Why is post-weld heat treatment (stress relieving) sometimes necessary for welded vessels?

Why is post-weld heat treatment (stress relieving) sometimes necessary for welded vessels?
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Answer :

During the welding process, the two metal pieces being joined are subject to extreme temperatures and  can cause the crystalline structure of the metal to pass through various metallurgical phases. As a result,  hardening (and embrittlement) of the metal can occur to varying degrees (usually dependent on carbon  content). Heat treatment is designed to reduce the hardness in the heat-affected zone of the metals and  increase ductility in these sections. Various pressure vessel codes contain the specifics regarding the  procedures for post-weld heat treatment. Heat is usually held for one hour per inch of thickness of the  metal. The temperature used is based on the “P-number” of the metals. P-numbers are assigned based  on the chemical composition of the metals. Holding temperatures can range from 1100-1350 °F (593- 732 °C).
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Related questions

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