Environmental testing of rolled and hot isostatically-pressed duplex stainless steels

Austenitic-ferritic (super)duplex stainless steels, (S)DSSs, are of particular interest in oil and gas applications, due to their combination of high strength and corrosion resistance. However, (S)DSSs are known to be susceptible to hydrogen embrittlement, via a mechanism commonly referred to as hydrogen-induced stress cracking (HISC). In subsea environments, (S)DSS components are often exposed to cathodic protection (CP), which is mainly applied to protect structural steel components, to which (S)DSS components are connected. CP can introduce hydrogen to metallic surfaces that are exposed to seawater and, in the case of (S)DSSs, the absorbed hydrogen can cause embrittlement via HISC, which has been found to be responsible for a number of subsea failures. During failure investigations of (S)DSS components, it has been observed that the microstructure, i.e. as manifested by the size, spacing and distribution of the austenite, ferrite and chromium nitride precipitates, obtained by various manufacturing and fabrication processes, is a key factor in resistance to HISC. However, because of the complexity of (S)DSS microstructures, the micro-mechanisms of hydrogen embrittlement have remained largely unknown. This work aims to evaluate and compare the resistance to HISC for two types of DSS products with significantly different microstructures: rolled and hot isostatically-pressed (HIPed), using the conventional fracture toughness test methods, i.e. unloading compliance testing of single edge notched bend specimens (SENBs). The two materials have been characterized in terms of composition, phase balance, austenite spacing and mechanical properties. An environmental-mechanical test program, largely based on fracture toughness testing, has been developed and performed to investigate the effect and significance of specimens' notch geometry, as well as hydrogen-charging conditions, highlighting the difficulties of evaluation of resistance to cracking of DSSs, using fracture toughness based tests in hydrogen-charging environments.