Post weld heat treatment methods is a common process used to relieve and redistribute residual stresses that have built up due to welding processes. It is also used to prevent hydrogen induced cracking in certain materials. It can be specified by various codes as a requirement for certain thicknesses of material or to mitigate specific micro-structural defects such as weld decay.

What is post weld heat treatment tempering?

Typical PWHT techniques include annealing and normalizing. Annealing involves heating the welded metal to a temperature and holding it there for a time that is based on the material type and thickness. This allows for diffusion of hydrogen and helps prevent hydrogen induced cracking when the weld is cooled to room temperature.

Normalizing involves heating the welded metal to a higher temperature than annealing and cooling it rapidly. This reduces the hardness of the weld and improves ductility in the weld zone. It can also be performed in combination with tempering or ageing to produce additional metallurgical changes that reduce the risk of brittle cracking in certain steels.

Another use of PWHT is to drive away moisture from the weld area. This is typically done by preheating the material to a lower temperature than the welding temperature range. This dries the surface and removes any contaminants that can cause porosity or hydrogen embrittlement.

Localized PWHT is often required for elongated components such as long pressure vessels or piping. This requires special equipment that can preheat the base material to a high temperature and control it to precise temperatures. Failure to do so can result in distortions of the structure during the PWHT cycle as different areas experience varying thermal expansions. Ceramic mat heaters are often used for this application as they can be positioned to provide a concentrated heat source to the area that needs the treatment.