In case of welded vessels why is stress relieving in the form of post-weld treatment necessary?

When a metal is welded together the point at which two surfaces are joined are subjected to very high temperatures. During this time period metals can go through a lot of random metallurgical processes. This causes the metal to be brittle and harder. This relies on the amount of carbon content. To relieve the metal of these properties heat treatment is done on affected areas. By doing so the affected areas ductility increases. In this process heat is maintained in affected metal according to thickness of the metal. Temperature of the heat is calculated from the P number that is unique for a particular composition.