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).

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