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Thermal applications

By Grant Laidlaw

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, the effects of shrinkage in various types of welded assemblies and how to curb it, and finally look at methods for distortion control.

What is weld distortion?

Distortion in a weld results from the expansion and the contraction of the weld metal and adjacent base metal during the heating and cooling cycle of the welding process. Doing all the welding on one side of a part will cause greater distortion than if the welds were to be alternated from one side to the other. During this heating and cooling cycle, many factors affect shrinkage of the metal and lead to distortion — these factors include 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 (Figure 1). These changes, in turn, affect heat flow and uniformity of heat distribution.

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What are the main types of distortion?

Distortion occurs in six main forms:

  1. Longitudinal shrinkage
  2. Transverse shrinkage
  3. Angular distortion
  4. Bowing and dishing
  5. Buckling

Contraction of the weld area on cooling results in both transverse and longitudinal shrinkage.

Non-uniform contraction (by way of thickness) produces angular distortion in addition to longitudinal and transverse shrinkage.

For example, in a single V-butt weld, the first weld run produces longitudinal and transverse shrinkage and rotation. The second run causes the plates to rotate using the first weld deposit as a fulcrum. Hence, balanced welding in a double side V-butt joint can be used to produce uniform contraction and prevent angular distortion.

Similarly, in a single side-fillet weld, non-uniform contraction produces angular distortion of the upstanding leg. Double-sided fillet welds can therefore be used to control distortion in the upstanding fillet, but because the weld is only deposited on one side of the base plate, angular distortion will now be produced in the plate.

Longitudinal bowing in welded plates happens when the weld centre is not coincident with the neutral axis of the section so that longitudinal shrinkage in the welds bends the section into a curved shape. Clad plate tends to bow in two directions due to longitudinal and transverse shrinkage of the cladding — this produces a dished shape. Dishing is also produced in stiffened plating. Plates usually dish inwards between the stiffeners, because of angular distortion at the stiffener attachment welds.

In plating, long range compressive stresses can cause elastic buckling in thin plates, resulting in dishing, bowing or rippling.

Distortion due to elastic buckling is unstable: if you attempt to flatten a buckled plate, it will probably 'snap' through and dish out in the opposite direction.

Twisting in a box section is caused by shear deformation at the corner joints. This is caused by unequal longitudinal thermal expansion of the abutting edges. Increasing the number of tack welds to prevent shear deformation often reduces the amount of twisting.

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Allowances for weld shrinkage

It is almost impossible to predict accurately the amount of shrinking. Nevertheless, a rule of thumb has been composed based on the size of the weld deposit. When welding steel, the following allowances should be made to cover shrinkage at the assembly stage.

Transverse shrinkage

  • Fillet welds:
  • Butt weld:
  • Fillet welds:
  • Butt welds: 3mm per 3m of weld.

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Longitudinal shrinkage

Increasing the leg length of fillet welds, in particular, increases shrinkage.

 

Factors affecting distortion

If a metal is uniformly heated and cooled there is almost no distortion. However, because the material is locally heated and restrained by the surrounding cold metal, stresses are generated higher than the material yield stress, causing permanent distortion. The principal factors affecting the type and the degree of distortion are the following:

  1. Parent material properties
  2. Amount of restraint
  3. Joint design
  4. Part fit-up
  5. Welding procedure.
  6. Do not over weld
  7. Use intermittent welding
  8. Use as few weld passes as possible
  9. Place welds near the neutral axis
  10. Balance welds around the neutral axis
  11. Use back step welding
  12. Anticipate the shrinkage forces
  13. Plan the welding sequence
  14. Remove shrinkage forces after welding
  15. Minimise welding time

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Reasons for distortion

To understand how and why distortion occurs during heating and cooling of a metal, consider the bar of steel shown in Figure 2. As the bar is uniformly heated, it expands in all directions, as shown in Figure 2(a). As the metal cools to room temperature it contracts
uniformly to its original dimensions.

But if the steel bar is restrained (as in a vice) while it is heated, as shown in Figure 2(b), lateral expansion cannot take place. But, since volume expansion must occur during the heating, the bar expands in a vertical direction and becomes thicker. As the deformed bar returns to room temperature, it will still tend to contract uniformly in all directions, as in Figure 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 weld.

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Shrinkage control

To prevent or minimise 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 minimise distortion caused by shrinkage:

The more metal placed in a joint, the greater the shrinkage forces. Correctly sizing a weld for the requirements of the joint not only minimises distortion, but also saves weld metal and time. The amount of weld metal in a fillet weld can be minimised by the use of a flat or slightly convex bead, and in a butt joint by proper edge preparation. 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 (more than 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.

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Another way to minimise weld metal is to use intermittent rather than continuous welds where possible, as in Figure 4(c). For attaching stiffeners to plate, for example, intermittent welds can reduce the weld metal by as much as 75% to provide the needed strength.

Fewer passes with large electrodes, Figure 4(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.

Distortion is minimised by providing a smaller leverage for the shrinkage forces to pull the plates out of alignment. Figure 4(e) illustrates this. Both design of the weld and welding sequence can be used effectively to control distortion.

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This practice, shown in Figure 4(f), offsets one shrinkage force with another to effectively minimise distortion of the weld. Here, too, design of the assembly and proper sequence of welding are important factors.

In the back step 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 Figure 5(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. Back-stepping may not be effective in all applications, and it cannot be used economically in automatic welding.

Pre-setting 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, pre‑set in this manner, are shown in Figure 5(h). The required amount of pre‑set for shrinkage to pull the plates into
alignment can be determined from a few trial welds.

Pre-bending, pre-setting or pre-springing the parts to be welded, Figure 6(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 pre‑set. 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.

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Another common practice for balancing shrinkage forces is to position identical weld back‑to‑back, Figure 6(j), clamping them tightly together. The welds are completed on both assemblies and allowed to cool before the clamps are released. Pre-bending can be combined with this method by inserting wedges at suitable positions between the parts before clamping.

In heavy weld, 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 absent, 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. 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.

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 Figure 6(k). Another example, in a fillet weld, consists of making intermittent welds according to the sequences shown in Figure 6(l). In these examples, the shrinkage in weld number one is balanced by the shrinkage in weld number two.

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 weld until the yield point of the weld metal is reached. For typical welds on low-carbon plate, this stress level would approximate 45 000psi. 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.

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 weld to an elevated temperature, followed by controlled cooling. Sometimes two identical weld 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 weld are thus minimised.

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 weld. The use of mechanised 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 175A, 25V, and 3ipm requires 87 500J 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 310A, 35V, and 8ipm requires 81 400J 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.)

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Other techniques for distortion control

Water-cooled jig

Various techniques have been developed to control distortion on specific weld. In sheet-metal welding, for example, a water-cooled jig (Figure 7) 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 to minimise distortion.


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Strongback

The strongback is another useful technique for distortion control during butt welding of plates, as in Figure 8(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 weld is finished.

 

Summary: a checklist to minimise distortion

Follow this checklist to minimise distortion in the design and fabrication of weld:

  • Do not over weld.
  • 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 minimise 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 by way of a planned welding sequence and weld positioning. Weld toward the unrestrained part of the member.
  • Use clamps, fixtures, and strongbacks to maintain alignment.
  • Pre‑bend the members or pre‑set 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 minimise the effects of distortion and residual stresses.

Forging and hardening

A chisel may be heated in a forge or with an oxyacetylene torch until it turns a cherry red colour for forging. It is hammered into shape on an anvil or suitable surface. This includes a rough version of the final cutting faces. After being shaped, it must be hardened. Heat the tip again, until cherry red. Plunge it for a few seconds into water. Pull it out and rub the edge briskly on a brick to remove surface residues and watch the colour. The earlier cherry red will darken with a series of colours. Eventually the metal will take on a mustard colour. This is your indication. Now plunge the end into water and keep it there until it cools. It will now be hardened and ready for sharpening on the grindstone.

This concludes the module on Thermal applications. Should you wish to be assessed against this module, please contact ACRA’s Anria Pieterse (SDF) on 0027 (0)11 393 1642.

 

References

  • ACRA training material.
    • Use intermittent welds where possible and consistent with design requirements.
    • Distribute the welding heat as evenly as possible by means of a planned welding sequence and weld positioning.

Grant Laidlaw F.SAIRAC

CEO, ACRA

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