Effects of Strain Rate and Microstructure on Fracture Toughness of Duplex Stainless Steels Under Hydrogen Charging Conditions

Failures of ferritic-austenitic stainless steel due to hydrogen induced stress cracking (HISC) have been very costly and raised concerns regarding subsea system integrity, some of which remain unresolved. The susceptibility to HISC crack initiation shows a strong correlation with austenite spacing and tests performed on smooth samples have shown that coarse-grained microstructures, with large austenite spacing, such as in forgings, are more susceptible to HISC than fine grained structures, eg as in pipe [1]. In all reported failures, cracking has been independent of the presence of fabrication flaws, even though welds were typically present, and initiated at external stress concentrators, so the importance of flaws remains undetermined. There is no well established method for determining fracture toughness values applicable to flaws in duplex stainless steel in the presence of hydrogen and hence reliable data do not exist, leading to a lack of understanding of the criticality of flaws and whether fine austenite spacing provides any benefit in resistance to extension of flaws. This paper provides new data from fracture toughness tests conducted on duplex pipe and forging parent materials, to explore the effect of product type/ microstructure and strain rate on fracture toughness under active charging in seawater under cathodic polarisation. This is part of ongoing work aimed at the development of an engineering critical assessment (ECA) approach for assessing flaw tolerance under hydrogen charging conditions.