Distortion Control
This domain tests why welded assemblies move and how to control longitudinal, transverse, angular and bowing distortion. The exam expects practical sequence controls, not just “weld slower”.
Core rules
- Distortion comes from non-uniform heating, shrinkage of weld metal and restraint. More heat input and more weld metal generally mean more distortion.
- Control methods: pre-set the parts opposite the expected movement, use balanced welding about the neutral axis, backstep or skip sequence, clamp or fixture, use heat sinks, and limit unnecessary weld size.
- Angular distortion is common in T-joints and fillet welds. Weld on both sides in a balanced sequence where possible.
- Restraint reduces movement but can raise residual stress and cracking risk.
Worked example
A 3 m beam is expected to shorten by 1.2 mm/m. Total shrinkage = 3 × 1.2 = 3.6 mm. The fabricator can cut the beam 3.6 mm longer or pre-set the assembly by 3.6 mm so the finished length is correct. The reasoning is that shrinkage is cumulative along the weld length.
Traps that cost marks
- Adding more weld metal to “strengthen” a joint and increasing distortion.
- Using the same sequence on both sides without balancing heat.
- Over-restraining thin plate and causing cracks.
- Forgetting that distortion can appear after cooling or machining.