Common defects arise from poor PWHT practice. Too rapid heating or cooling can reintroduce thermal stresses. Exceeding the upper transformation temperature may alter the base metal microstructure. Insufficient soak time leaves residual stresses unrelieved. These issues are not failures of PWHT itself but of process control.
For critical applications like pressure vessels and piping, codes such as ASME B31.3 and ASME Section VIII define mandatory PWHT requirements based on material, thickness, and service conditions. Compliance is verified through temperature charts and hardness testing. Hardness checks after PWHT confirm that the weld and HAZ meet specified limits.
NDT plays a key role in validating PWHT effectiveness. Hardness testing, often using portable instruments, provides a quick indication of proper tempering. In sour service, NACE MR0175 sets hardness limits that PWHT must achieve. Without controlled PWHT, the risk of hydrogen-induced cracking increases.
The takeaway is simple: follow the procedure, monitor thermal cycles, and verify results with NDT. Heat, when controlled, is a tool, not a threat.
Source: ASME B31
How does your team ensure PWHT parameters stay within code limits?