Post-Weld Heat Treat
Post-Weld Heat Treatment (PWHT) is any heat treatment that is performed after machining and welding to improve mechanical properties, reduce distortion, and reduce the risk of cracking after welding.
Welding can cause residual stresses in a component that can cause distortions of its shape later on in its lifespan. By reheating the metal after welding, we can relieve and redistribute these stresses to enhance the component’s resistance to fracture.
Common Alloys treated through PWHT
- Carbon Steels, e.g., 1018, 1045
- Structural Steels, e.g., A108, A572
- Alloy Steels, e.g., 4140, 4330, 4340
- Stainless Steels, e.g., 304, 316, 17-4
- Superalloys, e.g., Haynes, Hastelloy
TMT’s Equipment and Variations of PWHT
A standard post-weld heat treatment involves heating a weldment, holding it below the critical temperature for a time, and then slowly cooling it. Care must be taken for hardened materials to not exceed the original tempering temperature which would decrease the strength.
For critical weldments, a process with specified heating and cooling rates may be required per the applicable standard. Additionally, load thermocouples may be required to track the temperature of the furnace.
Some welds on stainless steels and superalloys may also require a solution annealing treatment. See our Solution Annealing page for more information.
Applications of PWHT
- Post-weld heat treatment can reduce subsequent dimensional changes by allowing a controlled redistribution of residual stresses. This may cause some movement of parts during the process as stresses are relieved, but will reduce the overall effects of residual stresses.
- Some welds will experience hardening in the fusion zone of the weld due to the rapid solidification of the weld. Higher carbon materials and filler rods will be most affected. To increase the toughness of the weld a post-weld heat treat will effectively temper the hardened area.
- As the weld area is solidifying and cooling, unfavorable stress states can be formed due to the thermal gradients across the weldment. This can lead to both dimensional and mechanical property deficiencies. Post-weld heat treating will allow the stress states to redistribute, lessening that effect.
Specs and Standards associated with PWHT
- AWS D1.1, D1.2, D1.5
- ASME B31.3, BPVS Section VIII, Section I, UCS 56
What To Consider When Specifying
- Heat Treat State
- Specification (if required)