Definition(s)
Postweld Heat Treatment
A work process which consists of heating an entire weldment or section of fabricated piping to an elevated temperature after completion of welding in order to relieve the detrimental effects of welding heat, such as reducing residual stresses, reducing hardness, and/or slightly modifying properties (See ASME B31.3, paragraph 331).
Source: API 570, Piping Inspection Code: In-service Inspection, Rating, Repair, and Alteration of Piping Systems, Fourth Edition, February 2016, with Addendum May 2017. Global Standards
Postweld Heat Treatment
Treatment that consists of heating an entire weldment or vessel to a specified elevated temperature after completion of welding in order to relieve the detrimental effects of welding heat, such as to reduce residual stresses, reduce hardness, stabilize chemistry and/or slightly modify properties.
Source: API 510, Pressure Vessel Inspection Code: In-service Inspection, Rating, Repair, and Alteration, Tenth Edition, May 2014, with Addendum May 2017. Global Standards
Post-Weld Heat Treatment (PWHT)
Any heat treatment subsequent to welding, including stress relief.
Source: API SPEC 16A, Specification for Drill-through Equipment, Third Edition, June 2004 (Errata/Supplement November 2004). Global Standards