The Influence of Post-Weld Heat Treatment and Weld Preheat on Brittle Fracture


Post-weld heat treatment (PWHT) and weld preheat can have a significant influence on the risk for brittle fracture in welded components. This topic is particularly relevant given the recent changes in PWHT requirements for P-No. 1 carbon steel materials in the 2014 Edition of ASME B31.3, Process Piping. Specifically, PWHT is no longer a mandatory requirement for any wall thickness, provided that multi-pass welding is employed for thicknesses greater than 3/16 of an inch and a minimum preheat of 200°F is implemented for thicknesses greater than 1 inch. Detailed fracture mechanics analyses have shown that the lack of a mandatory PWHT requirement for thicker carbon steel components may result in a significant increased risk for brittle fracture failures due to near-yield level weld residual stresses. Given the concern throughout industry regarding potential brittle fracture failures, this updated PWHT guidance is examined in this article, and commentary on the potential reduction in notch toughness due to PWHT is provided based on published data. This article provides an abbreviated summary of recently published Reference, wherein a rigorous approach to generate impact test exemption curves and to determine appropriate Charpy impact test temperatures by establishing separate as-welded and PWHT curves is provided.

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