Introduction to Eddy Current Testing

Magnetism, the underlying principle behind electric motors and generators, relays and stereo speakers, is also the force that enables an important category of NDT tools called eddy current instruments. Eddy current testing is widely used in the aerospace industry and in other manufacturing and service environments that require inspection of thin metal for potential safety-related or quality-related problems. In addition to crack detection in metal sheets and tubing, eddy current can be used for certain metal thickness measurements such as identifying corrosion under aircraft skin, to measure conductivity and monitor the effects of heat treatment, and to determine the thickness of nonconductive coatings over conductive substrates. Both field portable and fixed system instruments are available to meet a wide variety of test needs.

Eddy current NDT can examine large areas very quickly, and it does not require use of coupling liquids. In addition to finding cracks, eddy current can also be used to check metal hardness and conductivity in applications where those properties are of interest, and to measure thin layers of nonconductive coatings like paint on metal parts. At the same time, eddy current testing is limited to materials that conduct electricity and thus cannot be used on plastics. In some cases, eddy current and ultrasonic testing are used together as complementary techniques, with eddy current having an advantage for quick surface testing and ultrasonics having better depth penetration.

How it works

Eddy current testing is based on the physics phenomenon of electromagnetic induction. In an eddy current probe, an alternating current flows through a wire coil and generates an oscillating magnetic field. If the probe and its magnetic field are brought close to a conductive material like a metal test piece, a circular flow of electrons known as an eddy current will begin to move through the metal like swirling water in a stream. That eddy current flowing through the metal will in turn generate its own magnetic field, which will interact with the coil and its field through mutual inductance. Changes in metal thickness or defects like near-surface cracking will interrupt or alter the amplitude and pattern of the eddy current and the resulting magnetic field. This in turn affects the movement of electrons in the coil by varying the electrical impedance of the coil. The eddy current instrument plots changes in the impedance amplitude and phase angle, which can be used by a trained operator to identify changes in the test piece.

Eddy current density is highest near the surface of the part, so that is the region of highest test resolution. The standard depth of penetration is defined as the depth at which the eddy current density is 37% of its surface value, which in turn can be calculated from the test frequency and the magnetic permeability and conductivity of the test material. Thus, variations in the conductivity of the test material, its magnetic permeability, the frequency of the AC pulses driving the coil, and coil geometry will all have an effect on test sensitivity, resolution, and penetration.

There are many factors that will affect the capabilities of an eddy current inspection. Eddy currents traveling in materials with higher conductivity values will be more sensitive to surface defects but will have less penetration into the material, with penetration also being dependent on test frequency. Higher test frequencies increase near surface resolution but limit the depth of penetration, while lower test frequencies increase penetration. Larger coils inspect a greater volume of material from any given position, since the magnetic field flows deeper into the test piece, while smaller coils are more sensitive to small defects. Variations in permeability of a material generate noise that can limit flaw resolution because of greater background variations.

While conductivity and permeability are properties of the test material that are outside of the operator's control, the test frequency, coil type, and coil size can be chosen based on test requirements. In a given test, resolution will be determined by the probe type while detection capability will be controlled by material and equipment characteristics. Some inspections involve sweeping through multiple frequencies to optimize results, or inspection with multiple probes to obtain the best resolution and penetration required to detect all possible flaws. It is always important to select the right probe for each application in order to optimize test performance.

Impedance plane displays
While some older eddy current instruments used simple analog meter displays, the standard format now is an impedance plane plot that graphs coil resistance on the x-axis versus inductive reactance on the y-axis. Variations in the plot correspond to variations in the test piece. For example, the display below shows a setup for inspection for surface cracks in aluminum. The top curve represents a 0.040" deep surface crack, the middle curve is a 0.020" deep crack, and the smallest curve is a 0.008" deep crack. The horizontal line is the lift off in which the probe has been "nulled" (balanced) on the aluminum part and when it is lifted in the air, the signal moves directly to the left. This inspection is done with a pencil probe.
 

 
This display would be considered the calibration of the instrument. Once the parameters are set, they should not be changed during the inspection. The inspection measurements are dependent entirely on the comparison of the signal against the reference calibration.

Another common test involves measurement of nonconductive coatings like paint over metals. The screen display below shows a nonmetallic coating over aluminum. For this application, the probe is "nulled" (balanced) in air and then placed on the sample. The top line shows the signal on aluminum without any coating. The second line down is a 0.004" coating, then a 0.008" coating and the bottom line is a 0.012" coating. To create this image, the display position had to be changed between each measurement in order to display a separation between each signal. After this calibration is done, the inspector would measure on their parts and watch for the distance that the signal travels across the screen. Alarms could be used to alert the inspector when a coating is too thick or too thin.

A second way to measure the thickness of a nonconductive coating on a conductive material is using the conductivity measurement capability of the Olympus NDT N500 series instruments (N500C or higher). This measurement uses a special conductivity probe that displays the below screen instead of the standard impedance screen shown above. This measurement is most commonly used to determine the conductivity of a material but it will also provide the thickness of a coating which is considered the "Liftoff" from the material or how far the probe is above the surface of the conductive material. This example was a 0.004" coating on an aluminum test piece. Types of probes

Eddy current instruments can perform a wide variety of tests depending on the type of probe being used, and careful probe selection will help optimize performance. Some common probe types are listed below.

Surface probes - Used for identifying flaws on and below metal surfaces, usually large diameter to accommodate lower frequencies for deeper penetration, or for scanning larger areas.

Pencil probes - Smaller diameter probes housing coils built for high frequencies for high resolution of near surface flaws.

Bolt hole probes - Designed to inspect the inside of a bolt hole. These probes can be rotated by hand or automatically using a rotary scanner.

Donut probes - Designed to inspect aircraft fastener holes with fasteners in place.
Sliding probes - Also used in testing aircraft fastener holes, offering higher scan rates than donut probes.
ID probes - Used for inspection of heat exchangers and similar metal tubing from the inside, available in a variety of sizes.
OD probes - Used for inspection of metal tubing and bars from the outside, with the test piece passing through the coil

Reference standards
An eddy current system consisting of an instrument and a probe must always be calibrated with appropriate reference standards at the start of a test. This process involves identifying the baseline display from a given test piece and observing how it changes under the conditions that the test is intended to identify. In flaw detection applications, this calibration process typically involves the use of reference standards of the same material, shape, and size as the test piece, containing artificial defects such as saw cuts, drilled holes, or milled walls to simulate flaws. In thickness measurement applications the reference standards would consist of various samples of know thickness. The operator observes the response from the reference standards and then compares the indications from test pieces to these reference patterns to categorize parts. Proper calibration with appropriate reference standards is an essential part of any eddy current test procedure.

Common applications
Eddy current instruments can be used in a wide variety of tests. Some of the most common are listed below.
Weld Inspection - Many weld inspections employ ultrasonic NDT for subsurface testing and a complimentary eddy current method to scan the surface for open surface cracks on weld caps and in heat affected zones.
Conductivity Testing - Eddy current testing's ability to measure conductivity can be used to identify and sort ferrous and nonferrous alloys, and to verify heat treatment.
Surface Inspection - Surface cracks in machined parts and metal stock can be readily identified with eddy current. This includes inspection of the area around fasteners in aircraft and other critical applications.
Corrosion Detection - Eddy current instruments can be used to detect and quantify corrosion on the inside of thin metal such as aluminum aircraft skin. Low frequency probes can be used to locate corrosion on second and third layers of metal that cannot be inspected ultrasonically.
Bolt Hole Inspection - Cracking inside bolt holes can be detected using bolt hole probes, often with automated rotary scanners.
Tubing inspection - Both in-line inspection of tubing at the manufacturing stage and field inspection of tubing like heat exchangers are common eddy current applications. Both cracking and thickness variations can be detected.
Eddy current arrays
Eddy Current Array testing, or ECA, is a technology that provides the ability to simultaneously use multiple eddy current coils that are placed side by side in the same probe assembly. Each individual coil produces a signal relative to the phase and amplitude of the structure below it. This data is referenced to an encoded position and time and represented graphically as a C-scan image showing structures in a planar view. In addition to providing visualization through C-scan imaging, ECA allows coverage of larger areas in a single pass while maintaining high resolution. ECA can permit use of simpler fixturing, and can also simplify inspection of complex shapes through custom probes built to fit the profile of the test piece.

By Tom Nelligan and Cynthia Calderwood

Prevention and Control of Weld Distortion

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

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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.

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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.

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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.

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