Hydrogen embrittlement of catholically hydrogen-precharged 304L austenitic stainless steel: Effect of plastic pre-strain

Prior plastic deformation introduced by fabrication process such as cold working is an important factor for hydrogen embrittlement (HE) of austenitic stainless steels. In order to clarify the relationship between HE susceptibility of type 304L stainless steel and prior tensile plastic deformation, a group of flat specimens with different degrees of pre-strain (0 i.e. without pre-strain, 3, 6, 10, 15, 20 and 25% true strain) was catholically hydrogen charged and then was strained immediately to fracture by means of tensile testing. The resulting tensile properties and fracture morphologies were compared with those of a second reference group of specimens without charging. The results reveal a detrimental effect of high degrees of pre-strain on HE of the steel. When the pre-strain was less than 6%, the effect of pre-strain on HE of the steel was minor. While when the pre-strain was higher than 10%, the pre-strain greatly enhanced HE of the steel. This occurrence could be attributed to strain-induced α′ martensite transformation during pre-straining. The higher the pre-strain, the higher the content of α′ martensite transformed and so the HE occurred. Fracture morphology investigations suggest that the pre-strain-induced α′ martensite greatly enhanced the hydrogen diffusivity, and the mode of brittle fracture due to hydrogen strongly depended on the pre-strain level.