The 2014 Edition of ASME B31.3, Process Piping [1], introduced significant changes to the post weld heat treatment (PWHT) requirements for P-No. 1 carbon steel materials. In particular, PWHT is no longer a mandatory requirement for any wall thickness provided that multi-pass welding is employed for wall thicknesses greater than 5 mm (3/16 of an inch) and a minimum preheat of 95°C (200°F) is implemented for wall thicknesses greater than 25 mm (1 inch). Detailed fracture mechanics analyses have shown that the lack of a mandatory PWHT requirement for thicker P-No. 1 components may result in a significant increase in risk for brittle fracture failures due to near-yield level weld residual stresses. Given the concern throughout the pressure vessel and piping community regarding potential brittle fracture failures, this updated PWHT guidance is examined.
Impact testing requirements and exemption curves were introduced in the 1987 Addenda [2] of ASME Section VIII Division 1 (VIII-1) [3] in Paragraph UCS-66 and extended into ASME Section VIII Division 2 (VIII-2) [4]. During the VIII-2 rewrite in 2007 [5], the available technical and historical basis for the UCS-66 exemption curves was examined and improved to reflect modern fracture mechanics standards. The result of that effort was a systematic approach that can be modified for particular geometries and assumed flaws, if desired. The method used the most modern, fracture mechanics approach for welds in API 579-1/ASME FFS-1, Fitness-For-Service, (API 579) [6] based on the failure assessment diagram (FAD). As a result of explicitly accounting for weld residual stress, two separate sets of exemption curves are provided in VIII-2 [4]; one set for as-welded components and another set for PWHT components. In this paper, a similar approach is summarized to generate exemption curves by establishing newer as-welded and PWHT curves using the Fracture Toughness Master Curve (Master Curve) as documented in upcoming Welding Research Council (WRC) Bulletin 562 [7]. The increased propensity for brittle fracture in as-welded components versus PWHT components is clearly highlighted using this approach. The Master Curve, in conjunction with the elastic-plastic fracture mechanics employed in API 579 [6] provides a means to develop exemptions curves anchored in state-of-the-art fracture toughness technology that can be directly tied to different reference flaw sizes. Additionally, commentary on the appropriateness of the current ASME B31.3 [1] PWHT requirements is offered and the effectiveness of using weld preheat in lieu of PWHT as permitted in the National Board Inspection Code (NBIC) [8] is examined using simplified computational weld analysis.
