature complicate analysis of what happens during the welding cycle - and, thus, understanding of the factors contributing to weldment distortion.
What is Weld Distortion?
Distortion in a weld results from the expansion and contraction of the weld metal and adjacent base metal during the heating and cooling cycle of the welding process. Doing all welding on one side of a part will cause much more distortion than if the welds are alternated from one side to the other. During this heating and cooling cycle, many factors affect shrinkage of the metal and lead to distortion, such as physical and mechanical properties that change as heat is applied. For example, as the temperature of the weld area increases, yield strength, elasticity, and thermal conductivity of the steel plate decrease, while thermal expansion and specific heat increase (Fig. 3-1). These changes, in turn, affect heat flow and uniformity of heat distribution.
Reasons for Distortion
To understand how and why distortion occurs during heating and cooling of a metal, consider the bar of steel shown in Fig. 3-2. As the bar is uniformly heated, it expands in all directions, as shown in Fig. 3-2(a). As the metal cools to room temperature it contracts uniformly to its original dimensions.
Fig. 3-2 If a steel bar is uniformly heated while unrestrained, as in (a), it will expand in all directions and return to its original dimentions on cooling. If restrained, as in (b), during heating, it can expand only in the vertical direction - become thicker. On cooling, the deformed bar contracts uniformly, as shown in (c), and, thus, is permanently deformed. This is a simplified explanation of basic cause of distortion in welding assemblies.
But if the steel bar is restrained -as in a vise - while it is heated, as shown in Fig. 3-2(b), lateral expansion cannot take place. But, since volume expansion must occur during the heating, the bar expands in a vertical direction (in thickness) and becomes thicker. As the deformed bar returns to room temperature, it will still tend to contract uniformly in all directions, as in Fig. 3-2 (c). The bar is now shorter, but thicker. It has been permanently deformed, or distorted. (For simplification, the sketches show this distortion occurring in thickness only. But in actuality, length is similarly affected.)
In a welded joint, these same expansion and contraction forces act on the weld metal and on the base metal. As the weld metal solidifies and fuses with the base metal, it is in its maximum expanded from. On cooling, it attempts to contract to the volume it would normally occupy at the lower temperature, but it is restrained from doing so by the adjacent base metal. Because of this, stresses develop within the weld and the adjacent base metal. At this point, the weld stretches (or yields) and thins out, thus adjusting to the volume requirements of the lower temperature. But only those stresses that exceed the yield strength of the weld metal are relieved by this straining. By the time the weld reaches room temperature - assuming complete restraint of the base metal so that it cannot move - the weld will contain locked-in tensile stresses approximately equal to the yield strength of the metal. If the restraints (clamps that hold the workpiece, or an opposing shrinkage force) are removed, the residual stresses are partially relieved as they cause the base metal to move, thus distorting the weldment.
Shrinkage Control - What You Can Do to Minimize Distortion
To prevent or minimize weld distortion, methods must be used both in design and during welding to overcome the effects of the heating and cooling cycle. Shrinkage cannot be prevented, but it can be controlled. Several ways can be used to minimize distortion caused by shrinkage:
1. Do not overweld
The more metal placed in a joint, the greater the shrinkage forces. Correctly sizing a weld for the requirements of the joint not only minimizes distortion, but also saves weld metal and time. The amount of weld metal in a fillet weld can be minimized by the use of a flat or slightly convex bead, and in a butt joint by proper edge preparation and fitup. The excess weld metal in a highly convex bead does not increase the allowable strength in code work, but it does increase shrinkage forces.
When welding heavy plate (over 1 inch thick) bevelling or even double bevelling can save a substantial amount of weld metal which translates into much less distortion automatically.
In general, if distortion is not a problem, select the most economical joint. If distortion is a problem, select either a joint in which the weld stresses balance each other or a joint requiring the least amount of weld metal.
2. Use intermittent welding
Another way to minimize weld metal is to use intermittent rather than continuous welds where possible, as in Fig. 3-7(c). For attaching stiffeners to plate, for example, intermittent welds can reduce the weld metal by as much as 75 percent yet provide the needed strength.
3. Use as few weld passes as possible
Fewer passes with large electrodes, Fig. 3-7(d), are preferable to a greater number of passes with small electrodes when transverse distortion could be a problem. Shrinkage caused by each pass tends to be cumulative, thereby increasing total shrinkage when many passes are used.
4. Place welds near the neutral axis
Distortion is minimized by providing a smaller leverage for the shrinkage forces to pull the plates out of alignment. Figure 3-7(e) illustrates this. Both design of the weldment and welding sequence can be used effectively to control distortion.
5. Balance welds around the neutral axis
This practice, shown in Fig. 3-7(f), offsets one shrinkage force with another to effectively minimize distortion of the weldment. Here, too, design of the assembly and proper sequence of welding are important factors.
6. Use backstep welding
In the backstep technique, the general progression of welding may be, say, from left to right, but each bead segment is deposited from right to left as in Fig. 3-7(g). As each bead segment is placed, the heated edges expand, which temporarily separates the plates at B. But as the heat moves out across the plate to C, expansion along outer edges CD brings the plates back together. This separation is most pronounced as the first bead is laid. With successive beads, the plates expand less and less because of the restraint of prior welds. Backstepping may not be effective in all applications, and it cannot be used economically in automatic welding.
Fig. 3-7 Distortion can be prevented or minimized by techniques that defeat - or use constructively - the effects of the heating and cooling cycle. 7. Anticipate the shrinkage forces
Presetting parts (at first glance, I thought that this was referring to overhead or vertical welding positions, which is not the case) before welding can make shrinkage perform constructive work. Several assemblies, preset in this manner, are shown in Fig. 3-7(h). The required amount of preset for shrinkage to pull the plates into alignment can be determined from a few trial welds.
Prebending, presetting or prespringing the parts to be welded, Fig. 3-7(I), is a simple example of the use of opposing mechanical forces to counteract distortion due to welding. The top of the weld groove - which will contain the bulk of the weld metal - is lengthened when the plates are preset. Thus the completed weld is slightly longer than it would be if it had been made on the flat plate. When the clamps are released after welding, the plates return to the flat shape, allowing the weld to relieve its longitudinal shrinkage stresses by shortening to a straight line. The two actions coincide, and the welded plates assume the desired flatness.
Another common practice for balancing shrinkage forces is to position identical weldments back to back, Fig. 3-7(j), clamping them tightly together. The welds are completed on both assemblies and allowed to cool before the clamps are released. Prebending can be combined with this method by inserting wedges at suitable positions between the parts before clamping.
In heavy weldments, particularly, the rigidity of the members and their arrangement relative to each other may provide the balancing forces needed. If these natural balancing forces are not present, it is necessary to use other means to counteract the shrinkage forces in the weld metal. This can be accomplished by balancing one shrinkage force against another or by creating an opposing force through the fixturing. The opposing forces may be: other shrinkage forces; restraining forces imposed by clamps, jigs, or fixtures; restraining forces arising from the arrangement of members in the assembly; or the force from the sag in a member due to gravity.
8. Plan the welding sequence
A well-planned welding sequence involves placing weld metal at different points of the assembly so that, as the structure shrinks in one place, it counteracts the shrinkage forces of welds already made. An example of this is welding alternately on both sides of the neutral axis in making a complete joint penetration groove weld in a butt joint, as in Fig. 3-7(k). Another example, in a fillet weld, consists of making intermittent welds according to the sequences shown in Fig. 3-7(l). In these examples, the shrinkage in weld No. 1 is balanced by the shrinkage in weld No. 2.
Clamps, jigs, and fixtures that lock parts into a desired position and hold them until welding is finished are probably the most widely used means for controlling distortion in small assemblies or components. It was mentioned earlier in this section that the restraining force provided by clamps increases internal stresses in the weldment until the yield point of the weld metal is reached. For typical welds on low-carbon plate, this stress level would approximate 45,000 psi. One might expect this stress to cause considerable movement or distortion after the welded part is removed from the jig or clamps. This does not occur, however, since the strain (unit contraction) from this stress is very low compared to the amount of movement that would occur if no restraint were used during welding.
9. Remove shrinkage forces after welding
Peening is one way to counteract the shrinkage forces of a weld bead as it cools. Essentially, peening the bead stretches it and makes it thinner, thus relieving (by plastic deformation) the stresses induced by contraction as the metal cools. But this method must be used with care. For example, a root bead should never be peened, because of the danger of either concealing a crack or causing one. Generally, peening is not permitted on the final pass, because of the possibility of covering a crack and interfering with inspection, and because of the undesirable work-hardening effect. Thus, the utility of the technique is limited, even though there have been instances where between-pass peening proved to be the only solution for a distortion or cracking problem. Before peening is used on a job, engineering approval should be obtained.
Another method for removing shrinkage forces is by thermal stress relieving - controlled heating of the weldment to an elevated temperature, followed by controlled cooling. Sometimes two identical weldments are clamped back to back, welded, and then stress-relieved while being held in this straight condition. The residual stresses that would tend to distort the weldments are thus minimized.
10. Minimize welding time
Since complex cycles of heating and cooling take place during welding, and since time is required for heat transmission, the time factor affects distortion. In general, it is desirable to finish the weld quickly, before a large volume of surrounding metal heats up and expands. The welding process used, type and size of electrode, welding current, and speed of travel, thus, affect the degree of shrinkage and distortion of a weldment. The use of mechanized welding equipment reduces welding time and the amount of metal affected by heat and, consequently, distortion. For example, depositing a given-size weld on thick plate with a process operating at 175 amp, 25 volts, and 3 ipm requires 87,500 joules of energy per linear inch of weld (also known as heat input). A weld with approximately the same size produced with a process operating at 310 amp, 35 volts, and 8 ipm requires 81,400 joules per linear inch. The weld made with the higher heat input generally results in a greater amount of distortion. (note: I don't want to use the words "excessive" and "more than necessary" because the weld size is, in fact, tied to the heat input. In general, the fillet weld size (in inches) is equal to the square root of the quantity of the heat input (kJ/in) divided by 500. Thus these two welds are most likely not the same size.
Other Techniques for Distortion Control
Water-Cooled Jig
Fig. 3-33 A water-cooled jig for rapid removal of heat when welding sheet meta.
Fig. 3-34 Various strongback arrangements to control distortion during butt-welding.
Various techniques have been developed to control distortion on specific weldments. In sheet-metal welding, for example, a water-cooled jig (Fig. 3-33) is useful to carry heat away from the welded components. Copper tubes are brazed or soldered to copper holding clamps, and the water is circulated through the tubes during welding. The restraint of the clamps also helps minimize distortion.
Strongback
The "strongback" is another useful technique for distortion control during butt welding of plates, as in Fig. 3-34(a). Clips are welded to the edge of one plate and wedges are driven under the clips to force the edges into alignment and to hold them during welding.
Thermal Stress Relieving
Except in special situations, stress relief by heating is not used for correcting distortion. There are occasions, however, when stress relief is necessary to prevent further distortion from occurring before the weldment is finished.
Summary: A Checklist to Minimize Distortion
In summary, follow the checklist below in order to minimize distortion in the design and fabrication of weldments:
Do not overweld.
Control fitup.
Use intermittent welds where possible and consistent with design requirements.
Use the smallest leg size permissible when fillet welding.
For groove welds, use joints that will minimize the volume of weld metal. Consider double-sided joints instead of single-sided joints.
Weld alternately on either side of the joint when possible with multiple-pass welds.
Use minimal number of weld passes.
Use low heat input procedures. This generally means high deposition rates and higher travel speeds.
Use welding positioners to achieve the maximum amount of flat-position welding. The flat position permits the use of large-diameter electrodes and high-deposition-rate welding procedures.
Balance welds about the neutral axis of the member.
Distribute the welding heat as evenly as possible through a planned welding sequence and weldment positioning.
Weld toward the unrestrained part of the member.
Use clamps, fixtures, and strongbacks to maintain fitup and alignment.
Prebend the members or preset the joints to let shrinkage pull them back into alignment.
Sequence subassemblies and final assemblies so that the welds being made continually balance each other around the neutral axis of the section.
Following these techniques will help minimize the effects of distortion and residual stresses
source from
http://www.lincolnelectric.com/knowledge/articles/content/distortion.asp
Sunday, April 18, 2010
Visual Inspection - Welding Processes
Visual Inspection The finished weld should be inspected for undercut, overlap, surface irregularities, cracks, or other defects such as porosity. The root and second pass or hot pass should be inspected in process for the degree of penetration and side wall fusion especially if no further NDT/NDE is required. The extent of reinforcement and size and position of the welds in relationship to the fitted joint are important factors in the determination as to whether a welding job should be accepted or rejected, because collectively, the above all reflect the qualify of the weld.
Visual Inspection Defects and Causes by Welding Process
(SMAW) Stick Welding
INCOMPLETE PENETRATION
This term is used to describe the failure of the filler and base metal to fuse together at the root of the joint. Bridging occurs in groove welds when the deposited metal and base metal are not fused at the root of the joint. The frequent cause of incomplete penetration is a joint design which is not suitable for the welding process or the conditions of construction. When the groove is welded from one side only, incomplete penetration is likely to result under the following conditions.
a. The root face dimension is too big even though the root opening is adequate.
b. The root opening is too small.
c. The included angle of a V-groove is too small.
d. The electrode is too large.
e. The rate of travel is too high.
f. The welding current is too low.
LACK OF FUSION
Lack of fusion is the failure of a welding process to fuse together layers of weld metal or weld metal and base metal. The weld metal just rolls over the plate surfaces. This is generally referred to as overlap. Lack of fusion is caused by the following conditions:
a. Failure to raise to the melting point the temperature of the base metal or the previously deposited weld metal.
b. Improper fluxing, which fails to dissolve the oxide and other foreign material from the surfaces to which the deposited metal must fuse.
c. Dirty plate surfaces.
d. Improper electrode size or type.
e. Wrong current adjustment.
UNDERCUTTING
Undercutting is the burning away of the base metal at the toe of the weld. Undercutting may be caused by the following conditions:
a. Current adjustment that is too high.
b. Arc gap that is too long.
c. Failure to fill up the crater completely with weld metal.
d. Travel speed too fast.
SLAG INCLUSIONS
Slag inclusions are elongated or globular pockets of metallic oxides and other solids compounds. They produce porosity in the weld metal. In arc welding, slag inclusions are generally made up of electrode coating materials or fluxes. In multilayer welding operations, failure to remove the slag between the layers causes slag inclusions. Most slag inclusion can be prevented by:
a. Preparing the groove and weld properly before each bead is deposited.
b. Removing all slag.
c. Making sure that the slag rises to the surface of the weld pool.
d. Taking care to avoid leaving any contours such as a high crown which will be difficult to penetrate fully with the arc.
e. Avoiding travel speed that is too slow.
b. Avoiding current that is too low.
POROSITY
a. Porosity is the presence of pockets which do not contain any solid material. They differ from slag inclusions in that the pockets contain gas rather than a solid. The gases forming the voids are derived from:
(1) Gas released by cooling weld because of its reduced solubility temperature drops.
(2) Gases formed by the chemical reactions in the weld.
b. Porosity is best prevented by avoiding:
(1) Overheating and undercutting of the weld metal.
(2) Too high a current setting.
(3) Too long an arc.
OXY-FUEL GAS WELDING
a. The weld should be of consistent width throughout. The two edges should form straight parallel lines.
b. The face of the weld should be slightly convex with a reinforcement of not more than 1/16 in. (1.6 mm) above the plate surface. The convexity should be even along the entire length of the weld. It should not be high in one place and low in another.
c. The face of the weld should have fine, evenly spaced ripples. It should be free of excessive spatter, scale, and pitting.
d. The edges of the weld should be free of undercut or overlap.
e. Starts and stops should blend together so that it is difficult where they have taken place.
f. The crater at the end of the weld should be filled and show no holes, or cracks.
(1) If the joint is a butt joint, check the back side for complete penetration through the root of the joint. A slight bead should form on the back side.
(2) The root penetration and fusion of lap and T-joints can be checked by putting pressure on the upper plate until it is bent double. If the weld has not penetrated through the root, the plate will crack open at the joint as it is being bent. If it breaks, observe the extent of the penetration and fusion at the root. It will probably be lacking in fusion and penetration.
GAS METAL-ARC WELDING (GMAW) WITH SOLID-CORE WIRE
a. Lack of Penetration. Lack of penetration or fusion in the root area. This poor penetration is the result of too little heat corrected by:
(1) Increasing the wire-feed speed and reducing the stickout distance.
(2) Making sure that the fit-up is correct.
(3) Reducing the speed of travel.
(4) Using proper welding techniques such as correct lead angle and making sure that both toes of the bead fuse to the base metal.
b. Excessive Penetration. Excessive penetration usually causes burn through. It is the result of too much heat in the weld area. This can be corrected by:
(1) Reducing the wire size.
(2) Reducing the wire-feed speed and increasing the speed of travel.
(3) Making sure that the root opening and root face are correct.
(4) Increasing the stickout distance during welding and weaving the gun.
c. Whiskers. Whiskers are short lengths of electrode wire sticking through the weld on the root side of the joint. They are caused by pushing the electrode wire past the leading edge of the weld pool. Whiskers can be prevented by:
(1) Reducing the wire-feed speed and the speed of travel.
(2) Increasing the stickout distance and weaving the gun.
d. Voids. Voids are sometimes referred to as wagon tracks because of their resemblance to ruts in a dirt road. They may be continued along both sides of the weld deposit. They are found in multipass welding. Voids can be prevented by:
(1) Avoiding a large contoured crown and undercut.
(2) Making sure that all edges are filled in.
(3) On succeeding passes , using slightly higher arc voltage and increasing travel speed.
e. Lack of Fusion. Lack of fusion, also referred to as cold lap, is largely the result of improper torch handling, low heat, and higher speed travel. It is important that the arc be directed at the leading edge of the puddle. To prevent this defect, give careful consideration to the following:
(1) Direct the arc so that it covers all areas of the joint. The arc, not the puddle, should do the fusing.
(2) Keep the electrode at the leading edge of the puddle.
(3) Reduce the size of the puddle as necessary by reducing either the travel speed or wire-feed speed.
(4) Check current values carefully.
f. Porosity. The most common defect in welds produced by any welding process is porosity. Porosity that exists on the face of the weld is readily detected, but porosity in the weld metal below the surface must be determined by x-ray or other testing methods. The causes of most porosity are:
(1) Contamination by the atmosphere and other materials such as oil, dirt, rust, and paint.
(2) Changes in the physical qualities of the filler wire due to excessive current.
(3) Entrapment of the gas evolved during weld metal solidification.
(4) Loss of shielding gas because of too fast travel.
(5) Shielding gas flow rate too low, not providing full protection.
(6) Shielding gas flow rate too high, drawing air into the arc area.
(7) Wrong type of shielding gas being used.
(8) Gas shield blown away by wind or drafts.
(9) Defects in the gas system.
(10) Improper welding technique, excessive stickout, improper torch angle, and too fast removal of the gun and the shielding gas at the end of the weld.
g. Spatter. Spatter is made up of very fine particles of metal on the plate surface adjoining the weld area. It is usually caused by high current, a long arc, an irregular and unstable arc, improper shielding gas, or a clogged nozzle.
h. Irregular Weld Shape. Irregular welds include those that are too wide or too narrow, those that have an excessively convex or concave surface, and those that have coarse, irregular ripples. Such characteristics may be caused by poor torch manipulation, a speed of travel that is too slow, current that is too high or low, improper arc voltage, improper stickout, or improper shielding gas.
i. Undercutting. Undercutting is a cutting away of the base material along the edge of the weld. It may be present in the cover pass weld bead or in multipass welding. This condition is usually the result of high current, high voltage, excessive travel speed, low wire-feed speed, poor torch technique, improper gas shielding or the wrong filler wire. To correct undercutting, move the gun from side to side in the joint. Hesitate at each side before returning to the opposite side.
GAS METAL-ARC WELDING (GMAW) WITH FLUX-CORED WIRE
a. Burn-Through. Burn-through may be caused by the following:
(1) Current too high.
(2) Excessive gap between plates.
(3) Travel speed too s1ow.
(4) Bevel angle too large.
(5) Nose too small.
(6) Wire size too small.
(7) Insufficient metal hold-down or clamping.
b. Crown Too High or Too Low. The crown of the weld may be incorrect due to the following:
(1) Current too high or low.
(2) Voltage too high or low.
(3) Travel speed too high or low.
(4) Improper weld backing.
(5) Improper spacing in welds with backing.
(6) Workpiece not level.
c. Penetration Too Deep or Too Shallow. Incorrect penetration may be caused by any of the following:
(1) Current too high or low.
(2) Voltage too high or low.
(3) Improper gap between plates.
(4) Improper wire size.
(5) Travel speed too slow or fast.
d. Porosity and Gas Pockets. These defects may be the results of any of the following:
(1) Flux too shallow.
(2) Improper cleaning.
(3) Contaminated weld backing.
(4) Improper fitup in welds with manual backing.
(5) Insufficient penetration in double welds.
e. Reinforcement Narrow and Steep-Sloped (Pointed). Narrow and pointed reinforcements may be caused by the following:
(1) Insufficient width of flux.
(2) Voltage too low.
f. Mountain Range Reinforcement. If the reinforcement is ragged, the flux was too deep.
g. Undercutting. Undercutting may be caused by any of the following:
(1) Travel speed too high.
(2) Improper wire position (fillet welding).
(3) Improper weld backing.
(4) Current or voltage too high.
h. Voids and Cracks. These weld deficiencies may be caused by any of the following:
(1) Improper cooling.
(2) Failure to preheat.
(3) Improper fitup.
(4) Concave reinforcement (fillet weld).
(5) Excessive convexity (full penetration weld).
Practical Visual Inspection Tips
(1) Know the code or codes that apply to the job you are working on. They go hand in hand with the design specifications.
(2) Review all of the applicable weld procedures.
(3) Check each welder's qualification to the welding procedure that they are currently working with.
(4) Talk to the welders and fitters-not at them. Most of them already hate inspectors because of past bad experiences. They are your best source of information for potential quality problems because they are the ones actually doing the job. Be just as fast to tell welders when they have it right as you are when they have it wrong.
(5) Always carry a flashlight, magnifier glass, and inspection mirror. You will eventually need them all.
(6) Always inspect each weld 100% on both sides if possible. Clearly mark any defects on the work piece.
(7) Reject welds professionally and not personally because you don't like the welder or the boss.
(8) If you run into procedural problems or catch a blatant procedure deviation and it isn't corrected in a timely manner, do write an NCR (Non Conformance Report). Both the owner and design engineer have to sign it off. That relieves you of responsibility for the infraction if a failure should occur in the future. In today's finger pointing society you need to cover your butt.
http://www.weldprocedures.com/visins.html
Visual Inspection Defects and Causes by Welding Process
(SMAW) Stick Welding
INCOMPLETE PENETRATION
This term is used to describe the failure of the filler and base metal to fuse together at the root of the joint. Bridging occurs in groove welds when the deposited metal and base metal are not fused at the root of the joint. The frequent cause of incomplete penetration is a joint design which is not suitable for the welding process or the conditions of construction. When the groove is welded from one side only, incomplete penetration is likely to result under the following conditions.
a. The root face dimension is too big even though the root opening is adequate.
b. The root opening is too small.
c. The included angle of a V-groove is too small.
d. The electrode is too large.
e. The rate of travel is too high.
f. The welding current is too low.
LACK OF FUSION
Lack of fusion is the failure of a welding process to fuse together layers of weld metal or weld metal and base metal. The weld metal just rolls over the plate surfaces. This is generally referred to as overlap. Lack of fusion is caused by the following conditions:
a. Failure to raise to the melting point the temperature of the base metal or the previously deposited weld metal.
b. Improper fluxing, which fails to dissolve the oxide and other foreign material from the surfaces to which the deposited metal must fuse.
c. Dirty plate surfaces.
d. Improper electrode size or type.
e. Wrong current adjustment.
UNDERCUTTING
Undercutting is the burning away of the base metal at the toe of the weld. Undercutting may be caused by the following conditions:
a. Current adjustment that is too high.
b. Arc gap that is too long.
c. Failure to fill up the crater completely with weld metal.
d. Travel speed too fast.
SLAG INCLUSIONS
Slag inclusions are elongated or globular pockets of metallic oxides and other solids compounds. They produce porosity in the weld metal. In arc welding, slag inclusions are generally made up of electrode coating materials or fluxes. In multilayer welding operations, failure to remove the slag between the layers causes slag inclusions. Most slag inclusion can be prevented by:
a. Preparing the groove and weld properly before each bead is deposited.
b. Removing all slag.
c. Making sure that the slag rises to the surface of the weld pool.
d. Taking care to avoid leaving any contours such as a high crown which will be difficult to penetrate fully with the arc.
e. Avoiding travel speed that is too slow.
b. Avoiding current that is too low.
POROSITY
a. Porosity is the presence of pockets which do not contain any solid material. They differ from slag inclusions in that the pockets contain gas rather than a solid. The gases forming the voids are derived from:
(1) Gas released by cooling weld because of its reduced solubility temperature drops.
(2) Gases formed by the chemical reactions in the weld.
b. Porosity is best prevented by avoiding:
(1) Overheating and undercutting of the weld metal.
(2) Too high a current setting.
(3) Too long an arc.
OXY-FUEL GAS WELDING
a. The weld should be of consistent width throughout. The two edges should form straight parallel lines.
b. The face of the weld should be slightly convex with a reinforcement of not more than 1/16 in. (1.6 mm) above the plate surface. The convexity should be even along the entire length of the weld. It should not be high in one place and low in another.
c. The face of the weld should have fine, evenly spaced ripples. It should be free of excessive spatter, scale, and pitting.
d. The edges of the weld should be free of undercut or overlap.
e. Starts and stops should blend together so that it is difficult where they have taken place.
f. The crater at the end of the weld should be filled and show no holes, or cracks.
(1) If the joint is a butt joint, check the back side for complete penetration through the root of the joint. A slight bead should form on the back side.
(2) The root penetration and fusion of lap and T-joints can be checked by putting pressure on the upper plate until it is bent double. If the weld has not penetrated through the root, the plate will crack open at the joint as it is being bent. If it breaks, observe the extent of the penetration and fusion at the root. It will probably be lacking in fusion and penetration.
GAS METAL-ARC WELDING (GMAW) WITH SOLID-CORE WIRE
a. Lack of Penetration. Lack of penetration or fusion in the root area. This poor penetration is the result of too little heat corrected by:
(1) Increasing the wire-feed speed and reducing the stickout distance.
(2) Making sure that the fit-up is correct.
(3) Reducing the speed of travel.
(4) Using proper welding techniques such as correct lead angle and making sure that both toes of the bead fuse to the base metal.
b. Excessive Penetration. Excessive penetration usually causes burn through. It is the result of too much heat in the weld area. This can be corrected by:
(1) Reducing the wire size.
(2) Reducing the wire-feed speed and increasing the speed of travel.
(3) Making sure that the root opening and root face are correct.
(4) Increasing the stickout distance during welding and weaving the gun.
c. Whiskers. Whiskers are short lengths of electrode wire sticking through the weld on the root side of the joint. They are caused by pushing the electrode wire past the leading edge of the weld pool. Whiskers can be prevented by:
(1) Reducing the wire-feed speed and the speed of travel.
(2) Increasing the stickout distance and weaving the gun.
d. Voids. Voids are sometimes referred to as wagon tracks because of their resemblance to ruts in a dirt road. They may be continued along both sides of the weld deposit. They are found in multipass welding. Voids can be prevented by:
(1) Avoiding a large contoured crown and undercut.
(2) Making sure that all edges are filled in.
(3) On succeeding passes , using slightly higher arc voltage and increasing travel speed.
e. Lack of Fusion. Lack of fusion, also referred to as cold lap, is largely the result of improper torch handling, low heat, and higher speed travel. It is important that the arc be directed at the leading edge of the puddle. To prevent this defect, give careful consideration to the following:
(1) Direct the arc so that it covers all areas of the joint. The arc, not the puddle, should do the fusing.
(2) Keep the electrode at the leading edge of the puddle.
(3) Reduce the size of the puddle as necessary by reducing either the travel speed or wire-feed speed.
(4) Check current values carefully.
f. Porosity. The most common defect in welds produced by any welding process is porosity. Porosity that exists on the face of the weld is readily detected, but porosity in the weld metal below the surface must be determined by x-ray or other testing methods. The causes of most porosity are:
(1) Contamination by the atmosphere and other materials such as oil, dirt, rust, and paint.
(2) Changes in the physical qualities of the filler wire due to excessive current.
(3) Entrapment of the gas evolved during weld metal solidification.
(4) Loss of shielding gas because of too fast travel.
(5) Shielding gas flow rate too low, not providing full protection.
(6) Shielding gas flow rate too high, drawing air into the arc area.
(7) Wrong type of shielding gas being used.
(8) Gas shield blown away by wind or drafts.
(9) Defects in the gas system.
(10) Improper welding technique, excessive stickout, improper torch angle, and too fast removal of the gun and the shielding gas at the end of the weld.
g. Spatter. Spatter is made up of very fine particles of metal on the plate surface adjoining the weld area. It is usually caused by high current, a long arc, an irregular and unstable arc, improper shielding gas, or a clogged nozzle.
h. Irregular Weld Shape. Irregular welds include those that are too wide or too narrow, those that have an excessively convex or concave surface, and those that have coarse, irregular ripples. Such characteristics may be caused by poor torch manipulation, a speed of travel that is too slow, current that is too high or low, improper arc voltage, improper stickout, or improper shielding gas.
i. Undercutting. Undercutting is a cutting away of the base material along the edge of the weld. It may be present in the cover pass weld bead or in multipass welding. This condition is usually the result of high current, high voltage, excessive travel speed, low wire-feed speed, poor torch technique, improper gas shielding or the wrong filler wire. To correct undercutting, move the gun from side to side in the joint. Hesitate at each side before returning to the opposite side.
GAS METAL-ARC WELDING (GMAW) WITH FLUX-CORED WIRE
a. Burn-Through. Burn-through may be caused by the following:
(1) Current too high.
(2) Excessive gap between plates.
(3) Travel speed too s1ow.
(4) Bevel angle too large.
(5) Nose too small.
(6) Wire size too small.
(7) Insufficient metal hold-down or clamping.
b. Crown Too High or Too Low. The crown of the weld may be incorrect due to the following:
(1) Current too high or low.
(2) Voltage too high or low.
(3) Travel speed too high or low.
(4) Improper weld backing.
(5) Improper spacing in welds with backing.
(6) Workpiece not level.
c. Penetration Too Deep or Too Shallow. Incorrect penetration may be caused by any of the following:
(1) Current too high or low.
(2) Voltage too high or low.
(3) Improper gap between plates.
(4) Improper wire size.
(5) Travel speed too slow or fast.
d. Porosity and Gas Pockets. These defects may be the results of any of the following:
(1) Flux too shallow.
(2) Improper cleaning.
(3) Contaminated weld backing.
(4) Improper fitup in welds with manual backing.
(5) Insufficient penetration in double welds.
e. Reinforcement Narrow and Steep-Sloped (Pointed). Narrow and pointed reinforcements may be caused by the following:
(1) Insufficient width of flux.
(2) Voltage too low.
f. Mountain Range Reinforcement. If the reinforcement is ragged, the flux was too deep.
g. Undercutting. Undercutting may be caused by any of the following:
(1) Travel speed too high.
(2) Improper wire position (fillet welding).
(3) Improper weld backing.
(4) Current or voltage too high.
h. Voids and Cracks. These weld deficiencies may be caused by any of the following:
(1) Improper cooling.
(2) Failure to preheat.
(3) Improper fitup.
(4) Concave reinforcement (fillet weld).
(5) Excessive convexity (full penetration weld).
Practical Visual Inspection Tips
(1) Know the code or codes that apply to the job you are working on. They go hand in hand with the design specifications.
(2) Review all of the applicable weld procedures.
(3) Check each welder's qualification to the welding procedure that they are currently working with.
(4) Talk to the welders and fitters-not at them. Most of them already hate inspectors because of past bad experiences. They are your best source of information for potential quality problems because they are the ones actually doing the job. Be just as fast to tell welders when they have it right as you are when they have it wrong.
(5) Always carry a flashlight, magnifier glass, and inspection mirror. You will eventually need them all.
(6) Always inspect each weld 100% on both sides if possible. Clearly mark any defects on the work piece.
(7) Reject welds professionally and not personally because you don't like the welder or the boss.
(8) If you run into procedural problems or catch a blatant procedure deviation and it isn't corrected in a timely manner, do write an NCR (Non Conformance Report). Both the owner and design engineer have to sign it off. That relieves you of responsibility for the infraction if a failure should occur in the future. In today's finger pointing society you need to cover your butt.
http://www.weldprocedures.com/visins.html
Welding and Inspection
What is a design specification?
Virtually all major and many minor construction projects are engineered to meet established codes. Design specifications either meet or exceed these codes. There are simply too many organizations that specialize in codes to list them all, but I will give you some examples. Most large buildings are built to AWS (American Welding Society) code:D1.1. Steel bridge structures are built to AWS code D1.5. Gas and oil pipelines and storage tanks are are built to various API (American Petroleum Institute) codes. Power piping (pressurized piping), pressure vessels, nuclear and conventional power plants are built to various ASME (American Society of Mechanical Engineers) codes. Fire protection and sprinkler systems are built to NFPA (National Fire Protection Association) codes. Virtually all structural steel, piping, and alloys are produced to ASTM (American Society for Testing and Materials) specifications. There are other code organizations as well as state, local, and federal codes that govern all aspects of construction. Other codes would include plumbing, electrical, HVAC (Heating, Ventilation, and Air Conditioning) and military codes for a few more examples. These codes and the design specifications determine the method and extent of inspection required for a particular job or project. The first thing that an inspector has to do is establish the proper codes for the particular job at hand. Many complex construction projects fall under multiple codes.
Inspectors
An inspector shoulders an incredible amount of responsibility. He/she must be knowledgable about welding processes and procedures. The ability to determine the proper code or codes when not known is also needed. Knowledge of blueprints, specifications, and welding and non-destructive test symbols is required. Knowledge of various test methods is also necessary. The ability to keep and maintain test records is also important. One of the most important requirements is a fair and impartial attitude while performing inspection duties. Find Out More About Becoming an Inspector
What an Inspector looks for... Other Non-Destructive Test (NDT) methods are used to find indications which have to be interpreted according to the inspection procedure for that particular job. Indications are inspection lingo for possible defects. These discontinuities (a fancier name for indications) have to be evaluated with reference to the acceptance criteria for that particular job. After comparison to the criteria are they considered acceptable or rejectable. These other Non-Destructive Inspection (NDI) methods require special training, written and practical examinations and accumulation of experience. It is important that a welder also understands the flaw terminology to effectively repair any possible defects.
Visual Inspection (VT)
The importance of visual inspection is often over looked. A visual test (VT) will provide a wealth of information about a weld. Many weld defects such as porosity, cracks, incomplete fusion, inclusions, overlap, edge melt, and incomplete penetration can be observed with just a simple visual exam. A weld that passes a visual exam has a much higher probability of passing further Non-Destructive Evaluation (NDE) methods. Causes of welding defects by different welding processes.
X-Rays (RT)
Radiographic weld inspection is performed by pointing a radiographic source (an x-ray tube or a radioactive isotope) to the part of the weld to be inspected and by exposing for a predetermined time a radiographic film to the radiation on the opposite side of the source tip or tube. The resulting film contains information on the internal features of the weld. Variations in film density allow the film interpreter to accept or reject the weld based on comparison to specific hole or wire sizes in or on a penetrameter. These hole or wire sizes represent the largest acceptable defect size in a weld. Any indication that is larger than the acceptable wire or hole size is cause for rejection. All the relevant parameters including accept or reject are then recorded on an X-Ray Technique Sheet. The technique sheet and the processed film are usually turned over to the customer at the completion of the job.
Radiation Safety
I am including this section specifically to address a practice that I have seen at virtually every job site that I have worked at--people ducking under the radiation lines to take a short cut. Radiation lines are predetermined by the responsible radiographer based upon the strength of the radiation source, the direction of the shots, and available shielding, if any. Radiation is calculated using the inverse square law. Without getting into the actual math formulas involved, I will give you some examples of why these lines should never be crossed without the permission of the radiographer. First you need to know that ionizing radiation can cause measurable changes to the blood of anyone exposed. Example: The outer low radiation boundry is set to 2 mr (milli-rems) per hour and is 100 feet from the source. The radiographer only has one shot to take that will last for a minute. He calculates that the line he is aiming the shot at will read 30mr for that one minute. He is legal because the total exposure will not exceed 2mr per hour. Just as the shot is exposed, Mr. Curious decides to duck under the line to see how the shot was set up. He gets to 50 ft. and his exposure is now 120mr per hour. At 25 ft., it increases to 480mr per hour. At 12.5 ft it increases to 1920mr per hour. At 6 ft., the dose is 7680mr per hour. At 3 ft., the dose increases to 30720 mr per hour and as he stops at a foot and a half away, his exposure is a staggering 122880 mr per hour. One hour of exposure at that rate exceeds the whole body lethal dose of 1200 rems as currently set by the Nuclear Regulatory Commission (NRC). Radiation lines are set up to protect the general public, not the radiographers. They wear several types of dosimetry (radiation measuring devices) and carry a survey meter (geiger counter) to measure their own exposure. They are also the only ones that know what direction the shot is pointed at. Stay safe-stay out!
Ultrasonic (UT)
Ultrasonic weld inspection is based on the fact that high frequency sound waves out of the range of human hearing can propagate in different materials, and be reflected by internal interfaces and opposite wall surfaces. These waves are generated by piezoelectric transducers of different sizes and frequencies which transform electrical vibrations into mechanical vibrations and vice-versa. These transducers are selected to match the thickness, type, temperature, and configuration of the material to be tested. Signal reflections are evaluated on a computer screen, and by making reference to standard reflectors (normally flat bottom holes carefully machined on specimens of the same material) of given shape and size, the qualified inspector can conclude that if an echo is present where it should not be and if its reflection is larger than that of comparison, then there is an indication that must be evaluated. Additional techniques may be required to determine acceptance or rejection. Ultrasonic testing is capable of detecting thin interfaces normal to the line of propagation of the wave (that X-Rays cannot detect) so that both testing methods complement each other. Ultrasonic testing is becoming one of the most widely used methods of nondestructive testing. Its primary purpose is to detect and characterize internal discontinuities. UT can also measure thickness, detect surface discontinuities, and define bond characteristics.
Liquid Penetrant (PT)
Liquid Penetrant weld inspection is a sensitive method of detecting and locating discontinuities that are clear and open to the surface. A penetrating liquid dye is applied to the cleaned surface. This dye will seep into surface discontinuities. After a certain amount of time(dwell time), the excess penetrant dye is removed. A developer is then applied that acts like a blotter and draws the remaining penetrant out of the discontinuity. Liquid Penetrant inspection is used for both magnetic and non magnetic materials like aluminum, stainless steel, magnesium, titanium, bronze etc. and will detect extremely small cracks. There are three different types of penetrant used with both visible and flourescent methods. These are classified by how they are removed from the test surface: solvent removable, water washable, and post-emulsifiable. The solvent removable types are most common and highly portable making them ideal for "on site" inspections.
Leak Testing
Leak testing for weld inspection is done on containers and piping systems built to hold a liquid or a gas. The tank or piping system is usually pressurized above its design operating pressure and held at that pressure for a specific amount of time. The usual test mediums are air, gas(usually nitrogen), or water. These tests are performed mostly on new construction and are part of the ASME code.
http://www.weldprocedures.com/inspection.html
Virtually all major and many minor construction projects are engineered to meet established codes. Design specifications either meet or exceed these codes. There are simply too many organizations that specialize in codes to list them all, but I will give you some examples. Most large buildings are built to AWS (American Welding Society) code:D1.1. Steel bridge structures are built to AWS code D1.5. Gas and oil pipelines and storage tanks are are built to various API (American Petroleum Institute) codes. Power piping (pressurized piping), pressure vessels, nuclear and conventional power plants are built to various ASME (American Society of Mechanical Engineers) codes. Fire protection and sprinkler systems are built to NFPA (National Fire Protection Association) codes. Virtually all structural steel, piping, and alloys are produced to ASTM (American Society for Testing and Materials) specifications. There are other code organizations as well as state, local, and federal codes that govern all aspects of construction. Other codes would include plumbing, electrical, HVAC (Heating, Ventilation, and Air Conditioning) and military codes for a few more examples. These codes and the design specifications determine the method and extent of inspection required for a particular job or project. The first thing that an inspector has to do is establish the proper codes for the particular job at hand. Many complex construction projects fall under multiple codes.
Inspectors
An inspector shoulders an incredible amount of responsibility. He/she must be knowledgable about welding processes and procedures. The ability to determine the proper code or codes when not known is also needed. Knowledge of blueprints, specifications, and welding and non-destructive test symbols is required. Knowledge of various test methods is also necessary. The ability to keep and maintain test records is also important. One of the most important requirements is a fair and impartial attitude while performing inspection duties. Find Out More About Becoming an Inspector
What an Inspector looks for... Other Non-Destructive Test (NDT) methods are used to find indications which have to be interpreted according to the inspection procedure for that particular job. Indications are inspection lingo for possible defects. These discontinuities (a fancier name for indications) have to be evaluated with reference to the acceptance criteria for that particular job. After comparison to the criteria are they considered acceptable or rejectable. These other Non-Destructive Inspection (NDI) methods require special training, written and practical examinations and accumulation of experience. It is important that a welder also understands the flaw terminology to effectively repair any possible defects.
Visual Inspection (VT)
The importance of visual inspection is often over looked. A visual test (VT) will provide a wealth of information about a weld. Many weld defects such as porosity, cracks, incomplete fusion, inclusions, overlap, edge melt, and incomplete penetration can be observed with just a simple visual exam. A weld that passes a visual exam has a much higher probability of passing further Non-Destructive Evaluation (NDE) methods. Causes of welding defects by different welding processes.
X-Rays (RT)
Radiographic weld inspection is performed by pointing a radiographic source (an x-ray tube or a radioactive isotope) to the part of the weld to be inspected and by exposing for a predetermined time a radiographic film to the radiation on the opposite side of the source tip or tube. The resulting film contains information on the internal features of the weld. Variations in film density allow the film interpreter to accept or reject the weld based on comparison to specific hole or wire sizes in or on a penetrameter. These hole or wire sizes represent the largest acceptable defect size in a weld. Any indication that is larger than the acceptable wire or hole size is cause for rejection. All the relevant parameters including accept or reject are then recorded on an X-Ray Technique Sheet. The technique sheet and the processed film are usually turned over to the customer at the completion of the job.
Radiation Safety
I am including this section specifically to address a practice that I have seen at virtually every job site that I have worked at--people ducking under the radiation lines to take a short cut. Radiation lines are predetermined by the responsible radiographer based upon the strength of the radiation source, the direction of the shots, and available shielding, if any. Radiation is calculated using the inverse square law. Without getting into the actual math formulas involved, I will give you some examples of why these lines should never be crossed without the permission of the radiographer. First you need to know that ionizing radiation can cause measurable changes to the blood of anyone exposed. Example: The outer low radiation boundry is set to 2 mr (milli-rems) per hour and is 100 feet from the source. The radiographer only has one shot to take that will last for a minute. He calculates that the line he is aiming the shot at will read 30mr for that one minute. He is legal because the total exposure will not exceed 2mr per hour. Just as the shot is exposed, Mr. Curious decides to duck under the line to see how the shot was set up. He gets to 50 ft. and his exposure is now 120mr per hour. At 25 ft., it increases to 480mr per hour. At 12.5 ft it increases to 1920mr per hour. At 6 ft., the dose is 7680mr per hour. At 3 ft., the dose increases to 30720 mr per hour and as he stops at a foot and a half away, his exposure is a staggering 122880 mr per hour. One hour of exposure at that rate exceeds the whole body lethal dose of 1200 rems as currently set by the Nuclear Regulatory Commission (NRC). Radiation lines are set up to protect the general public, not the radiographers. They wear several types of dosimetry (radiation measuring devices) and carry a survey meter (geiger counter) to measure their own exposure. They are also the only ones that know what direction the shot is pointed at. Stay safe-stay out!
Ultrasonic (UT)
Ultrasonic weld inspection is based on the fact that high frequency sound waves out of the range of human hearing can propagate in different materials, and be reflected by internal interfaces and opposite wall surfaces. These waves are generated by piezoelectric transducers of different sizes and frequencies which transform electrical vibrations into mechanical vibrations and vice-versa. These transducers are selected to match the thickness, type, temperature, and configuration of the material to be tested. Signal reflections are evaluated on a computer screen, and by making reference to standard reflectors (normally flat bottom holes carefully machined on specimens of the same material) of given shape and size, the qualified inspector can conclude that if an echo is present where it should not be and if its reflection is larger than that of comparison, then there is an indication that must be evaluated. Additional techniques may be required to determine acceptance or rejection. Ultrasonic testing is capable of detecting thin interfaces normal to the line of propagation of the wave (that X-Rays cannot detect) so that both testing methods complement each other. Ultrasonic testing is becoming one of the most widely used methods of nondestructive testing. Its primary purpose is to detect and characterize internal discontinuities. UT can also measure thickness, detect surface discontinuities, and define bond characteristics.
Liquid Penetrant (PT)
Liquid Penetrant weld inspection is a sensitive method of detecting and locating discontinuities that are clear and open to the surface. A penetrating liquid dye is applied to the cleaned surface. This dye will seep into surface discontinuities. After a certain amount of time(dwell time), the excess penetrant dye is removed. A developer is then applied that acts like a blotter and draws the remaining penetrant out of the discontinuity. Liquid Penetrant inspection is used for both magnetic and non magnetic materials like aluminum, stainless steel, magnesium, titanium, bronze etc. and will detect extremely small cracks. There are three different types of penetrant used with both visible and flourescent methods. These are classified by how they are removed from the test surface: solvent removable, water washable, and post-emulsifiable. The solvent removable types are most common and highly portable making them ideal for "on site" inspections.
Leak Testing
Leak testing for weld inspection is done on containers and piping systems built to hold a liquid or a gas. The tank or piping system is usually pressurized above its design operating pressure and held at that pressure for a specific amount of time. The usual test mediums are air, gas(usually nitrogen), or water. These tests are performed mostly on new construction and are part of the ASME code.
http://www.weldprocedures.com/inspection.html
Inspection Certification
This article describes what it takes to become an ASNT certified NDT/NDE inspector. Certification in one process does not qualify an individual for another process. Full re-certification is required every three years for each process. Depending upon age, an annual vision test may also be required. Nondestructive testing personnel are often certified by their employer or other agency to meet specific qualification levels. Certification is a process of providing written proof that an individual is qualified to do a certain inspection task. The qualifications of an individual are based on education, level of training, work experience, and the ability to pass a vision test.
In the field of NDT/NDE, certification is important because NDT/NDE personnel are often making critical judgments that can have safety and/or significant financial consequences if not performed properly. NDT/NDE personnel must have a tremendous amount of confidence in the results of their work. Since many of the NDT/NDE methods do not produce a permanent record of the inspection results, certification presents objective evidence of the knowledge and skill level of the person performing an inspection.
The procedure used to assure that NDT/NDE personnel possess the qualifications necessary to do competent work includes:
Classroom training to gain the necessary knowledge
Practical experience under the guidance of an experienced inspector
Three separate qualification examinations to demonstrate that competancy has been achieved
Written certification to document successful demonstration of competency.
NDT/NDE Methods
Certification can be obtained in number of NDT/NDE methods, which are listed in the table below.
Acoustic Emission Testing AE
Eddy Current Testing ET
Leak Testing LT
Liquid Penetrant Testing PT
Magnetic Particle Testing MT
Neutron Radiographic Testing NRT
Radiographic Testing RT
Thermal/Infrared Testing TIR
Ultrasonic Testing UT
Vibration Analysis Testing VA
Visual Testing VT
NDT/NDE Certification Levels
NDT/NDE personnel are generally certified to several different levels of competence within each of the NDT/NDE methods they are working. The levels are Level I, Level I Special, Level II, and Level III.
Level I technicians are only qualified to perform specific calibrations and tests, and acceptance or rejection determinations allow little or no deviation from the procedure. Level I technicians are under close supervision and direction of a higher level tester. The level I position is not the trainee level, but the first level a trainee reaches upon demonstrating ability in specific tests. Level I Special personnel are limited even more in what they can do. They are usually trained to a specific procedure and can perform only certain types of inspections on a certain set of components.
Level II technicians are able to set up and calibrate equipment, conduct the inspection according to procedures, interpret, evaluate and document results in all the testing method(s) utilized by the certificate holder. The technician can provide on the job training for Level I and Level I Specials and act as a supervisor. The technician can also organize and document the results of the inspection. They must be familiar with all applicable codes, standards, inspection procedures, and other documents that control the NDT/NDE method being utilized.
Level III technicians are capable of establishing inspection techniques and procedures; interpreting codes, standards, and specifications; and designating the particular nondestructive testing methods, techniques, and procedures to be used. They must also have knowledge of materials, fabrication, and product technology. Level III technicians are responsible for training and examining Level I and Level II's. Usually level III technicians are in administration, supervision, or management positions, or are owners of a testing laboratory. Some Level III technicians also become consultants.
Certification Requirements
There are a number of organizations that have produced documents that recommended or specify the minimum qualifications for certification. The following is a partial list of documents pertaining to the certification of NDT/NDE personnel in the USA.
ASNT-SNT-TC-1A, The American Society for Nondestructive Testing, Recommended Practice, Personnel Qualification and Certification in Nondestructive Testing.
ATA-105 Aviation Transport Association, Guidelines for Training and Qualifying Personnel in Nondestructive Testing Methods.
AIA-NAS-410, Aerospace Industries Association, National Aerospace Standard, NAS Certification and Qualification of Nondestructive Test Personnel.
ISO 9712, International Organization for Standards, Nondestructive testing -- Qualification and certification of personnel.
The education and work experience requirements for the various specification are common or similar. Typical requirements are summarized in the table below for qualification levels I and II. Please consult the certification documents to assure that information is correct for your situation.
Examination Method Level Required Hours of NDT Training Minimum hours of work experience in a method Permitted time frame to obtain required work experience in a method
(In Months)
For those with high school diploma or equivalent
For those with at least 2 years of engineering or science study at a college or technical school
Acoustic Emission I 40 32 210 1.5-9
II 40 40 630 4.5-27
Eddy Current I 40 24 210 1.5-9
II 40 40 630 4.5-27
Liquid Penetrant I 4 4 70 0.5-3
II 8 4 140 1-6
Magnetic Particle I 12 8 70 0.5-3
II 8 4 210 1.5-9
Neutron Radiography I 28 20 420 3-18
II 40 40 1680 12-72
Radiography I 40 30 210 1.5-9
II 40 35 630 4.5-27
Thermal/Infrared I 32 30 210 1.5-9
II 34 32 1260 9-27
Ultrasonics I 40 30 210 1.5-9
II 40 40 840 4.5-27
Vibration Analysis I 24 24 420 2-18
II 72 48 1680 12-72
Visual I 8 4 70 0.5-3
II 16 8 140 1-6
NDT training can be obtained at colleges, vocational-technical schools, the Armed Forces, commercial training companies and through individual company training departments.
To be considered for certification as a Level III an individual must:
Have graduated from a university or college with a degree in engineering or science, and have at least one year of experience comparable to that of a Level II in the applicable NDT method(s). or
Have completed with passing grades at least two years of engineering or science study at a university, college or technical school and have two years of experience comparable to that of a Level II in the applicable NDT method(s). or
Have four years of experience comparable to that of a Level II in the applicable NDT method(s).
Certification Examinations
Once the education, training and work experience requirements have been met and documented, certification examinations must be taken. The examination process is three part and includes several exams. For certification to Levels I and II a general, a specific, and a practical, exam must be completed with a passing grade of 70 percent for each exam and a composite grade of 80 percent (determined by averaging the results of the three exams).
The general exam contains questions that are fairly specific to the particular inspection process. The specific exam contains interpretation questions that are specific to both the inspection process and one or more codes that would cover that process. The practical exam is a "hands on" test using a test specimen with known defects. The questions on the practical test address accept or reject criteria for the specimen to one or more codes using an inspection procedure for that particular inspection process. More on Exams below.
The Level III exam process includes completion of a basic, a method, and a specific examination with a passing grade of 70 percent for each exam and a composite grade of 80 percent. Level I and II exams must be administered by an NDT level III and this is usually done within companies that provide either in-house or public inspection services. Level I, II and III exams can also be taken through a central agency such as ASNT. Central certifications provides technicians with documentation of qualification that is recognized nationally and in some cases internationally.
Visual Examination
It is important that NDT personnel have good near visual acuity and color vision. Therefore, an eye test must be taken to insure that natural or corrected near distance acuity is acceptable. Depending on which specification the company uses for certification, an individual must be able to read a Jaeger Number 1 or 2 (or equivalent) type and size letter at the designated distance. Determining contrast of color(especially shades of red) or shades of gray is also generally required. Color blindness can prohibit an individual from qualifying for certain inspection processes.
Level I and II General Examinations
A general examination is a written test which covers the general principles of a particular NDT method. The test will range from a minimum of 20 to 40 questions. There are practice general exams with answers available in a convenient Ebook format for six of the most widely used inspection methods. These are: ET, MT, PT, RT, UT, and VT. These can be purchased for a nominal price.
Level I and II Specific Examinations
A specific examination is a test which is related to specifications, equipment, techniques and procedures which the employer uses and requires a specific level of expertise from their personnel. This examination should also cover the codes and procedures used by the employer.
Practical Examination (for Level I and II)
A practical exam tests an individuals ability to operate the necessary equipment, record and analyze the results and accept or reject the part under inspection. A part is selected and the individual is expected to find indications and accept or reject the part to a specific code.
Basic Examination (for Level III)
Covers questions concerning the following:
qualification and certification
materials, fabrication and product technology
general NDT methods
Method Examination (for Level III)
Covers questions concerning the following:
the fundamentals and principles for the NDT method covered by the exam
the application and establishment of techniques and procedures
the capability for interpreting codes, standards and specifications relating to the method covered by the exam
Specific Examination (for Level III)
Covers questions concerning the following:
the specifications, equipment, techniques and procedures applicable to specific products
methods employed to inspect the products
written practices
The above are the general requirements for inspector certification to ASNT-SNT-TC-1A that is the accepted certification associated with the ASME (American Society of Mechanical Engineers) code. The AWS (American Welding Society) and the API (American Petroleum Institute) code organizations have similar and also some totally different certification requirements for working with their respective codes.
AWS has specific requirements for visual inspection under its codes. Visual inspectors certified under ASNT-SNT-TC-1A are not certified to visually inspect any work performed under AWS codes. AWS does accept inspection certifications for most NDT/NDE personel that certified under ASNT-SNT-TC-1A for other inspection methods such as MT, PT, RT, and UT provided that the inspection procedures in the AWS codes are followed.
API will accept visual and NDT/NDE certifications from both AWS and ASME for certain work under their codes. Other specific inspection types require certification under API code requirements for items such as petroleum storage tanks.
Inspection requirements can be confusing without having access to the code books from each respective organization. AWS inspection requirements are addressed in AWS code D1.1. ASME inspection requirements are addressed in ASME Section V. API Standard 620 sets the requirements for an inspector under their codes.
In the field of NDT/NDE, certification is important because NDT/NDE personnel are often making critical judgments that can have safety and/or significant financial consequences if not performed properly. NDT/NDE personnel must have a tremendous amount of confidence in the results of their work. Since many of the NDT/NDE methods do not produce a permanent record of the inspection results, certification presents objective evidence of the knowledge and skill level of the person performing an inspection.
The procedure used to assure that NDT/NDE personnel possess the qualifications necessary to do competent work includes:
Classroom training to gain the necessary knowledge
Practical experience under the guidance of an experienced inspector
Three separate qualification examinations to demonstrate that competancy has been achieved
Written certification to document successful demonstration of competency.
NDT/NDE Methods
Certification can be obtained in number of NDT/NDE methods, which are listed in the table below.
Acoustic Emission Testing AE
Eddy Current Testing ET
Leak Testing LT
Liquid Penetrant Testing PT
Magnetic Particle Testing MT
Neutron Radiographic Testing NRT
Radiographic Testing RT
Thermal/Infrared Testing TIR
Ultrasonic Testing UT
Vibration Analysis Testing VA
Visual Testing VT
NDT/NDE Certification Levels
NDT/NDE personnel are generally certified to several different levels of competence within each of the NDT/NDE methods they are working. The levels are Level I, Level I Special, Level II, and Level III.
Level I technicians are only qualified to perform specific calibrations and tests, and acceptance or rejection determinations allow little or no deviation from the procedure. Level I technicians are under close supervision and direction of a higher level tester. The level I position is not the trainee level, but the first level a trainee reaches upon demonstrating ability in specific tests. Level I Special personnel are limited even more in what they can do. They are usually trained to a specific procedure and can perform only certain types of inspections on a certain set of components.
Level II technicians are able to set up and calibrate equipment, conduct the inspection according to procedures, interpret, evaluate and document results in all the testing method(s) utilized by the certificate holder. The technician can provide on the job training for Level I and Level I Specials and act as a supervisor. The technician can also organize and document the results of the inspection. They must be familiar with all applicable codes, standards, inspection procedures, and other documents that control the NDT/NDE method being utilized.
Level III technicians are capable of establishing inspection techniques and procedures; interpreting codes, standards, and specifications; and designating the particular nondestructive testing methods, techniques, and procedures to be used. They must also have knowledge of materials, fabrication, and product technology. Level III technicians are responsible for training and examining Level I and Level II's. Usually level III technicians are in administration, supervision, or management positions, or are owners of a testing laboratory. Some Level III technicians also become consultants.
Certification Requirements
There are a number of organizations that have produced documents that recommended or specify the minimum qualifications for certification. The following is a partial list of documents pertaining to the certification of NDT/NDE personnel in the USA.
ASNT-SNT-TC-1A, The American Society for Nondestructive Testing, Recommended Practice, Personnel Qualification and Certification in Nondestructive Testing.
ATA-105 Aviation Transport Association, Guidelines for Training and Qualifying Personnel in Nondestructive Testing Methods.
AIA-NAS-410, Aerospace Industries Association, National Aerospace Standard, NAS Certification and Qualification of Nondestructive Test Personnel.
ISO 9712, International Organization for Standards, Nondestructive testing -- Qualification and certification of personnel.
The education and work experience requirements for the various specification are common or similar. Typical requirements are summarized in the table below for qualification levels I and II. Please consult the certification documents to assure that information is correct for your situation.
Examination Method Level Required Hours of NDT Training Minimum hours of work experience in a method Permitted time frame to obtain required work experience in a method
(In Months)
For those with high school diploma or equivalent
For those with at least 2 years of engineering or science study at a college or technical school
Acoustic Emission I 40 32 210 1.5-9
II 40 40 630 4.5-27
Eddy Current I 40 24 210 1.5-9
II 40 40 630 4.5-27
Liquid Penetrant I 4 4 70 0.5-3
II 8 4 140 1-6
Magnetic Particle I 12 8 70 0.5-3
II 8 4 210 1.5-9
Neutron Radiography I 28 20 420 3-18
II 40 40 1680 12-72
Radiography I 40 30 210 1.5-9
II 40 35 630 4.5-27
Thermal/Infrared I 32 30 210 1.5-9
II 34 32 1260 9-27
Ultrasonics I 40 30 210 1.5-9
II 40 40 840 4.5-27
Vibration Analysis I 24 24 420 2-18
II 72 48 1680 12-72
Visual I 8 4 70 0.5-3
II 16 8 140 1-6
NDT training can be obtained at colleges, vocational-technical schools, the Armed Forces, commercial training companies and through individual company training departments.
To be considered for certification as a Level III an individual must:
Have graduated from a university or college with a degree in engineering or science, and have at least one year of experience comparable to that of a Level II in the applicable NDT method(s). or
Have completed with passing grades at least two years of engineering or science study at a university, college or technical school and have two years of experience comparable to that of a Level II in the applicable NDT method(s). or
Have four years of experience comparable to that of a Level II in the applicable NDT method(s).
Certification Examinations
Once the education, training and work experience requirements have been met and documented, certification examinations must be taken. The examination process is three part and includes several exams. For certification to Levels I and II a general, a specific, and a practical, exam must be completed with a passing grade of 70 percent for each exam and a composite grade of 80 percent (determined by averaging the results of the three exams).
The general exam contains questions that are fairly specific to the particular inspection process. The specific exam contains interpretation questions that are specific to both the inspection process and one or more codes that would cover that process. The practical exam is a "hands on" test using a test specimen with known defects. The questions on the practical test address accept or reject criteria for the specimen to one or more codes using an inspection procedure for that particular inspection process. More on Exams below.
The Level III exam process includes completion of a basic, a method, and a specific examination with a passing grade of 70 percent for each exam and a composite grade of 80 percent. Level I and II exams must be administered by an NDT level III and this is usually done within companies that provide either in-house or public inspection services. Level I, II and III exams can also be taken through a central agency such as ASNT. Central certifications provides technicians with documentation of qualification that is recognized nationally and in some cases internationally.
Visual Examination
It is important that NDT personnel have good near visual acuity and color vision. Therefore, an eye test must be taken to insure that natural or corrected near distance acuity is acceptable. Depending on which specification the company uses for certification, an individual must be able to read a Jaeger Number 1 or 2 (or equivalent) type and size letter at the designated distance. Determining contrast of color(especially shades of red) or shades of gray is also generally required. Color blindness can prohibit an individual from qualifying for certain inspection processes.
Level I and II General Examinations
A general examination is a written test which covers the general principles of a particular NDT method. The test will range from a minimum of 20 to 40 questions. There are practice general exams with answers available in a convenient Ebook format for six of the most widely used inspection methods. These are: ET, MT, PT, RT, UT, and VT. These can be purchased for a nominal price.
Level I and II Specific Examinations
A specific examination is a test which is related to specifications, equipment, techniques and procedures which the employer uses and requires a specific level of expertise from their personnel. This examination should also cover the codes and procedures used by the employer.
Practical Examination (for Level I and II)
A practical exam tests an individuals ability to operate the necessary equipment, record and analyze the results and accept or reject the part under inspection. A part is selected and the individual is expected to find indications and accept or reject the part to a specific code.
Basic Examination (for Level III)
Covers questions concerning the following:
qualification and certification
materials, fabrication and product technology
general NDT methods
Method Examination (for Level III)
Covers questions concerning the following:
the fundamentals and principles for the NDT method covered by the exam
the application and establishment of techniques and procedures
the capability for interpreting codes, standards and specifications relating to the method covered by the exam
Specific Examination (for Level III)
Covers questions concerning the following:
the specifications, equipment, techniques and procedures applicable to specific products
methods employed to inspect the products
written practices
The above are the general requirements for inspector certification to ASNT-SNT-TC-1A that is the accepted certification associated with the ASME (American Society of Mechanical Engineers) code. The AWS (American Welding Society) and the API (American Petroleum Institute) code organizations have similar and also some totally different certification requirements for working with their respective codes.
AWS has specific requirements for visual inspection under its codes. Visual inspectors certified under ASNT-SNT-TC-1A are not certified to visually inspect any work performed under AWS codes. AWS does accept inspection certifications for most NDT/NDE personel that certified under ASNT-SNT-TC-1A for other inspection methods such as MT, PT, RT, and UT provided that the inspection procedures in the AWS codes are followed.
API will accept visual and NDT/NDE certifications from both AWS and ASME for certain work under their codes. Other specific inspection types require certification under API code requirements for items such as petroleum storage tanks.
Inspection requirements can be confusing without having access to the code books from each respective organization. AWS inspection requirements are addressed in AWS code D1.1. ASME inspection requirements are addressed in ASME Section V. API Standard 620 sets the requirements for an inspector under their codes.
Duplex stainless steels
Typically twice the yield of austenitic stainless steels. Minimum Specified UTS typically 680 to 750N/mm2 (98.6 to 108ksi). Elongation typically > 25%.
Superior corrosion resistance than a 316. Good Resistance to stress corrosion cracking in a chloride environment.
Duplex materials have improved over the last decade; further additions of Nitrogen have been made improving weldability.
Because of the complex nature of this material it is important that it is sourced from good quality steel mills and is properly solution annealed. Castings and possibly thick sections may not cool fast when annealed causing sigma and other deleterious phases to form.
The material work hardens if cold formed; even the strain produced from welding can work harden the material particularly in multi pass welding. Therefore a full solution anneal is advantageous, particularly if low service temperatures are foreseen.
The high strength of this material can make joint fit up difficult.
Usable temperature range restricted to, -50 to 280°C
Used in Oil & Natural Gas production, chemical plants etc.
Standard Duplex
S31803 22Cr 5Ni 2.8Mo 0.15N PREn = 32-33
Super Duplex: Stronger and more corrosion resistant than standard duplex.
S32760(Zeron 100) 25Cr 7.5Ni 3.5Mo 0.23N PREn = 40
Micro Of Standard Duplex
Dark Areas:- Ferrite
Light Areas:- Austenite
Duplex solidifies initially as ferrite, then transforms on further cooling to a matrix of ferrite and austenite. In modern raw material the balance should be 50/50 for optimum corrosion resistance, particularly resistance to stress corrosion cracking. However the materials strength is not significantly effected by the ferrite / austenite phase balance.
The main problem with Duplex is that it very easily forms brittle intermetalic phases, such as Sigma, Chi and Alpha Prime. These phases can form rapidly, typically 100 seconds at 900°C. However shorter exposure has been known to cause a drop in toughness, this has been attribute to the formation of sigma on a microscopic scale.
Prolonged heating in the range 350 to 550°C can cause 475°C temper embrittlement.
For this reason the maximum recommended service temperature for duplex is about 280°C.
Sigma (55Fe 45Cr) can be a major problem when welding thin walled small bore pipe made of super duplex, although it can occur in thicker sections. It tends to be found in the bulk of the material rather than at the surface, therefore it probably has more effect on toughness than corrosion resistance. Sigma can also occur in thick sections, such as castings that have not been properly solution annealed (Not cooled fast enough).
However most standards accept that deleterious phases, such as sigma, chi and laves, may be tolerated if the strength and corrosion resistance are satisfactory.
Nitrogen is a strong austenite former and largely responsible for the balance between ferrite and austenite phases and the materials superior corrosion resistance. Nitrogen can’t be added to filler metal, as it does not transfer across the arc. It can also be lost from molten parent metal during welding. Its loss can lead to high ferrite and reduced corrosion resistance. Nitrogen can be added to the shielding gas and backing gas, Up to about 10%; however this makes welding difficult as it can cause porosity and contamination of the Tungsten electrode unless the correct welding technique is used. Too much Nitrogen will form a layer of Austenite on the weld surface. In my experience most duplex and super duplex are TIG welded using pure argon.
Backing / purge gas should contain less than 25ppm Oxygen for optimum corrosion resistance.
Fast cooling from molten will promote the formation of ferrite, slow cooling will promote austenite. During welding fast cooling is most likely, therefore welding consumables usually contain up to 2 - 4% extra Nickel to promote austenite formation in the weld. Duplex should never be welded without filler metal, as this will promote excessive ferrite, unless the welded component is solution annealed. Acceptable phase balance is usually 30 – 70% Ferrite
Duplex welding consumables are suitable for joining duplex to austenitic stainless steel or carbon steel; they can also be used for corrosion resistant overlays. Nickel based welding consumables can be used but the weld strength will not be as good as the parent metal, particularly on super duplex.
• Low levels of austenite: - Poor toughness and general corrosion resistance.
• High levels of austenite: - Some Reduction in strength and reduced resistance to stress corrosion cracking.
Good impact test results are a good indication that the material has been successfully welded. The parent metal usually exceeds 200J. The ductile to brittle transition temperature is about –50°C. The transition is not as steep as that of carbon steel and depends on the welding process used. Flux protected processes, such as MMA; tend to have a steeper transition curve and lower toughness. Multi run welds tend to promote austenite and thus exhibit higher toughness
Tight controls and the use of arc monitors are recommended during welding and automatic or mechanised welding is preferred. Repair welding can seriously affect corrosion resistance and toughness; therefore any repairs should follow specially developed procedures. See BS4515 Part 2 for details.
Production control test plates are recommended for all critical poduction welds.
Welding procedures should be supplemented by additional tests, depending on the application and the requirements of any application code:-
• A ferrite count using a Ferro scope is probably the most popular. For best accuracy the ferrite count should be performed manually and include a check for deleterious phases.
• Good impact test results are also a good indication of a successful welding procedure and are mandatory in BS4515 Part 2.
• A corrosion test, such as the G48 test, is highly recommended. The test may not model the exact service corrosion environment, but gives a good qualative assessment of the welds general corrosion resistance; this gives a good indication that the welding method is satisfactory. G48 test temperature for standard duplex is typically 22°C, for super duplex 35°C
________________________________________
Typical Welding Procedure For Zeron 100 (Super Duplex)
Pipe 60mm Od x 4mm Thick Position 6G
Maximum Interpass 100°C Temperature at the end of welding < 250°C
1.6mm Filler Wire 85 amps 2 weld runs (Root and Cap)
Arc energy 1 to 1,5 KJ/mm Travel speed 0.75 to 1 mm/sec
Recommended Testing
1. Ferric Chloride Pitting Test To ASTM G48 : Method A
2. Chemical analysis of root
3. Ferrite count
Superior corrosion resistance than a 316. Good Resistance to stress corrosion cracking in a chloride environment.
Duplex materials have improved over the last decade; further additions of Nitrogen have been made improving weldability.
Because of the complex nature of this material it is important that it is sourced from good quality steel mills and is properly solution annealed. Castings and possibly thick sections may not cool fast when annealed causing sigma and other deleterious phases to form.
The material work hardens if cold formed; even the strain produced from welding can work harden the material particularly in multi pass welding. Therefore a full solution anneal is advantageous, particularly if low service temperatures are foreseen.
The high strength of this material can make joint fit up difficult.
Usable temperature range restricted to, -50 to 280°C
Used in Oil & Natural Gas production, chemical plants etc.
Standard Duplex
S31803 22Cr 5Ni 2.8Mo 0.15N PREn = 32-33
Super Duplex: Stronger and more corrosion resistant than standard duplex.
S32760(Zeron 100) 25Cr 7.5Ni 3.5Mo 0.23N PREn = 40
Micro Of Standard Duplex
Dark Areas:- Ferrite
Light Areas:- Austenite
Duplex solidifies initially as ferrite, then transforms on further cooling to a matrix of ferrite and austenite. In modern raw material the balance should be 50/50 for optimum corrosion resistance, particularly resistance to stress corrosion cracking. However the materials strength is not significantly effected by the ferrite / austenite phase balance.
The main problem with Duplex is that it very easily forms brittle intermetalic phases, such as Sigma, Chi and Alpha Prime. These phases can form rapidly, typically 100 seconds at 900°C. However shorter exposure has been known to cause a drop in toughness, this has been attribute to the formation of sigma on a microscopic scale.
Prolonged heating in the range 350 to 550°C can cause 475°C temper embrittlement.
For this reason the maximum recommended service temperature for duplex is about 280°C.
Sigma (55Fe 45Cr) can be a major problem when welding thin walled small bore pipe made of super duplex, although it can occur in thicker sections. It tends to be found in the bulk of the material rather than at the surface, therefore it probably has more effect on toughness than corrosion resistance. Sigma can also occur in thick sections, such as castings that have not been properly solution annealed (Not cooled fast enough).
However most standards accept that deleterious phases, such as sigma, chi and laves, may be tolerated if the strength and corrosion resistance are satisfactory.
Nitrogen is a strong austenite former and largely responsible for the balance between ferrite and austenite phases and the materials superior corrosion resistance. Nitrogen can’t be added to filler metal, as it does not transfer across the arc. It can also be lost from molten parent metal during welding. Its loss can lead to high ferrite and reduced corrosion resistance. Nitrogen can be added to the shielding gas and backing gas, Up to about 10%; however this makes welding difficult as it can cause porosity and contamination of the Tungsten electrode unless the correct welding technique is used. Too much Nitrogen will form a layer of Austenite on the weld surface. In my experience most duplex and super duplex are TIG welded using pure argon.
Backing / purge gas should contain less than 25ppm Oxygen for optimum corrosion resistance.
Fast cooling from molten will promote the formation of ferrite, slow cooling will promote austenite. During welding fast cooling is most likely, therefore welding consumables usually contain up to 2 - 4% extra Nickel to promote austenite formation in the weld. Duplex should never be welded without filler metal, as this will promote excessive ferrite, unless the welded component is solution annealed. Acceptable phase balance is usually 30 – 70% Ferrite
Duplex welding consumables are suitable for joining duplex to austenitic stainless steel or carbon steel; they can also be used for corrosion resistant overlays. Nickel based welding consumables can be used but the weld strength will not be as good as the parent metal, particularly on super duplex.
• Low levels of austenite: - Poor toughness and general corrosion resistance.
• High levels of austenite: - Some Reduction in strength and reduced resistance to stress corrosion cracking.
Good impact test results are a good indication that the material has been successfully welded. The parent metal usually exceeds 200J. The ductile to brittle transition temperature is about –50°C. The transition is not as steep as that of carbon steel and depends on the welding process used. Flux protected processes, such as MMA; tend to have a steeper transition curve and lower toughness. Multi run welds tend to promote austenite and thus exhibit higher toughness
Tight controls and the use of arc monitors are recommended during welding and automatic or mechanised welding is preferred. Repair welding can seriously affect corrosion resistance and toughness; therefore any repairs should follow specially developed procedures. See BS4515 Part 2 for details.
Production control test plates are recommended for all critical poduction welds.
Welding procedures should be supplemented by additional tests, depending on the application and the requirements of any application code:-
• A ferrite count using a Ferro scope is probably the most popular. For best accuracy the ferrite count should be performed manually and include a check for deleterious phases.
• Good impact test results are also a good indication of a successful welding procedure and are mandatory in BS4515 Part 2.
• A corrosion test, such as the G48 test, is highly recommended. The test may not model the exact service corrosion environment, but gives a good qualative assessment of the welds general corrosion resistance; this gives a good indication that the welding method is satisfactory. G48 test temperature for standard duplex is typically 22°C, for super duplex 35°C
________________________________________
Typical Welding Procedure For Zeron 100 (Super Duplex)
Pipe 60mm Od x 4mm Thick Position 6G
Maximum Interpass 100°C Temperature at the end of welding < 250°C
1.6mm Filler Wire 85 amps 2 weld runs (Root and Cap)
Arc energy 1 to 1,5 KJ/mm Travel speed 0.75 to 1 mm/sec
Recommended Testing
1. Ferric Chloride Pitting Test To ASTM G48 : Method A
2. Chemical analysis of root
3. Ferrite count
SLAG INCLUSIONS/LACK OF FUSION
SLAG INCLUSIONS
CAUSES
• when two adjacent beads are deposited without proper over lap
• multi-pass welding with excessive undercuts in weld toe
• uneven profile of the preceding weld runs
• poor welding technique and type of flux (properties)
• access restrictions in doing a weld and cleaning the flux
• poor convex weld bead profile
PREVENTION
• clean the slag between runs by using circular wire brush (cup brush not allowed)
• use correct welding techniques to produce a smooth bead
• plan your sequence of the deposits with proper over lap
• Use the correct current and travel speed to avoid undercutting.
LACK OF FUSION
CAUSES
• narrow joint preparation
• incorrect welding parameter setting
• poor welding technique
• magnetic arc blow
• insufficient cleaning of oily or scaled surface
• LOF mainly appears in the vertical position
PREVENTION
• ensure the joint preparation is wide enough as per joint geometry
• ensure the correct welding parameters(high current, high speed, short arc length is used
• ensure the correct electrode/torch angle is used for manipulation of the arc in the joint
• Use the correct weaving limits
Seam weld orientation
• -check before make any fit up that the seam weld of the pipe at least 45 degree from the bottom of the pipe specially for field run avoiding the seam to intersect with any support will be install later.
• -check the distance between to seam welds is 2 times the thickness of the thicker member at least
CAUSES
• when two adjacent beads are deposited without proper over lap
• multi-pass welding with excessive undercuts in weld toe
• uneven profile of the preceding weld runs
• poor welding technique and type of flux (properties)
• access restrictions in doing a weld and cleaning the flux
• poor convex weld bead profile
PREVENTION
• clean the slag between runs by using circular wire brush (cup brush not allowed)
• use correct welding techniques to produce a smooth bead
• plan your sequence of the deposits with proper over lap
• Use the correct current and travel speed to avoid undercutting.
LACK OF FUSION
CAUSES
• narrow joint preparation
• incorrect welding parameter setting
• poor welding technique
• magnetic arc blow
• insufficient cleaning of oily or scaled surface
• LOF mainly appears in the vertical position
PREVENTION
• ensure the joint preparation is wide enough as per joint geometry
• ensure the correct welding parameters(high current, high speed, short arc length is used
• ensure the correct electrode/torch angle is used for manipulation of the arc in the joint
• Use the correct weaving limits
Seam weld orientation
• -check before make any fit up that the seam weld of the pipe at least 45 degree from the bottom of the pipe specially for field run avoiding the seam to intersect with any support will be install later.
• -check the distance between to seam welds is 2 times the thickness of the thicker member at least
Saturday, April 17, 2010
How often should I have my Ultrasonic Test Blocks recertified?
This question is becoming more and more common. Ten years ago, customers rarely sent UT blocks back to the manufacturer for recertification. Now, many do. We estimate that at this time, 15-20% of buyers send their blocks back to us to recertify. It may be that auditing or certifying agencies are beginning to look at test blocks as Measuring & Test Equipment (M&TE) and require that they be verified as time passes. We have seen situations where blocks that have been used extensively begin to exhibit dimensional changes. We have even observed blocks that are worn to the point that they no longer meet the intended specifications. Conversely, some blocks still look absolutely new after 10 years. Clearly, block condition, and the need to recertify, is influenced by the amount of use/abuse to which the block has been subjected.
PH Tool does not impose any calibration frequency on its blocks. We know they are perfect when they leave here, and we trust our customers to decide what makes sense for them. If you are considering having us recertify your test blocks, our suggestion is a 24 month calibration frequency. Some of our customers do this as frequently as every 12 months. It is really up to you to decide, based on your evaluation of the blocks and their use, and your internal Quality Assurance program.
We have a recertification reminder system in place at PH Tool that sends email reminders 60 days before the due date. Recertification reports are modified a bit from those for other blocks to include the original manufacture date, the recalibration date, the calibration frequency, and the next recalibration due date. In addition, much more dimensional data is provided on a recertification report compared to a new block cert, as every feature of the block is reported. We are happy to recertify PH Tool manufactured blocks as well as those made by others. Should you want to order new blocks with full dimensional certs upfront, let us know that when your order is placed. The cost of this service is discounted significantly at the time of new order placement. Contact us for recertification pricing and with any questions you may still have about this service.
PH Tool does not impose any calibration frequency on its blocks. We know they are perfect when they leave here, and we trust our customers to decide what makes sense for them. If you are considering having us recertify your test blocks, our suggestion is a 24 month calibration frequency. Some of our customers do this as frequently as every 12 months. It is really up to you to decide, based on your evaluation of the blocks and their use, and your internal Quality Assurance program.
We have a recertification reminder system in place at PH Tool that sends email reminders 60 days before the due date. Recertification reports are modified a bit from those for other blocks to include the original manufacture date, the recalibration date, the calibration frequency, and the next recalibration due date. In addition, much more dimensional data is provided on a recertification report compared to a new block cert, as every feature of the block is reported. We are happy to recertify PH Tool manufactured blocks as well as those made by others. Should you want to order new blocks with full dimensional certs upfront, let us know that when your order is placed. The cost of this service is discounted significantly at the time of new order placement. Contact us for recertification pricing and with any questions you may still have about this service.
Welding Distortion
Welding Distortion
There was a time when the welding operator used to pick up his shield and electrode holder and commenced welding a job, beginning and finishing at any place. If the completed work became distorted, it was taken for granted that it could not be avoided. The impression was that all welding caused distortion, so why worry.
This was purely ignorance, because distortion can be controlled and minimized by approaching the job in the correct manner. Today, welded work is being completed with minimum or no distortion. For example, large machine beds are being fabricated out of rolled steel sections and plates and welded within a tolerance of 1.5 mm.
The minimization of distortion is one of the most important factors in the production of a successful and economical weldment, or in the repair of a broken part. Uncontrolled or excessive distortion increases the job cost due to the expense of rectification or may render the job useless.
Distortion and Residual Stresses
When a metal is heated, it expands. If this expansion is resisted, deformation will occur. After welding/heating when the metal cools, it contracts.
If this contraction is resisted, a stress is applied. If this applied stress causes movement, distortion occurs. If this applied stress causes no movement, it is left as residual stress.
Concept of Distortion - An object is said to be distorted when it is put out of shape or it becomes unshapely. During welding, when weld metal is deposited, the base metal is heated and thus it expands, and, when after the welding, it cools, the base metal plus the weld metal shrinks.
It is therefore obvious that the shrinkage of a welded joint is far greater than the expansion.
This non uniform expansion and contraction of the weld metal and the adjacent base metal which occurs during the heating and cooling cycle of the welding process results in the distortion of a weldment.
To gain insight into how base metal expansion and base metal plus weld metal shrinkage cause distortion, it is helpful to look at
(i) What happens to base metal, and
(ii) What happens to base metal plus weld metal.
(i) During welding, the base metal near the arc is heated to the melting point. A few centimeters away, the temperature of the base metal is substantially lower.
This sharp temperature differential causes non uniform expansion followed by base metal movement, or metal displacement if the parts being joined are restrained. Also, the expansion of the hotter base metal (i.e., which is nearer the welding arc) is subject to restraint, due to the resistance of comparatively colder metal away from the welding arc. The metal nearer the arc expands more than that away from the arc.
As the arc passes down the joint, thus removing the source of heat, the base metal begins to cool and shrink. If the surrounding metal restrains the adjacent base metal from contracting normally, internal stresses build up. These combine with the stresses developed in the weld metal and increase the tendency to distort.
The volume of this adjacent base metal which contributes to distortion can be controlled by welding procedures. Achieving higher welding speeds through the use of powdered iron type manual electrodes and semiautomatic or fully automatic equipment using submerged arc or self-shielded welding reduces the amount of adjacent material that is affected by the heat of the arc and progressively decreases distortion.
(ii) During most of the welding, filler metal is added from the electrode. The molten filler metal and melted base metal combine to form the weld metal. Just as the weld metal solidifies, it is in its maximum expanded state actually occupying the greatest volume it can occupy as a solid.
Upon cooling, it attempts to contract to the volume it would normally occupy at the lower temperature, but is restrained from doing so by the adjacent base metal. At the time the weld reaches room temperature assuming complete restraint of the base metal so that it cannot move the weld tends to have lockedin tensile stresses approximately equal to the yield strength.
If one or more of the restraints are removed such as clamps holding the workpiece the locked in stresses find partial relief by causing the base metal to move thus deforming or distorting the weldment.
To Conclude
(i) Unequal expansion and contraction due to non-uniform (welding) heating, restraint from within the base metal, restraint due to other structural members joined with the base metal being welded tend to pull base metal out of original alignment and cause distortion.
(ii) Distortion of all kinds increases with the volume of metal deposited.
Types of Distortion
Distortion in weldments takes place by three dimensional changes that occur during welding:
(a) Longitudinal shrinkage that occurs parallel to the weld line.
(b) Transverse shrinkage that occurs perpendicular to the weld line.
(c) Angular change that consists of rotation around the weld line.
Longitudinal Type Shrinkage Distortion
When a weld is deposited lengthwise on a light, narrow and perfectly flat strip of metal that is neither clamped nor held in any way, the strip will tend to bow upward in the direction of bead.
This is due to the longitudinal contraction of the weld metal as it cools. Longitudinal contraction is maximum along the weld centre line and decreases towards the edges.
Longitudinal distortion depends upon the
(i) Contraction forces.
(ii) Stiffness of the section being welded.
(iii) Distance between the centroids of weld and section.
Transverse Shrinkage Distortion
When two plates being butt welded together are neither too heavy nor held together, and are thus free to move, they will be drawn closer together by the contraction of the weld metal. This is called transverse contraction. Transverse contraction exists all along the weld length and it depends upon the permanent contraction of elements in the weld zone.
The transverse contraction can be prevented by
(i) Proper tack welding.
(ii) Placing a wedge between the plates.
(iii) Separating the plates (before welding) to provide allowance (about 1 mm/100 mm of weld) for contraction.
(iv)Increasing the arc travel speed.
Angular Shrinkage
When two beveled plates are welded, it is found that the plates are pulled out of line with each other.
Since the opening at the top of the single Vee groove is greater than at the bottom, a greater portion of the weld metal is deposited there, and thus the drawing or pulling is greatest on that side of the joint.
Angular contraction is related to the shape and size of the cooling weld metal zone and the stiffness of the remaining unused part. Double groove joints tend to minimize angular distortion because the contraction effects of the two sides, i.e., top and bottom of the plate, get cancelled with each other.
Control of Welding Distortion
If distortion is to be prevented or minimized in a weldment, strategies must be used in the design and in shop practices to overcome the effects of the heating and cooling cycles. Shrinkage or contraction cannot be prevented, but it can be controlled.
There are various practical ways for minimizing the distortion caused by contraction:
1. Keep the contraction forces as low as possible by using only that amount of weld metal as is required by the joint. Another way to state this is doing not overweld. The more the metal placed in a joint, the greater the contraction forces will be.
Correctly sizing the weld for the service requirements of the joint helps to control distortion. The amount of weld metal can be minimized in a fillet joint by use of a flat or slightly convex bead, and in a butt joint by proper edge preparation, fit up and reinforcement.
A bevel not exceeding 30 degrees on each side will give proper fusion at the root of the weld, yet require minimal weld metal. J or U preparations further reduce weld metal for thicker plates. A double joint requires about one half the weld metal of a single joint.
When attaching stiffeners to plate, intermittent welds (in place of continuous welds) will enable reduction of weld metal to one fourth, yet give all the strength needed.To summarize, keep weld as small as possible.
2. Use as few weld passes as possible
The more the number of passes, the more is resulting shrink age (because shrinkage of each pass tends to be cumulative), and hence the distortion. Apply fewer passes with large electrodes. Select electrodes for highest deposition efficiency.
3. Place welds near the neutral axis
This reduces distortion by providing a smaller leverage for the shrink age (contraction) forces to pull the plates out of alignment.
4. Balance welds around the neutral axis
This will balance one shrinkage force against another. Design and welding sequence can be used to effectively control distortion.
5. Use of backs step welding or skip method of welding
With this welding technique, weld bead increments are deposited in the direction opposite to the progress of welding the joint e.g., each bead is deposited from right to left, but the welding progresses from left to right.
As each bead is placed, die heat from the weld along the edges causes expansion there, which temporarily separates the plates; but as the heat moves out across the plate, the expansion along the outer edges brings the plate back together.
The expansion of the plate is most pronounced when the first bead is laid. With successive beads, the plates expand less and less because of the locking effect of prior welds. Back stepping may have less effect in some cases and cannot be economically used in fully automatic welding.
6. Make shrinkage forces work in the desired direction
Several assemblies can be preset out of position before welding so that the shrinkage forces will pull the plates into alignment. Prebending or prespringing the parts to be welded is a simple example of the use of mechanically induced opposing forces to counteract weld shrinkage.
7. Balance shrinkage (contraction) forces with opposing forces.
The opposing forces may be
(i) Other shrinkage forces.
(ii) Restraining forces imposed by clamps, jigs and fixtures.
(iii) Restraining forces arising from the arrangement of members in the assembly.
(iv)The counterforce from the sag in a member produced by the force of gravity.
A common practice to balance shrinkage forces is to position identical weldments back to back and clamp them tightly together. The welds are completed on both assemblies and allowed to cool before the clamps are released.
Clamps, jigs and fixtures, that lock parts into a desired position and hold them until welding is finished, are probably the most widely used means of controlling distortion in small assemblies or component parts.
8. Welding sequences
Welding sequence implies the order of making the welds in a weldment. The weld metal is placed at different points about the structure so that as it shrinks at one place it will counteract the shrinkage forces of weld already made. Also, weld down hand whenever possible. Weld outward, from a central point. Restrict heat affected zone by keeping metal adjacent to joint as cool as possible.
9. Removal of shrinkage forces during or after welding
Peening is one method, in which force is applied to the weld (with the help of a hammer) to make it thinner thereby making it longer and relieving residual stresses.
Stress relief by controlled heating of the weldment to an elevated temperature followed by controlled cooling is another way to remove contraction forces.
10. Reduce the welding time
It is desirable to finish the weld quickly before too great a volume of surrounding metal becomes expanded by the heat. Welding should be carried out as fast as possible.
11. Breakdown large weldments into subassemblies.
In this manner, distortion errors can be rectified on each subassembly before final erection.
Minimizing Distortion in Repair Work - Control of distortion for repair welding is vitally important if the alignment of machined surfaces, bolt holes, bearings, etc., is to be preserved. Most of the basic principles mentioned under section 33.5 apply also to repair work, although some modification may be necessary in order to allow for various conditions which it may not be easy to control as in new construction. Bevel angles, for instance, may not be so accurately prepared and balancing of welds is generally not possible.
In the case of castings, the main precaution must be to avoid stressing the brittle cast metal and to allow weld metal contraction to take place without restraint.
As in the production of a weldment, distortion and stressing of the cast metal will be caused by local expansion at the weld point; the remedy is to keep the difference in temperature between the weld point and the remainder of the casting as small as possible.
This can be done by either keeping the heat input very low or by preheating. One of the most valuable points of these procedures, however, is the ductility of the weld metal during cooling.
When the fracture is between two parts which may be set as required before welding, allowance is easily made for angular distortion and contraction. Often, however, a fracture is tied, i.e., surrounded by other parts of a casting, and allowance must be made for expansion of the heated joint edges and contraction of the weld metal.
If much heat is applied to the fracture, the resultant expansion may crack the casting at another place: similarly, the rigidity of the casting may prevent weld metal contraction with the possibility of the weld metal cracking during cooling.
Effects of Metal Properties on Welding Distortion
1. Higher coefficients of thermal expansion mean greater amounts of expansion, therefore greater subsequent contraction and increased possibility for weldment distortion.
2. A metal with relatively low thermal conductivity will allow heat to flow out from a source at a low rate. When welding, this results in a steep temperature gradient, increases the shrinkage effect of the weld and plate adjacent to it and thus increases distortion.
3. The higher the yield strength of material in the weld area, the greater the amount of residual stress that can act to distort the weld assembly. Conversely, the lower the yield, the less likely (or severe) the possible distortion.
4. If modulus of elasticity is high, the material is more likely to resist movement or distortion.
Calculation of Shrinkage
1. Transverse shrinkage of butt welds
S = 5.16 x Aw / t + 1.27 d
where
S = transverse shrinkage
Aw = cross-sectional area of weld
t = thickness of plates
d = root openin
2. Longitudinal shrinkage of butt welds:
∆ L/L = 3.17 x I x L/100,000 x t
where
∆ L is the longitudinal shrinkage (mm)
L is length of weld (mm)
t is the plate thickness(mm)
I is welding current(Amp).
http://www.welding-technology-machines.info/welding-distortion/minimizing-distortion-in-repair-work.htm
There was a time when the welding operator used to pick up his shield and electrode holder and commenced welding a job, beginning and finishing at any place. If the completed work became distorted, it was taken for granted that it could not be avoided. The impression was that all welding caused distortion, so why worry.
This was purely ignorance, because distortion can be controlled and minimized by approaching the job in the correct manner. Today, welded work is being completed with minimum or no distortion. For example, large machine beds are being fabricated out of rolled steel sections and plates and welded within a tolerance of 1.5 mm.
The minimization of distortion is one of the most important factors in the production of a successful and economical weldment, or in the repair of a broken part. Uncontrolled or excessive distortion increases the job cost due to the expense of rectification or may render the job useless.
Distortion and Residual Stresses
When a metal is heated, it expands. If this expansion is resisted, deformation will occur. After welding/heating when the metal cools, it contracts.
If this contraction is resisted, a stress is applied. If this applied stress causes movement, distortion occurs. If this applied stress causes no movement, it is left as residual stress.
Concept of Distortion - An object is said to be distorted when it is put out of shape or it becomes unshapely. During welding, when weld metal is deposited, the base metal is heated and thus it expands, and, when after the welding, it cools, the base metal plus the weld metal shrinks.
It is therefore obvious that the shrinkage of a welded joint is far greater than the expansion.
This non uniform expansion and contraction of the weld metal and the adjacent base metal which occurs during the heating and cooling cycle of the welding process results in the distortion of a weldment.
To gain insight into how base metal expansion and base metal plus weld metal shrinkage cause distortion, it is helpful to look at
(i) What happens to base metal, and
(ii) What happens to base metal plus weld metal.
(i) During welding, the base metal near the arc is heated to the melting point. A few centimeters away, the temperature of the base metal is substantially lower.
This sharp temperature differential causes non uniform expansion followed by base metal movement, or metal displacement if the parts being joined are restrained. Also, the expansion of the hotter base metal (i.e., which is nearer the welding arc) is subject to restraint, due to the resistance of comparatively colder metal away from the welding arc. The metal nearer the arc expands more than that away from the arc.
As the arc passes down the joint, thus removing the source of heat, the base metal begins to cool and shrink. If the surrounding metal restrains the adjacent base metal from contracting normally, internal stresses build up. These combine with the stresses developed in the weld metal and increase the tendency to distort.
The volume of this adjacent base metal which contributes to distortion can be controlled by welding procedures. Achieving higher welding speeds through the use of powdered iron type manual electrodes and semiautomatic or fully automatic equipment using submerged arc or self-shielded welding reduces the amount of adjacent material that is affected by the heat of the arc and progressively decreases distortion.
(ii) During most of the welding, filler metal is added from the electrode. The molten filler metal and melted base metal combine to form the weld metal. Just as the weld metal solidifies, it is in its maximum expanded state actually occupying the greatest volume it can occupy as a solid.
Upon cooling, it attempts to contract to the volume it would normally occupy at the lower temperature, but is restrained from doing so by the adjacent base metal. At the time the weld reaches room temperature assuming complete restraint of the base metal so that it cannot move the weld tends to have lockedin tensile stresses approximately equal to the yield strength.
If one or more of the restraints are removed such as clamps holding the workpiece the locked in stresses find partial relief by causing the base metal to move thus deforming or distorting the weldment.
To Conclude
(i) Unequal expansion and contraction due to non-uniform (welding) heating, restraint from within the base metal, restraint due to other structural members joined with the base metal being welded tend to pull base metal out of original alignment and cause distortion.
(ii) Distortion of all kinds increases with the volume of metal deposited.
Types of Distortion
Distortion in weldments takes place by three dimensional changes that occur during welding:
(a) Longitudinal shrinkage that occurs parallel to the weld line.
(b) Transverse shrinkage that occurs perpendicular to the weld line.
(c) Angular change that consists of rotation around the weld line.
Longitudinal Type Shrinkage Distortion
When a weld is deposited lengthwise on a light, narrow and perfectly flat strip of metal that is neither clamped nor held in any way, the strip will tend to bow upward in the direction of bead.
This is due to the longitudinal contraction of the weld metal as it cools. Longitudinal contraction is maximum along the weld centre line and decreases towards the edges.
Longitudinal distortion depends upon the
(i) Contraction forces.
(ii) Stiffness of the section being welded.
(iii) Distance between the centroids of weld and section.
Transverse Shrinkage Distortion
When two plates being butt welded together are neither too heavy nor held together, and are thus free to move, they will be drawn closer together by the contraction of the weld metal. This is called transverse contraction. Transverse contraction exists all along the weld length and it depends upon the permanent contraction of elements in the weld zone.
The transverse contraction can be prevented by
(i) Proper tack welding.
(ii) Placing a wedge between the plates.
(iii) Separating the plates (before welding) to provide allowance (about 1 mm/100 mm of weld) for contraction.
(iv)Increasing the arc travel speed.
Angular Shrinkage
When two beveled plates are welded, it is found that the plates are pulled out of line with each other.
Since the opening at the top of the single Vee groove is greater than at the bottom, a greater portion of the weld metal is deposited there, and thus the drawing or pulling is greatest on that side of the joint.
Angular contraction is related to the shape and size of the cooling weld metal zone and the stiffness of the remaining unused part. Double groove joints tend to minimize angular distortion because the contraction effects of the two sides, i.e., top and bottom of the plate, get cancelled with each other.
Control of Welding Distortion
If distortion is to be prevented or minimized in a weldment, strategies must be used in the design and in shop practices to overcome the effects of the heating and cooling cycles. Shrinkage or contraction cannot be prevented, but it can be controlled.
There are various practical ways for minimizing the distortion caused by contraction:
1. Keep the contraction forces as low as possible by using only that amount of weld metal as is required by the joint. Another way to state this is doing not overweld. The more the metal placed in a joint, the greater the contraction forces will be.
Correctly sizing the weld for the service requirements of the joint helps to control distortion. The amount of weld metal can be minimized in a fillet joint by use of a flat or slightly convex bead, and in a butt joint by proper edge preparation, fit up and reinforcement.
A bevel not exceeding 30 degrees on each side will give proper fusion at the root of the weld, yet require minimal weld metal. J or U preparations further reduce weld metal for thicker plates. A double joint requires about one half the weld metal of a single joint.
When attaching stiffeners to plate, intermittent welds (in place of continuous welds) will enable reduction of weld metal to one fourth, yet give all the strength needed.To summarize, keep weld as small as possible.
2. Use as few weld passes as possible
The more the number of passes, the more is resulting shrink age (because shrinkage of each pass tends to be cumulative), and hence the distortion. Apply fewer passes with large electrodes. Select electrodes for highest deposition efficiency.
3. Place welds near the neutral axis
This reduces distortion by providing a smaller leverage for the shrink age (contraction) forces to pull the plates out of alignment.
4. Balance welds around the neutral axis
This will balance one shrinkage force against another. Design and welding sequence can be used to effectively control distortion.
5. Use of backs step welding or skip method of welding
With this welding technique, weld bead increments are deposited in the direction opposite to the progress of welding the joint e.g., each bead is deposited from right to left, but the welding progresses from left to right.
As each bead is placed, die heat from the weld along the edges causes expansion there, which temporarily separates the plates; but as the heat moves out across the plate, the expansion along the outer edges brings the plate back together.
The expansion of the plate is most pronounced when the first bead is laid. With successive beads, the plates expand less and less because of the locking effect of prior welds. Back stepping may have less effect in some cases and cannot be economically used in fully automatic welding.
6. Make shrinkage forces work in the desired direction
Several assemblies can be preset out of position before welding so that the shrinkage forces will pull the plates into alignment. Prebending or prespringing the parts to be welded is a simple example of the use of mechanically induced opposing forces to counteract weld shrinkage.
7. Balance shrinkage (contraction) forces with opposing forces.
The opposing forces may be
(i) Other shrinkage forces.
(ii) Restraining forces imposed by clamps, jigs and fixtures.
(iii) Restraining forces arising from the arrangement of members in the assembly.
(iv)The counterforce from the sag in a member produced by the force of gravity.
A common practice to balance shrinkage forces is to position identical weldments back to back and clamp them tightly together. The welds are completed on both assemblies and allowed to cool before the clamps are released.
Clamps, jigs and fixtures, that lock parts into a desired position and hold them until welding is finished, are probably the most widely used means of controlling distortion in small assemblies or component parts.
8. Welding sequences
Welding sequence implies the order of making the welds in a weldment. The weld metal is placed at different points about the structure so that as it shrinks at one place it will counteract the shrinkage forces of weld already made. Also, weld down hand whenever possible. Weld outward, from a central point. Restrict heat affected zone by keeping metal adjacent to joint as cool as possible.
9. Removal of shrinkage forces during or after welding
Peening is one method, in which force is applied to the weld (with the help of a hammer) to make it thinner thereby making it longer and relieving residual stresses.
Stress relief by controlled heating of the weldment to an elevated temperature followed by controlled cooling is another way to remove contraction forces.
10. Reduce the welding time
It is desirable to finish the weld quickly before too great a volume of surrounding metal becomes expanded by the heat. Welding should be carried out as fast as possible.
11. Breakdown large weldments into subassemblies.
In this manner, distortion errors can be rectified on each subassembly before final erection.
Minimizing Distortion in Repair Work - Control of distortion for repair welding is vitally important if the alignment of machined surfaces, bolt holes, bearings, etc., is to be preserved. Most of the basic principles mentioned under section 33.5 apply also to repair work, although some modification may be necessary in order to allow for various conditions which it may not be easy to control as in new construction. Bevel angles, for instance, may not be so accurately prepared and balancing of welds is generally not possible.
In the case of castings, the main precaution must be to avoid stressing the brittle cast metal and to allow weld metal contraction to take place without restraint.
As in the production of a weldment, distortion and stressing of the cast metal will be caused by local expansion at the weld point; the remedy is to keep the difference in temperature between the weld point and the remainder of the casting as small as possible.
This can be done by either keeping the heat input very low or by preheating. One of the most valuable points of these procedures, however, is the ductility of the weld metal during cooling.
When the fracture is between two parts which may be set as required before welding, allowance is easily made for angular distortion and contraction. Often, however, a fracture is tied, i.e., surrounded by other parts of a casting, and allowance must be made for expansion of the heated joint edges and contraction of the weld metal.
If much heat is applied to the fracture, the resultant expansion may crack the casting at another place: similarly, the rigidity of the casting may prevent weld metal contraction with the possibility of the weld metal cracking during cooling.
Effects of Metal Properties on Welding Distortion
1. Higher coefficients of thermal expansion mean greater amounts of expansion, therefore greater subsequent contraction and increased possibility for weldment distortion.
2. A metal with relatively low thermal conductivity will allow heat to flow out from a source at a low rate. When welding, this results in a steep temperature gradient, increases the shrinkage effect of the weld and plate adjacent to it and thus increases distortion.
3. The higher the yield strength of material in the weld area, the greater the amount of residual stress that can act to distort the weld assembly. Conversely, the lower the yield, the less likely (or severe) the possible distortion.
4. If modulus of elasticity is high, the material is more likely to resist movement or distortion.
Calculation of Shrinkage
1. Transverse shrinkage of butt welds
S = 5.16 x Aw / t + 1.27 d
where
S = transverse shrinkage
Aw = cross-sectional area of weld
t = thickness of plates
d = root openin
2. Longitudinal shrinkage of butt welds:
∆ L/L = 3.17 x I x L/100,000 x t
where
∆ L is the longitudinal shrinkage (mm)
L is length of weld (mm)
t is the plate thickness(mm)
I is welding current(Amp).
http://www.welding-technology-machines.info/welding-distortion/minimizing-distortion-in-repair-work.htm
Types of Welding Processes
The links to the topics below are the best informational articles that I could find on each of the subjects.
Carbon Arc Gouging
Carbon Arc Gouging or Air Arc Gouging is not a welding process, but is an effective process to quickly remove base or weld metal where needed. A copper coated carbon electrode is placed into a gouging rig that is designed for that purpose. A high voltage and amperage power source coupled with an air source of about 80-100 psi melts the metal and expels the molten metal from the work piece. The process is loud, smokey, and sprays molten metal in the direction that the air jet is pointed at. The process is normally used for weld repairs, back gouging the back side of full penetration welds, beveling plate edges, and removing excess weld.
..... Click the link for more information.
Cold welding
Cold or contact welding was first recognized as a general materials phenomenon in the 1940s. It was then discovered that two clean, flat surfaces of similar metal would strongly adhere if brought into contact under vacuum. It is now known that the force of adhesion following first contact can be augmented by pressing the metals tightly together, increasing the duration of contact, raising the temperature of the work pieces, or any combination of the preceding.
..... Click the link for more information.
Electron beam welding
Electron beam welding is a welding process where the energy to melt the material is applied by an electron beam. To avoid dispersion of the electron beam, the workpiece is typically placed in a vacuum chamber, although electron beam welding under atmospheric pressure is attempted too. Electron beam welding is an established branch of Electron Beam Technology.
..... Click the link for more information.
Explosive welding
Explosive welding uses the force of a controlled detonation to atomically fuse one metal object to another. The process is popular for the joining of dissimilar metals. Explosive welding is considered a cold welding process that allows metals to be joined without losing their pre-welded metalurgical properties. This process allows the joining of different metals that would be impossible by any other welding process.
..... Click the link for more information.
Forge welding
Forge welding , the oldest known form of welding, is a welding process of heating two or more pieces of wrought iron or steel until their surfaces are malleable and then hammering them together. Often a flux is used to keep the welding surfaces from oxidizing and producing a poor quality weld. A simple flux can be made from borax, sometimes with the addition of iron filings. Care must be taken to avoid "burning" the metal, which is overheating to the point that it gives off sparks from rapid oxidation.
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Friction welding
Friction welding Rotary friction welding was the first of the friction welding methods to be developed and commercially used. There are two method variations: continuous drive rotary friction welding and stored energy friction welding. In the first method, a piece is rotated at a set speed while the joining stationary piece is fed into it at a pre-determined pressure until the metal in the joint area reaches a temperature high enough to melt it. The other method, also known as inertia welding adds a flywheel to the rotating piece and power is cut as the two pieces are forced together with the same end result-a welded joint. Parts with a non-rotational geometry can be joined by linear reciprocating frictional welding which is similar in form to a reciprocating saw.
..... Click the link for more information.
Friction-stir welding
Friction-stir welding was invented and experimentally proven by Wayne Thomas and a team of his colleagues at the TWI Welding Institute, U. K., in December 1991. TWI holds a patent for the process. In FSW, a cylindrical-shouldered tool, with a profiled threaded / unthreaded probe (nib) is rotated at a constant speed and fed at a constant traverse rate into the joint line between two pieces
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Fusion welding
Fusion welding is any welding process that uses a heat source to weld a material and also usually uses a protective shield from the atmosphere by a gas shield or flux or both. This would include gas, stick, mig, tig, sub-arc, laser, orbital, plasma, spot, stud, thermite, and electron beam welding.
..... Click the link for more information.
Gas welding
In gas welding, the heat energy and high temperature needed to melt the metal is obtained by the combustion of a fuel gas with oxygen. Gas Fuels--The most commonly used fuel gas is acetylene. Other gases used are liquified petroleum gas (LPG), natural gas, hydrogen and MAPP gas. Acetylene is obtained from the action of water upon calcium carbide. Calcium carbide and water combine to yield acetylene gas and lime as a by-product.
..... Click the link for more information.
Induction welding
Induction welding is a form of welding that uses electromagnetic induction to heat the workpiece. The welding apparatus contains an induction coil that is energized with a radio-frequency electric current. This generates a high-frequency electromagnetic field that acts on either an electrically conductive or a ferromagnetic workpiece. In an electrically conductive workpiece, such as steel, the main heating effect is resistive heating, which is due to magnetically induced currents called eddy currents. Nonmagnetic materials such as plastics can be induction-welded by implanting them with metallic or ferro-magnetic compounds called susceptors, that absorb the electromagnetic energy from the induction coil, become hot, and lose their heat energy to the surrounding material by thermal conduction.
..... Click the link for more information.
Laser welding
Laser welding was in its infancy 20 years ago. Today, laser welding is an integral part of the plastics and metal working industries. A wide variety of cutting and welding operations can be performed on a variety of hard to weld and dissimilar materials with this process. A benefit of laser cutting is the ability to cut a wide range of materials such as metal, polymers, ceramics, wood, leather, cloth, and more. Cladding, heat-treating and hard-surfacing can also be accomplished with laser welding.
..... Click the link for more information.
Manual Metal Arc Welding
Manual Metal Arc welding, also known as stick or SMAW-Shielded Metal Arc Welding is one of the most common and reliable forms of welding. An electric current (either alternating current or direct current) is used to form an arc between an electrode coated in flux and the metals to be joined. The flux gives off gases to prevent oxygen reacting with the weld metal. The flux then solidifies to form slag on top of the weld. Once cool the slag can easily be chipped off provided that the weld is properly applied.
..... Click the link for more information.
Gas Metal Arc Welding
Metal Inert Gas or MIG welding, also known as gas metal arc welding, is a type of welding which utilizes a welding gun through which a continuous wire electrode and a shielding gas is fed. The wires used in the electrodes are typically 0.7, 1.0, 1.2 or 1.6 mm diameter, either solid or 'flux' filled. To prevent nitrogen and oxygen contaminating the weld, an inert shielding gas is fed around the arc, either argon or helium.
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Plasma welding
Plasma welding
is a process that utilizes a stream of ionized particles. It originated in 1955 as an aluminum cutting process and used as such until the first successful welds were produced in 1963. The plasma torch uses a water-cooled copper nozzle and tungsten electrode. An electric arc is produced between the electrode and copper nozzle while a gas suchas helium or hydrogen is forced through the arc. The gas becomes super-heated and ionizes into a plasma stream. Click the link for more information.
Resistance Welding
Resistance Spot Welding is a quick and simple method of welding metal. It uses two large electrodes which are placed on either side of the surface to be welded, and passes a large electrical current through them that heats up the metal in-between. The result is a small "spot" that is quickly heated to the melting point, forming a small dot of welded metal. Applying the current for too long can burn a hole right through the material.
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Stud welding
Stud welding is an electric arc process for attaching studs and other fasteners to steel and other surfaces. Stud welding eliminates the need for drilling or punching holes in the structure. A special collet on the stud gun holds a ceramic ferrule in place around the stud. This ferrule holds the molten metal in place and helps form the fillet weld as the stud cools after it is shot onto the structure. Click the link for more information.
Submerged arc welding Submerged arc welding is a type of welding which utilises a large diameter wire electrode, typically 3 or 4mm diameter. The electode is fed into the arc at a controlled rate. The arc is shielded by a granular flux which is poured to form a pile of flux surrounding the arc. Unlike other types of arc welding, eye protection is not required, since the arc is covered by the flux. Some of the flux is converted to slag by the arc, which protects the weld as it cools. The slag can easily be chipped off the weld when cool. Surplus flux is collected for re-use.
..... Click the link for more information.
Thermite
A thermite or thermit reaction is one in which aluminum metal is oxidized by an oxide of another metal, most commonly that of iron. (The name thermite is also used to refer to a mixture of two such chemicals.) The products are aluminium oxide, free elemental metal and a great deal of heat. The reactants are commonly powdered and mixed with a binder to keep the material solid and prevent separation.
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Tungsten Inert Gas Welding
Tungsten inert gas welding or TIG is also known as gas tungsten arc welding (GTAW) or HELIARC, a trade name of Linde. A fixed tungsten electrode protected by a shielding gas is used to create an arc that melts the metal of the parts to be joined. As there is no continuous feed wire electrode as with MIG welding, a filler rod is dipped in the puddle of molten metal to join the two parts.
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(TIG)
Ultrasonic welding In ultrasonic welding, energy is delivered to the joint area in the form of high-power ultrasound. This type of welding is used to build assemblies that are too small, complex, or delicate for more common welding techniques to be appropriate. It is also used to weld plastics and materials that are dissimilar. For joining complex injection molded parts, ultrasonic welding requires expensive custom equipment specially designed for the parts being welded. The parts are sandwiched between shaped mandrel and the horn. One of the plastic parts has a spiked energy director which contacts the second plastic part. The ultrasonic energy melts the point contact between and the parts and they are joined. This process replaces a glued joint.
..... Click the link for more information.
Underwater SMAW Welding In underwater SMAW welding, a coated welding electrode along with an insulated electrode holder is used to make sound welds. This type of welding is used to weld assemblies that are impractical or too expensive to move above water. Specific techniques are used to insure a sound weld.
..... Click the link for more information.
Welding differs from soldering Soldering is a method of applying a lower melting point metal to join other metal parts using solder. Soldering can be performed in a number of ways, including bulk liquification, or by using a point source such as an electric soldering iron or brazing torch. One application of soldering is making connections between electronic parts and printed circuit boards, another is in plumbing.
..... Click the link for more information. brazing Brazing is a joining process whereby a non-ferrous filler metal and an alloy are heated to melting temperature (above 450 °C) and distributed between two or more close-fitting parts by capillary attraction. At its liquidus temperature, the molten filler metal interacts with a thin layer of the base metal, cooling to form an exceptionally strong, sealed joint due to grain structure interaction. The brazed joint becomes a sandwich of different layers, each metallurgicaly linked to each other. If silver alloy is used, brazing can be referred to as Silver Brazing or Sil-brazing. Colloquially, the inaccurate terms "Silver Soldering" or "Hard Soldering" are used.
..... Click the link for more information.
Silver (Brazing) Soldering uses a material called silver solder. A solder is a metal alloy (often of silver, tin and lead), usually with a low melting point, that is melted and used to join metallic surfaces, especially in the fields of electronics and plumbing, in a process called soldering. In electronics, tin/lead solders are normally 60/40 by weight in order to produce a near-eutectic mixture (lowest melting point - below 190°C).
..... Click the link for more information.
http://www.weldprocedures.com/weldingprocesses.html
Carbon Arc Gouging
Carbon Arc Gouging or Air Arc Gouging is not a welding process, but is an effective process to quickly remove base or weld metal where needed. A copper coated carbon electrode is placed into a gouging rig that is designed for that purpose. A high voltage and amperage power source coupled with an air source of about 80-100 psi melts the metal and expels the molten metal from the work piece. The process is loud, smokey, and sprays molten metal in the direction that the air jet is pointed at. The process is normally used for weld repairs, back gouging the back side of full penetration welds, beveling plate edges, and removing excess weld.
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Cold welding
Cold or contact welding was first recognized as a general materials phenomenon in the 1940s. It was then discovered that two clean, flat surfaces of similar metal would strongly adhere if brought into contact under vacuum. It is now known that the force of adhesion following first contact can be augmented by pressing the metals tightly together, increasing the duration of contact, raising the temperature of the work pieces, or any combination of the preceding.
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Electron beam welding
Electron beam welding is a welding process where the energy to melt the material is applied by an electron beam. To avoid dispersion of the electron beam, the workpiece is typically placed in a vacuum chamber, although electron beam welding under atmospheric pressure is attempted too. Electron beam welding is an established branch of Electron Beam Technology.
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Explosive welding
Explosive welding uses the force of a controlled detonation to atomically fuse one metal object to another. The process is popular for the joining of dissimilar metals. Explosive welding is considered a cold welding process that allows metals to be joined without losing their pre-welded metalurgical properties. This process allows the joining of different metals that would be impossible by any other welding process.
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Forge welding
Forge welding , the oldest known form of welding, is a welding process of heating two or more pieces of wrought iron or steel until their surfaces are malleable and then hammering them together. Often a flux is used to keep the welding surfaces from oxidizing and producing a poor quality weld. A simple flux can be made from borax, sometimes with the addition of iron filings. Care must be taken to avoid "burning" the metal, which is overheating to the point that it gives off sparks from rapid oxidation.
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Friction welding
Friction welding Rotary friction welding was the first of the friction welding methods to be developed and commercially used. There are two method variations: continuous drive rotary friction welding and stored energy friction welding. In the first method, a piece is rotated at a set speed while the joining stationary piece is fed into it at a pre-determined pressure until the metal in the joint area reaches a temperature high enough to melt it. The other method, also known as inertia welding adds a flywheel to the rotating piece and power is cut as the two pieces are forced together with the same end result-a welded joint. Parts with a non-rotational geometry can be joined by linear reciprocating frictional welding which is similar in form to a reciprocating saw.
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Friction-stir welding
Friction-stir welding was invented and experimentally proven by Wayne Thomas and a team of his colleagues at the TWI Welding Institute, U. K., in December 1991. TWI holds a patent for the process. In FSW, a cylindrical-shouldered tool, with a profiled threaded / unthreaded probe (nib) is rotated at a constant speed and fed at a constant traverse rate into the joint line between two pieces
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Fusion welding
Fusion welding is any welding process that uses a heat source to weld a material and also usually uses a protective shield from the atmosphere by a gas shield or flux or both. This would include gas, stick, mig, tig, sub-arc, laser, orbital, plasma, spot, stud, thermite, and electron beam welding.
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Gas welding
In gas welding, the heat energy and high temperature needed to melt the metal is obtained by the combustion of a fuel gas with oxygen. Gas Fuels--The most commonly used fuel gas is acetylene. Other gases used are liquified petroleum gas (LPG), natural gas, hydrogen and MAPP gas. Acetylene is obtained from the action of water upon calcium carbide. Calcium carbide and water combine to yield acetylene gas and lime as a by-product.
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Induction welding
Induction welding is a form of welding that uses electromagnetic induction to heat the workpiece. The welding apparatus contains an induction coil that is energized with a radio-frequency electric current. This generates a high-frequency electromagnetic field that acts on either an electrically conductive or a ferromagnetic workpiece. In an electrically conductive workpiece, such as steel, the main heating effect is resistive heating, which is due to magnetically induced currents called eddy currents. Nonmagnetic materials such as plastics can be induction-welded by implanting them with metallic or ferro-magnetic compounds called susceptors, that absorb the electromagnetic energy from the induction coil, become hot, and lose their heat energy to the surrounding material by thermal conduction.
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Laser welding
Laser welding was in its infancy 20 years ago. Today, laser welding is an integral part of the plastics and metal working industries. A wide variety of cutting and welding operations can be performed on a variety of hard to weld and dissimilar materials with this process. A benefit of laser cutting is the ability to cut a wide range of materials such as metal, polymers, ceramics, wood, leather, cloth, and more. Cladding, heat-treating and hard-surfacing can also be accomplished with laser welding.
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Manual Metal Arc Welding
Manual Metal Arc welding, also known as stick or SMAW-Shielded Metal Arc Welding is one of the most common and reliable forms of welding. An electric current (either alternating current or direct current) is used to form an arc between an electrode coated in flux and the metals to be joined. The flux gives off gases to prevent oxygen reacting with the weld metal. The flux then solidifies to form slag on top of the weld. Once cool the slag can easily be chipped off provided that the weld is properly applied.
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Gas Metal Arc Welding
Metal Inert Gas or MIG welding, also known as gas metal arc welding, is a type of welding which utilizes a welding gun through which a continuous wire electrode and a shielding gas is fed. The wires used in the electrodes are typically 0.7, 1.0, 1.2 or 1.6 mm diameter, either solid or 'flux' filled. To prevent nitrogen and oxygen contaminating the weld, an inert shielding gas is fed around the arc, either argon or helium.
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Plasma welding
Plasma welding
is a process that utilizes a stream of ionized particles. It originated in 1955 as an aluminum cutting process and used as such until the first successful welds were produced in 1963. The plasma torch uses a water-cooled copper nozzle and tungsten electrode. An electric arc is produced between the electrode and copper nozzle while a gas suchas helium or hydrogen is forced through the arc. The gas becomes super-heated and ionizes into a plasma stream. Click the link for more information.
Resistance Welding
Resistance Spot Welding is a quick and simple method of welding metal. It uses two large electrodes which are placed on either side of the surface to be welded, and passes a large electrical current through them that heats up the metal in-between. The result is a small "spot" that is quickly heated to the melting point, forming a small dot of welded metal. Applying the current for too long can burn a hole right through the material.
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Stud welding
Stud welding is an electric arc process for attaching studs and other fasteners to steel and other surfaces. Stud welding eliminates the need for drilling or punching holes in the structure. A special collet on the stud gun holds a ceramic ferrule in place around the stud. This ferrule holds the molten metal in place and helps form the fillet weld as the stud cools after it is shot onto the structure. Click the link for more information.
Submerged arc welding Submerged arc welding is a type of welding which utilises a large diameter wire electrode, typically 3 or 4mm diameter. The electode is fed into the arc at a controlled rate. The arc is shielded by a granular flux which is poured to form a pile of flux surrounding the arc. Unlike other types of arc welding, eye protection is not required, since the arc is covered by the flux. Some of the flux is converted to slag by the arc, which protects the weld as it cools. The slag can easily be chipped off the weld when cool. Surplus flux is collected for re-use.
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Thermite
A thermite or thermit reaction is one in which aluminum metal is oxidized by an oxide of another metal, most commonly that of iron. (The name thermite is also used to refer to a mixture of two such chemicals.) The products are aluminium oxide, free elemental metal and a great deal of heat. The reactants are commonly powdered and mixed with a binder to keep the material solid and prevent separation.
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Tungsten Inert Gas Welding
Tungsten inert gas welding or TIG is also known as gas tungsten arc welding (GTAW) or HELIARC, a trade name of Linde. A fixed tungsten electrode protected by a shielding gas is used to create an arc that melts the metal of the parts to be joined. As there is no continuous feed wire electrode as with MIG welding, a filler rod is dipped in the puddle of molten metal to join the two parts.
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(TIG)
Ultrasonic welding In ultrasonic welding, energy is delivered to the joint area in the form of high-power ultrasound. This type of welding is used to build assemblies that are too small, complex, or delicate for more common welding techniques to be appropriate. It is also used to weld plastics and materials that are dissimilar. For joining complex injection molded parts, ultrasonic welding requires expensive custom equipment specially designed for the parts being welded. The parts are sandwiched between shaped mandrel and the horn. One of the plastic parts has a spiked energy director which contacts the second plastic part. The ultrasonic energy melts the point contact between and the parts and they are joined. This process replaces a glued joint.
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Underwater SMAW Welding In underwater SMAW welding, a coated welding electrode along with an insulated electrode holder is used to make sound welds. This type of welding is used to weld assemblies that are impractical or too expensive to move above water. Specific techniques are used to insure a sound weld.
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Welding differs from soldering Soldering is a method of applying a lower melting point metal to join other metal parts using solder. Soldering can be performed in a number of ways, including bulk liquification, or by using a point source such as an electric soldering iron or brazing torch. One application of soldering is making connections between electronic parts and printed circuit boards, another is in plumbing.
..... Click the link for more information. brazing Brazing is a joining process whereby a non-ferrous filler metal and an alloy are heated to melting temperature (above 450 °C) and distributed between two or more close-fitting parts by capillary attraction. At its liquidus temperature, the molten filler metal interacts with a thin layer of the base metal, cooling to form an exceptionally strong, sealed joint due to grain structure interaction. The brazed joint becomes a sandwich of different layers, each metallurgicaly linked to each other. If silver alloy is used, brazing can be referred to as Silver Brazing or Sil-brazing. Colloquially, the inaccurate terms "Silver Soldering" or "Hard Soldering" are used.
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Silver (Brazing) Soldering uses a material called silver solder. A solder is a metal alloy (often of silver, tin and lead), usually with a low melting point, that is melted and used to join metallic surfaces, especially in the fields of electronics and plumbing, in a process called soldering. In electronics, tin/lead solders are normally 60/40 by weight in order to produce a near-eutectic mixture (lowest melting point - below 190°C).
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THE DUTIES OF THE WELDING INSPECTOR
VISUAL INSPECTION
At any point in the course of welding, i.e. tacking, root pass, filler pass or capping pass, but particularly for the root and cap, a detailed inspection may be required. British Standard 5289: 1976 gives guidance on tools and responsibilities together with sketches of typical defects.
The inspector at this point must -
a) observe, identify and perhaps record (measure) the features of the weld.
b) decide whether the weld is acceptable in terms of the particular levels that are permitted; defect levels may be ‘in-house’ or national codes of practice.
When the defect size is in excess of the permitted level then either a concession must be applied for (from a competent person), or the weld rejected.
INSPECTION BEFORE WELDING
Before Assembly:
Check * All applicable documents.
* Quality plan is authorised and endorsed with signature, date and company stamp.
* Application standard is up to date with the latest edition, revision or amendment.
* The drawings are clear, the issue number is marked and the latest revision is used.
* Welding procedure sheets (specifications) are available, have been approved and are
employed in production.
* Welder qualifications with identification and range of approval are verified and that only approved welders as required are employed in production.
* Calibration certificates, material certificates (mill sheets) and consumer certificates are available and valid.
* Parent material identification is verified against documentation and markings.
* Material composition, type and condition.
* Correct methods are applied for cutting and machining.
* Identification of welding consumables such as electrodes, filler wire, fluxes, shielding and backing gases and any special requirements (e.g. drying) are met.
* Plant and equipment are in a safe condition and adequate for the job.
* Safety permits e.g. hot work permit, gas free permit, enclosed space certificate are
available and valid.
After Assembly
Check * Dimensions, tolerances, preparation, fit-up and alignment are in accordance with the
Approved drawings and standards.
* Tack welds, bridging pieces, clamping and type of backing – if any used are correct.
* Cleanliness of work area is maintained.
* Preheat in accordance with procedure.
NOTE Good inspection prior to welding can eliminate conditions that lead to the formation of defects.
INSPECTION DURING WELDING
Check * The welding process must be monitored.
* Preheat and interpass temperatures must be monitored.
* Interpass cleaning – chipping, grinding, gouging, must be monitored.
* Root and subsequent run sequence.
* Essential variables such as current, voltage, travel speed to be monitored.
* Filler metals, fluxes and shielding gases are correct.
* Welding is in compliance with weld procedure sheet and application standard.
INSPECTION AFTER WELDING
Check * Visual inspection to be carried out to ascertain acceptability of appearance of welds.
* Dimensional accuracy to be ascertained.
* Conformity with drawings and standards requirements.
* Post weld heat treatment, if any, monitored and recorded.
* NDT carried out and reports assessed.
* Assess defects as to either repairing, or application for concession.
* Carry out any necessary repairs.
* Control of distortion
REPAIRS
* Repair procedure and welding code should be authorised.
* Defect area should be marked positively and clearly.
* Check when partially removed and fully removed (visual and NDT).
* Re-welding should be monitored.
* Re-inspect completed repair.
Collate all documents and reports. Pass the document package on to a higher authority for final inspection, approval and storage
http://hazwelding.wordpress.com/2007/11/05/welding-inspector-duties/
At any point in the course of welding, i.e. tacking, root pass, filler pass or capping pass, but particularly for the root and cap, a detailed inspection may be required. British Standard 5289: 1976 gives guidance on tools and responsibilities together with sketches of typical defects.
The inspector at this point must -
a) observe, identify and perhaps record (measure) the features of the weld.
b) decide whether the weld is acceptable in terms of the particular levels that are permitted; defect levels may be ‘in-house’ or national codes of practice.
When the defect size is in excess of the permitted level then either a concession must be applied for (from a competent person), or the weld rejected.
INSPECTION BEFORE WELDING
Before Assembly:
Check * All applicable documents.
* Quality plan is authorised and endorsed with signature, date and company stamp.
* Application standard is up to date with the latest edition, revision or amendment.
* The drawings are clear, the issue number is marked and the latest revision is used.
* Welding procedure sheets (specifications) are available, have been approved and are
employed in production.
* Welder qualifications with identification and range of approval are verified and that only approved welders as required are employed in production.
* Calibration certificates, material certificates (mill sheets) and consumer certificates are available and valid.
* Parent material identification is verified against documentation and markings.
* Material composition, type and condition.
* Correct methods are applied for cutting and machining.
* Identification of welding consumables such as electrodes, filler wire, fluxes, shielding and backing gases and any special requirements (e.g. drying) are met.
* Plant and equipment are in a safe condition and adequate for the job.
* Safety permits e.g. hot work permit, gas free permit, enclosed space certificate are
available and valid.
After Assembly
Check * Dimensions, tolerances, preparation, fit-up and alignment are in accordance with the
Approved drawings and standards.
* Tack welds, bridging pieces, clamping and type of backing – if any used are correct.
* Cleanliness of work area is maintained.
* Preheat in accordance with procedure.
NOTE Good inspection prior to welding can eliminate conditions that lead to the formation of defects.
INSPECTION DURING WELDING
Check * The welding process must be monitored.
* Preheat and interpass temperatures must be monitored.
* Interpass cleaning – chipping, grinding, gouging, must be monitored.
* Root and subsequent run sequence.
* Essential variables such as current, voltage, travel speed to be monitored.
* Filler metals, fluxes and shielding gases are correct.
* Welding is in compliance with weld procedure sheet and application standard.
INSPECTION AFTER WELDING
Check * Visual inspection to be carried out to ascertain acceptability of appearance of welds.
* Dimensional accuracy to be ascertained.
* Conformity with drawings and standards requirements.
* Post weld heat treatment, if any, monitored and recorded.
* NDT carried out and reports assessed.
* Assess defects as to either repairing, or application for concession.
* Carry out any necessary repairs.
* Control of distortion
REPAIRS
* Repair procedure and welding code should be authorised.
* Defect area should be marked positively and clearly.
* Check when partially removed and fully removed (visual and NDT).
* Re-welding should be monitored.
* Re-inspect completed repair.
Collate all documents and reports. Pass the document package on to a higher authority for final inspection, approval and storage
http://hazwelding.wordpress.com/2007/11/05/welding-inspector-duties/
History of Welding
History of Welding
Middle Ages
Welding can trace its historic development back to ancient times. The earliest examples come from the Bronze Age. Small gold circular boxes were made by pressure welding lap joints together. It is estimated that these boxes were made more than 2000 years ago. During the Iron Age the Egyptians and people in the eastern Mediterranean area learned to weld pieces of iron together. Many tools were found which were made approximately 1000 B.C.
During the Middle Ages, the art of blacksmithing was developed and many items of iron were produced which were welded by hammering. It was not until the 19th century that welding, as we know it today was invented.
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1800
Edmund Davy of England is credited with the discovery of acetylene in 1836. The production of an arc between two carbon electrodes using a battery is credited to Sir Humphry Davy in 1800. In the mid-nineteenth century, the electric generator was invented and arc lighting became popular. During the late 1800s, gas welding and cutting was developed. Arc welding with the carbon arc and metal arc was developed and resistance welding became a practical joining process.
1880
Auguste De Meritens, working in the Cabot Laboratory in France, used the heat of an arc for joining lead plates for storage batteries in the year 1881. It was his pupil, a Russian, Nikolai N. Benardos, working in the French laboratory, who was granted a patent for welding. He, with a fellow Russian, Stanislaus Olszewski, secured a British patent in 1885 and an American patent in 1887. The patents show an early electrode holder. This was the beginning of carbon arc welding. Bernardos' efforts were restricted to carbon arc welding, although he was able to weld iron as well as lead. Carbon arc welding became popular during the late 1890s and early 1900s.
1890
In 1890, C.L. Coffin of Detroit was awarded the first U.S. patent for an arc welding process using a metal electrode. This was the first record of the metal melted from the electrode carried across the arc to deposit filler metal in the joint to make a weld. About the same time, N.G. Slavianoff, a Russian, presented the same idea of transferring metal across an arc, but to cast metal in a mold.
1900
Approximately 1900, Strohmenger introduced a coated metal electrode in Great Britain. There was a thin coating of clay or lime, but it provided a more stable arc. Oscar Kjellberg of Sweden invented a covered or coated electrode during the period of 1907 to 1914. Stick electrodes were produced by dipping short lengths of bare iron wire in thick mixtures of carbonates and silicates, and allowing the coating to dry.
Meanwhile, resistance welding processes were developed, including spot welding, seam welding, projection welding and flash butt welding. Elihu Thompson originated resistance welding. His patents were dated 1885-1900. In 1903, a German named Goldschmidt invented thermite welding that was first used to weld railroad rails.
Gas welding and cutting were perfected during this period as well. The production of oxygen and later the liquefying of air, along with the introduction of a blow pipe or torch in 1887, helped the development of both welding and cutting. Before 1900, hydrogen and coal gas were used with oxygen. However, in about 1900 a torch suitable for use with low-pressure acetylene was developed.
World War I brought a tremendous demand for armament production and welding was pressed into service. Many companies sprang up in America and in Europe to manufacture welding machines and electrodes to meet the requirements.
1919
Immediately after the war in 1919, twenty members of the Wartime Welding Committee of the Emergency Fleet Corporation under the leadership of Comfort Avery Adams, founded the American Welding Society as a nonprofit organization dedicated to the advancement of welding and allied processes.
Alternating current was invented in 1919 by C.J. Holslag; however it did not become popular until the 1930s when the heavy-coated electrode found widespread use.
1920
In 1920, automatic welding was introduced. It utilized bare electrode wire operated on direct current and utilized arc voltage as the basis of regulating the feed rate. Automatic welding was invented by P.O. Nobel of the General Electric Company. It was used to build up worn motor shafts and worn crane wheels. It was also used by the automobile industry to produce rear axle housings.
During the 1920s, various types of welding electrodes were developed. There was considerable controversy during the 1920s about the advantage of the heavy-coated rods versus light-coated rods. The heavy-coated electrodes, which were made by extruding, were developed by Langstroth and Wunder of the A.O. Smith Company and were used by that company in 1927. In 1929, Lincoln Electric Company produced extruded electrode rods that were sold to the public. By 1930, covered electrodes were widely used. Welding codes appeared which required higher-quality weld metal, which increased the use of covered electrodes.
During the 1920s there was considerable research in shielding the arc and weld area by externally applied gases. The atmosphere of oxygen and nitrogen in contact with the molten weld metal caused brittle and sometime porous welds. Research work was done utilizing gas shielding techniques. Alexander and Langmuir did work in chambers using hydrogen as a welding atmosphere. They utilized two electrodes starting with carbon electrodes but later changing to tungsten electrodes. The hydrogen was changed to atomic hydrogen in the arc. It was then blown out of the arc forming an intensely hot flame of atomic hydrogen during to the molecular form and liberating heat. This arc produced half again as much heat as an oxyacetylene flame. This became the atomic hydrogen welding process. Atomic hydrogen never became popular but was used during the 1930s and 1940s for special applications of welding and later on for welding of tool steels.
H.M. Hobart and P.K. Devers were doing similar work but using atmospheres of argon and helium. In their patents applied for in 1926, arc welding utilizing gas supplied around the arc was a forerunner of the gas tungsten arc welding process. They also showed welding with a concentric nozzle and with the electrode being fed as a wire through the nozzle. This was the forerunner of the gas metal arc welding process. These processes were developed much later.
1930
Stud welding was developed in 1930 at the New York Navy Yard, specifically for attaching wood decking over a metal surface. Stud welding became popular in the shipbuilding and construction industries.
The automatic process that became popular was the submerged arc welding process. This "under powder" or smothered arc welding process was developed by the National Tube Company for a pipe mill at McKeesport, Pennsylvania. It was designed to make the longitudinal seams in the pipe. The process was patented by Robinoff in 1930 and was later sold to Linde Air Products Company, where it was renamed Unionmelt® welding. Submerged arc welding was used during the defense buildup in 1938 in shipyards and in ordnance factories. It is one of the most productive welding processes and remains popular today.
1940
Gas tungsten arc welding (GTAW) had its beginnings from an idea by C.L. Coffin to weld in a nonoxidizing gas atmosphere, which he patented in 1890. The concept was further refined in the late 1920s by H.M.Hobart, who used helium for shielding, and P.K. Devers, who used argon. This process was ideal for welding magnesium and also for welding stainless and aluminum. It was perfected in 1941, patented by Meredith, and named Heliarc® welding. It was later licensed to Linde Air Products, where the water-cooled torch was developed. The gas tungsten arc welding process has become one of the most important.
The gas shielded metal arc welding (GMAW) process was successfully developed at Battelle Memorial Institute in 1948 under the sponsorship of the Air Reduction Company. This development utilized the gas shielded arc similar to the gas tungsten arc, but replaced the tungsten electrode with a continuously fed electrode wire. One of the basic changes that made the process more usable was the small-diameter electrode wires and the constant-voltage poser source. This principle had been patented earlier by H.E. Kennedy. The initial introduction of GMAW was for welding nonferrous metals. The high deposition rate led users to try the process on steel. The cost of inert gas was relatively high and the cost savings were not immediately available.
1950
In 1953, Lyubavskii and Novoshilov announced the use of welding with consumable electrodes in an atmosphere of CO2 gas. The CO2 welding process immediately gained favor since it utilized equipment developed for inert gas metal arc welding, but could now be used for economically welding steels. The CO2 arc is a hot arc and the larger electrode wires required fairly high currents. The process became widely used with the introduction of smaller-diameter electrode wires and refined power supplies. This development was the short-circuit arc variation which was known as Micro-wire®, short-arc, and dip transfer welding, all of which appeared late in 1958 and early in 1959. This variation allowed all-position welding on thin materials and soon became the most popular of the gas metal arc welding process variations.
1960
Another variation was the use of inert gas with small amounts of oxygen that provided the spray-type arc transfer. It became popular in the early 1960s. A recent variation is the use of pulsed current. The current is switched from a high to a low value at a rate of once or twice the line frequency.
Soon after the introduction of CO2 welding, a variation utilizing a special electrode wire was developed. This wire, described as an inside-outside electrode, was tubular in cross section with the fluxing agents on the inside. The process was called Dualshield®, which indicated that external shielding gas was utilized, as well as the gas produced by the flux in the core of the wire, for arc shielding. This process, invented by Bernard, was announced in 1954, but was patented in 1957, when the National Cylinder Gas Company reintroduced it.
In 1959, an inside-outside electrode was produced which did not require external gas shielding. The absence of shielding gas gave the process popularity for noncritical work. This process was named Innershield®.
The electroslag welding process was announced by the Soviets at the Brussels World Fair in Belgium in 1958. It had been used in the Soviet Union since 1951, but was based on work done in the United States by R.K. Hopkins, who was granted patents in 1940. The Hopkins process was never used to a very great degree for joining. The process was perfected and equipment was developed at the Paton Institute Laboratory in Kiev, Ukraine, and also at the Welding Research Laboratory in Bratislava, Czechoslovakia. The first production use in the U.S. was at the Electromotive Division of General Motors Corporation in Chicago, where it was called the Electro-molding process. It was announced in December 1959 for the fabrication of welded diesel engine blocks. The process and its variation, using a consumable guide tube, is used for welding thicker materials.
The Arcos Corporation introduced another vertical welding method, called Electrogas, in 1961. It utilized equipment developed for electroslag welding, but employed a flux-cored electrode wire and an externally supplied gas shield. It is an open arc process since a slag bath is not involved. A newer development uses self-shielding electrode wires and a variation uses solid wire but with gas shielding. These methods allow the welding of thinner materials than can be welded with the electroslag process.
Gage invented plasma arc welding in 1957. This process uses a constricted arc or an arc through an orifice, which creates an arc plasma that has a higher temperature than the tungsten arc. It is also used for metal spraying and for cutting.
The electron beam welding process, which uses a focused beam of electrons as a heat source in a vacuum chamber, was developed in France. J.A. Stohr of the French Atomic Energy Commission mad the first public disclosure of the process on November 23, 1957. In the United States, the automotive and aircraft engine industries are the major users of electron beam welding.
Most Recent
Friction welding, which uses rotational speed and upset pressure to provide friction heat, was developed in the Soviet Union. It is a specialized process and has applications only where a sufficient volume of similar parts is to be welded because of the initial expense for equipment and tooling. This process is called inertia welding.
Laser welding is one of the newest processes. The laser was originally developed at the Bell Telephone Laboratories as a communications device. Because of the tremendous concentration of energy in a small space, it proved to be a powerful heat source. It has been used for cutting metals and nonmetals. Continuous pulse equipment is available. The laser is finding welding applications in automotive metalworking operations.
--------------------------------------------------------------------------------
Information courtesy of Hobart Institute Of Welding Technology.
This article was excerpted from Modern Welding Technology, 4th edition, 1998, by Howard B. Cary. Published by Prentice-Hall, the book may be ordered from the Training Materials Dept., Hobart Institute of Welding Technology, 400 Trade Square East, Troy, OH 45373. http://www.welding.org
http://www.rodovens.com/welding_articles/history.htm
Middle Ages
Welding can trace its historic development back to ancient times. The earliest examples come from the Bronze Age. Small gold circular boxes were made by pressure welding lap joints together. It is estimated that these boxes were made more than 2000 years ago. During the Iron Age the Egyptians and people in the eastern Mediterranean area learned to weld pieces of iron together. Many tools were found which were made approximately 1000 B.C.
During the Middle Ages, the art of blacksmithing was developed and many items of iron were produced which were welded by hammering. It was not until the 19th century that welding, as we know it today was invented.
Prevent Welding Defects with a Rod Oven — FREE Shipping — Learn More
1800
Edmund Davy of England is credited with the discovery of acetylene in 1836. The production of an arc between two carbon electrodes using a battery is credited to Sir Humphry Davy in 1800. In the mid-nineteenth century, the electric generator was invented and arc lighting became popular. During the late 1800s, gas welding and cutting was developed. Arc welding with the carbon arc and metal arc was developed and resistance welding became a practical joining process.
1880
Auguste De Meritens, working in the Cabot Laboratory in France, used the heat of an arc for joining lead plates for storage batteries in the year 1881. It was his pupil, a Russian, Nikolai N. Benardos, working in the French laboratory, who was granted a patent for welding. He, with a fellow Russian, Stanislaus Olszewski, secured a British patent in 1885 and an American patent in 1887. The patents show an early electrode holder. This was the beginning of carbon arc welding. Bernardos' efforts were restricted to carbon arc welding, although he was able to weld iron as well as lead. Carbon arc welding became popular during the late 1890s and early 1900s.
1890
In 1890, C.L. Coffin of Detroit was awarded the first U.S. patent for an arc welding process using a metal electrode. This was the first record of the metal melted from the electrode carried across the arc to deposit filler metal in the joint to make a weld. About the same time, N.G. Slavianoff, a Russian, presented the same idea of transferring metal across an arc, but to cast metal in a mold.
1900
Approximately 1900, Strohmenger introduced a coated metal electrode in Great Britain. There was a thin coating of clay or lime, but it provided a more stable arc. Oscar Kjellberg of Sweden invented a covered or coated electrode during the period of 1907 to 1914. Stick electrodes were produced by dipping short lengths of bare iron wire in thick mixtures of carbonates and silicates, and allowing the coating to dry.
Meanwhile, resistance welding processes were developed, including spot welding, seam welding, projection welding and flash butt welding. Elihu Thompson originated resistance welding. His patents were dated 1885-1900. In 1903, a German named Goldschmidt invented thermite welding that was first used to weld railroad rails.
Gas welding and cutting were perfected during this period as well. The production of oxygen and later the liquefying of air, along with the introduction of a blow pipe or torch in 1887, helped the development of both welding and cutting. Before 1900, hydrogen and coal gas were used with oxygen. However, in about 1900 a torch suitable for use with low-pressure acetylene was developed.
World War I brought a tremendous demand for armament production and welding was pressed into service. Many companies sprang up in America and in Europe to manufacture welding machines and electrodes to meet the requirements.
1919
Immediately after the war in 1919, twenty members of the Wartime Welding Committee of the Emergency Fleet Corporation under the leadership of Comfort Avery Adams, founded the American Welding Society as a nonprofit organization dedicated to the advancement of welding and allied processes.
Alternating current was invented in 1919 by C.J. Holslag; however it did not become popular until the 1930s when the heavy-coated electrode found widespread use.
1920
In 1920, automatic welding was introduced. It utilized bare electrode wire operated on direct current and utilized arc voltage as the basis of regulating the feed rate. Automatic welding was invented by P.O. Nobel of the General Electric Company. It was used to build up worn motor shafts and worn crane wheels. It was also used by the automobile industry to produce rear axle housings.
During the 1920s, various types of welding electrodes were developed. There was considerable controversy during the 1920s about the advantage of the heavy-coated rods versus light-coated rods. The heavy-coated electrodes, which were made by extruding, were developed by Langstroth and Wunder of the A.O. Smith Company and were used by that company in 1927. In 1929, Lincoln Electric Company produced extruded electrode rods that were sold to the public. By 1930, covered electrodes were widely used. Welding codes appeared which required higher-quality weld metal, which increased the use of covered electrodes.
During the 1920s there was considerable research in shielding the arc and weld area by externally applied gases. The atmosphere of oxygen and nitrogen in contact with the molten weld metal caused brittle and sometime porous welds. Research work was done utilizing gas shielding techniques. Alexander and Langmuir did work in chambers using hydrogen as a welding atmosphere. They utilized two electrodes starting with carbon electrodes but later changing to tungsten electrodes. The hydrogen was changed to atomic hydrogen in the arc. It was then blown out of the arc forming an intensely hot flame of atomic hydrogen during to the molecular form and liberating heat. This arc produced half again as much heat as an oxyacetylene flame. This became the atomic hydrogen welding process. Atomic hydrogen never became popular but was used during the 1930s and 1940s for special applications of welding and later on for welding of tool steels.
H.M. Hobart and P.K. Devers were doing similar work but using atmospheres of argon and helium. In their patents applied for in 1926, arc welding utilizing gas supplied around the arc was a forerunner of the gas tungsten arc welding process. They also showed welding with a concentric nozzle and with the electrode being fed as a wire through the nozzle. This was the forerunner of the gas metal arc welding process. These processes were developed much later.
1930
Stud welding was developed in 1930 at the New York Navy Yard, specifically for attaching wood decking over a metal surface. Stud welding became popular in the shipbuilding and construction industries.
The automatic process that became popular was the submerged arc welding process. This "under powder" or smothered arc welding process was developed by the National Tube Company for a pipe mill at McKeesport, Pennsylvania. It was designed to make the longitudinal seams in the pipe. The process was patented by Robinoff in 1930 and was later sold to Linde Air Products Company, where it was renamed Unionmelt® welding. Submerged arc welding was used during the defense buildup in 1938 in shipyards and in ordnance factories. It is one of the most productive welding processes and remains popular today.
1940
Gas tungsten arc welding (GTAW) had its beginnings from an idea by C.L. Coffin to weld in a nonoxidizing gas atmosphere, which he patented in 1890. The concept was further refined in the late 1920s by H.M.Hobart, who used helium for shielding, and P.K. Devers, who used argon. This process was ideal for welding magnesium and also for welding stainless and aluminum. It was perfected in 1941, patented by Meredith, and named Heliarc® welding. It was later licensed to Linde Air Products, where the water-cooled torch was developed. The gas tungsten arc welding process has become one of the most important.
The gas shielded metal arc welding (GMAW) process was successfully developed at Battelle Memorial Institute in 1948 under the sponsorship of the Air Reduction Company. This development utilized the gas shielded arc similar to the gas tungsten arc, but replaced the tungsten electrode with a continuously fed electrode wire. One of the basic changes that made the process more usable was the small-diameter electrode wires and the constant-voltage poser source. This principle had been patented earlier by H.E. Kennedy. The initial introduction of GMAW was for welding nonferrous metals. The high deposition rate led users to try the process on steel. The cost of inert gas was relatively high and the cost savings were not immediately available.
1950
In 1953, Lyubavskii and Novoshilov announced the use of welding with consumable electrodes in an atmosphere of CO2 gas. The CO2 welding process immediately gained favor since it utilized equipment developed for inert gas metal arc welding, but could now be used for economically welding steels. The CO2 arc is a hot arc and the larger electrode wires required fairly high currents. The process became widely used with the introduction of smaller-diameter electrode wires and refined power supplies. This development was the short-circuit arc variation which was known as Micro-wire®, short-arc, and dip transfer welding, all of which appeared late in 1958 and early in 1959. This variation allowed all-position welding on thin materials and soon became the most popular of the gas metal arc welding process variations.
1960
Another variation was the use of inert gas with small amounts of oxygen that provided the spray-type arc transfer. It became popular in the early 1960s. A recent variation is the use of pulsed current. The current is switched from a high to a low value at a rate of once or twice the line frequency.
Soon after the introduction of CO2 welding, a variation utilizing a special electrode wire was developed. This wire, described as an inside-outside electrode, was tubular in cross section with the fluxing agents on the inside. The process was called Dualshield®, which indicated that external shielding gas was utilized, as well as the gas produced by the flux in the core of the wire, for arc shielding. This process, invented by Bernard, was announced in 1954, but was patented in 1957, when the National Cylinder Gas Company reintroduced it.
In 1959, an inside-outside electrode was produced which did not require external gas shielding. The absence of shielding gas gave the process popularity for noncritical work. This process was named Innershield®.
The electroslag welding process was announced by the Soviets at the Brussels World Fair in Belgium in 1958. It had been used in the Soviet Union since 1951, but was based on work done in the United States by R.K. Hopkins, who was granted patents in 1940. The Hopkins process was never used to a very great degree for joining. The process was perfected and equipment was developed at the Paton Institute Laboratory in Kiev, Ukraine, and also at the Welding Research Laboratory in Bratislava, Czechoslovakia. The first production use in the U.S. was at the Electromotive Division of General Motors Corporation in Chicago, where it was called the Electro-molding process. It was announced in December 1959 for the fabrication of welded diesel engine blocks. The process and its variation, using a consumable guide tube, is used for welding thicker materials.
The Arcos Corporation introduced another vertical welding method, called Electrogas, in 1961. It utilized equipment developed for electroslag welding, but employed a flux-cored electrode wire and an externally supplied gas shield. It is an open arc process since a slag bath is not involved. A newer development uses self-shielding electrode wires and a variation uses solid wire but with gas shielding. These methods allow the welding of thinner materials than can be welded with the electroslag process.
Gage invented plasma arc welding in 1957. This process uses a constricted arc or an arc through an orifice, which creates an arc plasma that has a higher temperature than the tungsten arc. It is also used for metal spraying and for cutting.
The electron beam welding process, which uses a focused beam of electrons as a heat source in a vacuum chamber, was developed in France. J.A. Stohr of the French Atomic Energy Commission mad the first public disclosure of the process on November 23, 1957. In the United States, the automotive and aircraft engine industries are the major users of electron beam welding.
Most Recent
Friction welding, which uses rotational speed and upset pressure to provide friction heat, was developed in the Soviet Union. It is a specialized process and has applications only where a sufficient volume of similar parts is to be welded because of the initial expense for equipment and tooling. This process is called inertia welding.
Laser welding is one of the newest processes. The laser was originally developed at the Bell Telephone Laboratories as a communications device. Because of the tremendous concentration of energy in a small space, it proved to be a powerful heat source. It has been used for cutting metals and nonmetals. Continuous pulse equipment is available. The laser is finding welding applications in automotive metalworking operations.
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Information courtesy of Hobart Institute Of Welding Technology.
This article was excerpted from Modern Welding Technology, 4th edition, 1998, by Howard B. Cary. Published by Prentice-Hall, the book may be ordered from the Training Materials Dept., Hobart Institute of Welding Technology, 400 Trade Square East, Troy, OH 45373. http://www.welding.org
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