The Effect of Post-Weld Heat Treatment and Weld Preheat on Material Properties and Protection Against Brittle Fracture

by Phillip E. Prueter, P.E. / September 2017

 

The intent of this webinar is to highlight the effect of post-weld heat treatment (PWHT) and weld preheat 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 webinar, commentary on the potential reduction in notch toughness due to PWHT is provided based on published data.

 

Additionally, an approach to generating exemption curves and to determine appropriate Charpy impact test temperatures by establishing separate as-welded and PWHT curves is summarized.  These comparisons use the Fracture Toughness Master Curve (Master Curve) as documented in recently published Welding Research Council (WRC) Bulletin 562.  The increased propensity for brittle fracture in as-welded components versus PWHT components is clearly highlighted using this methodology. The Master Curve, in conjunction with the elastic-plastic fracture mechanics employed in API 579-1/ASME FFS-1, Fitness-For-Service (API 579) provides a means to quantify the crack driving force associated with weld residual stress and is anchored in state-of-the-art fracture mechanics.  Lastly, commentary on the appropriateness of the current ASME B31.3 PWHT requirements is offered and the effectiveness of using weld preheat in lieu of PWHT as permitted in the National Board Inspection Code (NBIC) is scrutinized using computational weld analysis.

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