post weld heat treatment is the heating of a welded structure or component to a controlled temperature to relieve residual stresses and micro-structural changes that occur as a result of welding. PWHT is often required by codes for structures of certain thicknesses and grades. In addition, many customers require PWHT because of the risk of hydrogen-induced cracking (HIC) which can cause catastrophic failures like the collapse of pressure equipment or major piping components.

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PWHT is typically a furnace based process that cycles the metal through a series of temperatures – usually well below the transformation range for the material. The temperature cycle can be varied based on the desired effects and service requirements for the component or structure. For example, if the material is high strength steel, a lower temperature cycle is needed to reduce the chance of hydrogen embrittlement which occurs with rapid cooling.

To avoid this, a preheating period is normally applied to the weld area before beginning the PWHT cycle. This can be achieved using gas burners, oxy-gas flames, electric blankets, induction heating or in the furnace itself. However, it is important that the heating is uniform throughout the weld and HAZ areas. Intense non-uniform heating is not effective in retarding cooling and can lead to higher residual stresses, distortion or undesirable metallurgical changes in the base material.

Research into the effect of post weld heat treatment on the microstructure and mechanical properties of various materials has been carried out by several scientists. For example, Alipooramirabad et al investigated the effect of low-temperature PWHT on Quenched and Tempered (QT) S690QL1 high strength steel welds. They found that PWHT can significantly decrease the residual stress levels, drop the hardness level in the weld and HAZ and improve the tensile strength and ductility of the weld zone.