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Chemical and Process Engineering Resources

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Category: Corrosion
Question: Why is post-weld heat treatment (stress relieving) sometimes necessary for welded vessels?
Keywords: v1i14,post,weld,heat,treatment,stress,relieving,carbon,steel,vessel,fabrication
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 also 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).


PWHT is required to do for various equipments either as per the code requirment or it is required to do in some of the sevices like for carbon steel it is to be done in caustics, amine, methanol etc.