The effect of hydrogen on the mechanical behaviour of duplex stainless steel.

Abstract

Duplex stainless steels are commonly used in environments that are expected to produce hydrogen i. e. in sour environments and sea water applications, often under cathodic protection. Under these conditions there is a concern about their susceptibility to hydrogen embrittlement. The effect of hydrogen, both external and internal, on the mechanical properties and the fracture characteristics of duplex stainless steels Type 2205 and 2507 have been studied by slow strain rate techniques using smooth tensile specimens. Specimens were strained to failure in air after high pressure hydrogen thermal charging, in a hydrogen atmosphere, in a hydrogen sulphide environment under open circuit potential condition, and whilst cathodically polarized at different potentials in distilled water with 100 wppm potassium sulphate added, in 3.5% aqueous sodium chloride, or in NACE solution. All the environments produced a major reduction in ductility that increases linearly with decrease in strain rate. The severity of the embrittlement depended upon whether the supply of hydrogen was external or internal. Internal hydrogen, as in thermally charged specimens, produced a more profound loss in ductility than straining in a hydrogen atmosphere and prolonged room temperature aging of these specimens, for up to 3 years, resulted in insignificant recovery of ductility, emphasizing the role of the austenite as a hydrogen reservoir. Provision of hydrogen at very high fugacities (cathodic polarization) during straining indicated that the potential at which loss in ductility is first noted corresponds to the hydrogen evolution potential for the particular solution involved. The presence of chloride ion seems to have no significant effect on the loss in ductility- The presence of hydrogen sulphide in the environment, however, introduced the complication of extensive chemical attack during and after crack propagation. The loss in ductility increased as the pH of the solution decreased and, irrespective of pH, maximum embrittlement occurred at some particular temperature between 20 and 90'C. The latter is attributed to the two competing processes of hydrogen ABSTRACT embrittlement and corrosion. A minimum chloride ion concentration of 300 wppm seems necessary to maintain the maximum embrittlement. The ultimate tensile strength of the steel is not affected by hydrogen since cracking only occurs after it is exceeded. Cracks initiate and grow preferentially through the ferrite phase, with fracture surfaces exhibiting quasi-cleavage features; the austenite often failed in a ductile mode. The proportion and distribution of the two phases has a significant effect on the degree of embrittlement. The presence of greater amounts of austenite seems to inhibit crack propagation, but may act as a hydrogen source or reservoir for the embrittlement of the ferrite phase. Straining of the as received weldments, which had been annealed after welding, showed no evidence of hydrogen embrittlement, but an attempt was made to simulate via heattreatment the structures that could occur in the heat affected zone of the weld and these structures had inferior mechanical properties in the presence of hydrogen.