Postweld heat treatment, most often stress relief of carbon and low-alloy steel at about 595 to 675 °C, held for a time based on thickness and cooled at a controlled rate. It lowers residual stress, tempers hard HAZ structure and drives off hydrogen. Codes require it above certain thicknesses and for some alloys and services, and it is done in furnaces or locally with resistance heating pads, insulation and chart recorders.
Heat treaters are wrapping the nozzle welds tonight for PWHT, 1,150 for two hours on the chart.
Also heard as
- postweld heat treatment
- stress relief
- SR
Related terms
HAZ
Heat-affected zone: base metal next to a weld whose structure changed from the heat but did not melt.
Preheat
Heating base metal before welding to slow cooling and drive off moisture, preventing hydrogen cracks.
Hydrogen cracking
Delayed cracking in steel welds caused by hydrogen, hard microstructure and stress acting together.
WPS
Welding procedure specification: the written instructions that tell welders exactly how to make a weld.