Tensile flow and fracture behaviour of austenitic stainless steels after thermal aging in a hydrogen atmosphere

A hydrogen content of about 300 at.ppm introduced by concurrent sensitization and precharging from the gas phase at 600°C of type 304L and 316L austenitic stainless steels induced a noticeable degradation of the mechanical properties, changes in fracture mode or morphology and an increase in the amount of deformation martensite. The various changes in properties were revealed on post-treatment tensile testing in air at ambient temperature, scanning electron microscopy fractography and X-ray diffractometry. The degradation of the mechanical properties was described quantitatively by a detailed examination of the entire range of uniform deformation. The relevant effects were (a) ductility losses of the order of 30%, (b) flow stress increases (hydrogen hardening) of the order of from 7% (type 304L steel) to 10% (type 316L steel) at moderate and large strains and (c) a decrease in the strain-hardening capacity of the order of 30% at moderate strains. The yield stress and ultimate tensile strength of both steels were little affected by hydrogenation and were little affected by hydrogenation and were thus shown to be of doubtful value as criteria of hydrogen effects when examined out of the context of overall plastic behaviour. Strain-induced martensite is suggested to be the dominant factor in the behaviour of the type 304L steel. The large susceptibility to hydrogen effects shown by the type 316L steel is discussed in terms of nickel content, surface and geometry effects and also void growth.