The Post Weld Heat Treatment (PWHT) of socket welds shall be performed in the following two cases:

1. When required by the line specification (i.e. caustic lines), in this case the holding temperature for carbon steel and killed carbon steel pipes shall be maximum 600°C for approximately 5 minutes.
2. When the material is alloy steel and the line is in hydrogen service. In this case the PWHT can be carried out after the postheating. The holding temperature shall be max 650°C for approximately 15 minutes.

The temperature may be checked by temperature indicating crayons, digital thermometers or other means.

The cooling shall be made under insulation.

Special care shall be taken during valves welding: the torch used shall be of a small size and the valve body shall be protected by direct flame contact by metal disc or by insulating material placed besides the weld itself.