Post-Weld Heat Treatment — What It Is and Why It Matters
Post-weld heat treatment (PWHT, also called stress relief) is a controlled heating cycle applied to a weldment after welding to reduce residual stresses, temper hard microstructures in the HAZ, and improve the mechanical properties of the joint. The weldment is heated to a specified temperature, held for a specified time, and cooled at a controlled rate.
PWHT is required by code for many pressure vessel, piping, and structural applications, particularly on thick sections of carbon and alloy steels where residual stresses from welding are high enough to risk stress corrosion cracking, distortion under service loads, or brittle fracture at low temperatures.
Typical PWHT for carbon steel involves heating to 1,100-1,250°F (600-675°C), holding for one hour per inch of thickness (with minimums specified by code), and cooling at a controlled rate not exceeding about 500°F per hour. PWHT is performed using electric resistance heaters, furnaces, or induction heating equipment, with temperature monitored by thermocouples attached to the weldment.
Frequently Asked Questions
Why is post-weld heat treatment necessary?
Welding creates residual stresses from uneven heating and cooling, and can produce hard, brittle microstructures in the heat-affected zone. PWHT reduces these residual stresses to acceptable levels and tempers hard zones to restore ductility, reducing the risk of cracking during service — especially under pressure, cyclic loading, or at low temperatures.
Can I PWHT with a torch?
For small, non-code applications, local stress relief with a carefully controlled oxy-fuel torch can provide some benefit. However, proper code-compliant PWHT requires controlled heating rates, uniform temperature distribution (monitored by thermocouples), specified hold times, and controlled cooling rates — which typically requires resistance heaters or a furnace.