Observations on Flaw Tolerance and Brittle Fracture: The Effect of Post Weld Heat Treatment

Introduction

Post weld heat treatment (PWHT) can have a significant influence on the risk for brittle fracture in welded components. Furthermore, 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 [1]. 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 applied for thicknesses greater than 1 inch. Fracture mechanics calculations have shown that the lack of a mandatory PWHT requirement for thicker carbon steel components may result in a significantly increased risk for brittle fracture failures due to near-yield level weld residual stresses. Given the concern throughout industry regarding the potential for brittle fracture failures, PWHT guidance to address potential issues arising from the change in the 2014 edition as cited previously, is examined in this article, and commentary on the potential reduction in fracture toughness due to PWHT is provided based on a review of published literature.

This article summarizes aspects of recently published Reference [2], where 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 presented. This approach permits direct comparison of flaw tolerance for as-welded and PWHT components using the Fracture Toughness Master Curve (Master Curve) as documented in recently published Welding Research Council (WRC) Bulletin 562 [3].  The increased overall propensity for brittle fracture in as-welded components versus PWHT components is clearly highlighted using this methodology.  Furthermore, the Master Curve, in conjunction with the elastic-plastic fracture mechanics methodologies described in API 579-1/ASME FFS-1, Fitness-For-Service (API 579) [4] provides a means to quantify the crack driving force associated with weld residual stress using modern fracture mechanics. Lastly, commentary on the appropriateness of the current ASME B31.3 PWHT requirements is offered, and observations on the consequence of using weld preheat in lieu of PWHT, as permitted by the National Board Inspection Code (NBIC) [5], are provided.

PWHT Fundamentals

Weld residual stresses in pressure retaining equipment are an artifact of highly localized transient heat input that occurs during the welding process. As discussed in Reference [6], weld residual stresses are the result of internal forces occurring without any external forces when the heating of the weld area relative to the adjacent material experiences restrained thermal expansion. Plastic strains then develop and during the cooling process, tensile residual stresses are induced in areas near the weld deposit due to the restraint of the adjacent (colder) base metal. These residual stresses increase the likelihood for crack initiation and propagation and depending on the process conditions and service environment, may increase the risk of stress corrosion cracking, fatigue cracking, and ultimately brittle fracture.