scope:

Postweld heat treatment produces both mechanical and metallurgical effects in pressure vessel steels that will vary widely with the composition of the steel, its past thermal and mechanical history including welding, the temperature and duration of the postheat, and the heating and cooling rates accompanying postheat. When the majority of welded pressure vessels were fabricated from carbon steels, postweld heat treatment (PWHT) was performed to relieve the locked-up stresses resulting from fabrication, and the process was identified as "stress relief." As higher-strength alloy steels were applied to pressure vessels, it was realized that there were other effects involved, some of which were beneficial and others detrimental to the performance of the vessels. This review is an attempt to clarify the present status of PWHT as a fabrication tool and to provide a background for defining the conditions under which postheating is necessary and those in which it may be detrimental. The discussion of PWHT will be limited to treatments performed in the temperature range of 400C to just below the transformation temperature of the steeland to steels with minimum yield strengths up to 700 MPa (100 ksi). Temperatures below this range largely fail to accomplish the desired effects, while treatments into or above the transformation range evoke an entirely different set of responses in the steels that are accompanied by an equally different set of procedures and problems