Evaluating the Flaw Tolerance and Ductile Tearing Resistance of Austenitic Stainless Steel Welds

Abstract In general, austenitic stainless steels behave in a more ductile than brittle manner and do not typically experience a ductile-to-brittle transition like typical ferritic steels. Furthermore, the fracture toughness of austenitic stainless steels is ordinarily high, even at low temperatures, provided the material has not experienced any notable in-service degradation in fracture toughness. Often, it is more likely for an existing crack-like flaw in an austenitic stainless steel pressure vessel or piping component to experience ductile tearing than to initiate a brittle fracture. For this reason, the Level 3 ductile tearing analysis methodologies described in Part 9 of API 579-1/ASME FFS-1, Fitness-For-Service (API 579) [1] are summarized in this paper and compared to conventional elastic-plastic fracture mechanics (based in the use of the failure assessment diagram, per API 579). Additionally, in this paper, a case study is summarized where identified crack-like flaws in an austenitic stainless steel reactor are analyzed for ductile tearing to gain perspective on the propensity of the existing cracks to cause a catastrophic failure of the pressure boundary. A conventional brittle fracture assessment is performed for comparison purposes using a lower bound fracture toughness value for stainless steel weldments, based on published data. These calculations account for primary stress (from internal pressure) and weld residual stresses. Understanding the fracture and ductile tearing resistance of austenitic stainless steel welds in pressure equipment is an important aspect of managing the risk associated with operating components that may be prone to crack initiation and propagation due to operational and environmental conditions. Relevant damage mechanisms include reheat (stress relaxation) cracking, high-temperature creep, thermal or mechanical fatigue, and stress corrosion cracking. Leveraging fracture mechanics-based fitness-for-service approaches, as discussed in this paper, for qualifying crack-like flaws can extend equipment life and minimize the need to perform costly repairs or component replacement.