A study on the susceptibility of high strength tempered martensite steels to hydrogen embrittlement (HE) based on incremental step load (ISL) testing methodology

Incremental step load (ISL) technique was used to determine the material susceptibility of three different grades of tempered martensite steels 35CrMo4, 41Cr4 and 36NiCrMo4 to hydrogen embrittlement (HE). In addition to testing the steel grades in air, the environmental hydrogen embrittlement susceptibilities (EHE) of each material were determined by in-situ charging of hydrogen at three different cathodic potentials −0.85 V_SCE, −1V_SCE and −1.2V_SCE to obtain the entire threshold curves for these materials. Overall investigation implies that 35CrMo4 is more susceptible than 41Cr4 and 36NiCrMo4. The fracture surface morphology in case of 35CrMo4 is entirely intergranular at −1V_SCE and −1.2V_SCE showing the severity of embrittlement. The higher matrix-cementite interfaces along with high cementite and low manganese sulphide (MnS) inclusion content of 36NiCrMo4 influencing the hydrogen transport kinetics within the material are responsible for the lower material susceptibility. The thermal desorption analyses (TDA) further corroborate the observations.