Heat Treatment and Post-Weld Operations
This domain tests preheat, post-weld heat treatment and post-weld cleaning. You must know why heat treatment is done and what can go wrong.
Core rules
- Preheat slows cooling and helps prevent hydrogen cracking in hardenable or thick steel. It is specified in the WPS and checked with a thermometer or temperature-indicating crayons.
- Post-weld heat treatment (PWHT), often stress relief, reduces residual stress and can soften the heat-affected zone. For many carbon steels, stress relief is commonly in the 600–650°C range, with soak time often about 1 hour per 25 mm of thickness and a minimum of 1 hour.
- Heat and cool slowly to avoid thermal gradients. Control heating rate, cooling rate and soak temperature.
- Post-weld cleaning removes slag, spatter and heat tint. Use stainless brushes for stainless steel.
Worked example
A 50 mm thick carbon-steel joint is stress-relieved at 620°C. Using 1 hour per 25 mm, soak time = 50/25 = 2 hours. The part is heated and cooled slowly through the specified rates. The reasoning is that time at temperature allows stress relaxation without overheating.
Traps that cost marks
- Treating PWHT as a cure for every defect; it does not repair cracks or lack of fusion.
- Heating stainless steel unnecessarily and causing sensitisation.
- Cooling rapidly after stress relief and creating new stress.
- Ignoring the WPS heating and cooling rates.