Welding Inspection & Metallurgy API ICP Self Study Notes

The primary reason for post-weld heat treatment is to relieve the residual
stresses in a welded fabrication. Stresses occur during welding due to the
localized heating and severe temperature changes that occur. PWHT
releases these stresses by allowing the metal to creep slightly at the elevated
temperature.
However there may also be in-service conditions that require particular
PWHT conditions. These may not be so closely detailed in construction
specifications and inspectors should therefore be particularly aware of these
potential requirements when allowing, authorizing or inspecting in-service
repairs.