What is distortion?
Weld shrinkage plagues experienced and amateur welders alike. Shrinkage causes distortion of the weldment. Warping of the base plate is caused by heat from the welding arc. Distortion results from the expansion and contraction of the weld metal and adjacent base metal during the welding process.
Reasons for distortion
As a bar of metal is uniformly heated, it expands in all directions. When the bar cools, it uniformly contracts to its original dimensions. Restraining the bar in any way–using a vice or other means–uniform expansion can’t occur. Because expansion must still occur, the bar becomes distorted as it expands in any unhindered direction. Contraction occurs uniformly regardless and permanently retains the deformation. The same basic process occurs when welding.
10 Tips for controlling weld shrinkage
1. Don’t over-weld
The greater the amount of metal in a joint, the greater the shrinkage forces. Reducing the amount of weld saves metal, saves time, and protects the weld from distortion. If the metal plate is thicker than 0.25”, beveling can help ward off distortion.
2. Use intermittent welding
Using intermittent welding, rather than continuous welding, where possible greatly decreases distortion. Even though weld metal and heat transfer are reduced by as much as 75%, the strength of the weld remains relatively uncompromised.
3. Use as few weld passes as possible
Fewer passes with a larger electrode are preferable to many passes with smaller electrodes when transverse distortion could be a problem. Shrinkage is cumulative with each pass. More passes lead to more shrinkage.
4. Place welds near the neutral axis
Providing a smaller leverage for shrinkage forces to pull the places out of alignment by placing welds near the neutral axis minimizes distortion.
5. Balance welds around the neutral axis
Offset one shrinkage force with another to effectively minimize distortion. The design of the assembly and sequence of welding are both important factors to consider.
6. Use backstep welding
The general progression of the weld may be, for example, left to right, but each sequential bead segment is placed right to left. After placement of each bead, the heated edges expand on the far side of the weld. Once the heat spreads across the plate, expansion on the opposite edge brings the plates back together. This process greatly reduces distortion.
7. Anticipate the shrinkage forces
Presetting the parts before welding can make shrinkage perform constructive work. Presetting, prebending, or prespringing the parts to weld uses opposing mechanical forces to counteract distortion. For example, lengthening the top of a weld groove when presetting the plates can help reduce distortion. The resulting weld is slightly longer than it would have been on a flat plate. After removal of the clamps, the final weld relieves its longitudinal shrinkage stresses by shortening to a straight line. This process results in a flat plate. Another technique balances shrinkage by positioning identical weldments back-to-back and clamping them tightly together. Welds are placed on both assemblies and allowed to cool before removing clamps. Combining this process with prebending can further prevent distortion. A diagram would help
8. Planning the weld sequence
The proper weld sequence involves placing weld metal at different points of the assembly so that shrinkage occurs in just one place at a time and is counteracted by sequential welds. Alternating welds on both sides of the neutral axis is especially useful in complete joint penetration groove welds. In fillet welds, intermittent welds alternating down weldment balance shrinkage.
9. Remove shrinkage forces after welding
Peening counteracts the shrinkage forces of a weld bead as it cools. Peening stretches the bead and makes it thinner, thus relieving the stresses induced by contraction as the metal cools. This process requires caution; peening is inapplicable on a root bead or on a final pass as the process could conceal or cause cracks. Another technique is thermal stress relieving. Thermal stress relieving involves controlled heating of the weldment followed by controlled cooling to removed weld-introduced stresses.
10. Minimize welding time
Minimizing time while welding minimized the amount of metal heated. Reducing heat reduces shrinkage and thus distortion.