Mechanisms of high pressure hydrogen environment embrittlement in austenitic stainless steels under tensile and fatigue loading

Abstract

Hydrogen is known to degrade the mechanical performance of many engineering materials. The effects of its entry into metal matrices from manufacturing processes and service environments has been reported previously to result in loss of ductility and fracture toughness as well as increased fatigue crack propagation rates. One of these damage mechanisms, hydrogen environment embrittlement, was explored in stainless steels in order to provide better understanding of the role of the composition and microstructure in susceptibility to the effects of high-pressure hydrogen atmosphere on tensile and fatigue performance.

Current knowledge in the field has been extended by investigating the influence of a high pressure hydrogen environment on monotonic tensile failure and fatigue crack propagation processes in the austenitic stainless steels, 304L and 316L, and to explore the effects of secondary variables on damage severity (temperature, pressure, frequency). Assessment of the role of microstructure and composition on susceptibility to damage was completed by comparison of alloys’ relative performance and their fracture characteristics by conducting tensile and fatigue testing in high pressure hydrogen environment at pressures ranging from 200 to 1000 bar and temperatures between -50 and +50?C. Fatigue testing work at high pressure (above 450 bar) and in the low temperature regime was completed using equipment designed as a part of the EngD project.

Testing under high pressure hydrogen environment resulted in pronounced loss in ductility and increase in fatigue crack propagation rates in both materials, 304L steel was more adversely affected in all testing conditions than 316L. The degree of damage was observed to increase with increasing hydrogen pressure and reducing temperature in both load regimes. Increased testing temperature resulted in partial recovery of global ductility measurements in tensile tests while fatigue crack propagation rates were still significantly increased.

The embrittlement mechanisms differed between 304L and 316L steels due to the different phase stability and deformation mechanisms characterising these alloys. In 304L, hydrogen was seen to facilitate crack propagation along microstructural features such as slip bands, phase and twin boundaries, with some indication of the effects of localised plasticity. While some of these mechanisms were observed to be operative in 316L, it was difficult to attribute the fracture of this steel to a particular mechanism. It appears that martensite formation and planar slip processes were not the only necessary conditions for hydrogen embrittlement. Features of interfacial fracture were noted in this steel, particularly at ferrite stringers and austenite matrix, possibly indicating fracture due to local accumulation of hydrogen and consequent ferrite embrittlement and localised fracture.

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ORCID for Philippa Reed: orcid.org/0000-0002-2258-0347

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