Postweld Heat Treatment for Service Conditions

See Section 600 for specific recommendations where service environments warrant postweld heat treatment (PWHT). PWHT is performed to reduce residual stresses, hold dimensional tolerances, improve resistance to brittle fracture, reduce susceptibility to stress corrosion cracking, or provide added safety in certain hazardous services.

In addition to thickness requirements, the Code requires PWHT for carbon or lowalloy vessels operating below minus 50°F, for unfired steam boilers at pressures above 50 psig, and for lethal-service vessels. Where service conditions might promote stress corrosion cracking, the need for PWHT should be investigated. This subject is discussed in the Corrosion Prevention and Metallurgy Manual.

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