Judgment of PWHT Necessity

Whether the pressure vessel is necessary for post-weld heat treatment should be clearly specified in the design, and the current pressure vessel design specifications have requirements for this.

For welded pressure vessels, there is a large residual stress in the welding zone, which is adversely affected by the residual stress. It only manifests under certain conditions. When the residual stress combines with the hydrogen in the weld, it will promote the hardening of the heat-affected zone, leading to cold cracks and delayed cracks.

When the residual static stress in the weld or the dynamic load stress in the load operation is combined with the corrosive effect of the medium, it may cause crack-like corrosion, the so-called stress corrosion. Welding residual stress and the hardening of the base metal caused by welding are important factors for stress corrosion cracking.

The research results show that the main effect of deformation and residual stress on metal materials is to transform the metal from uniform corrosion to local corrosion, that is, to intergranular or transgranular corrosion. Of course, both corrosion cracking and intergranular corrosion of metals appear in the medium with certain characteristics of the metal.

In the presence of residual stress, it varies according to the composition, concentration and temperature of the corrosive medium, as well as the difference between the composition, structure, surface state, and stress state of the base metal and the welding zone, thereby causing corrosion The nature of the damage may change.