The Effects of Prior Austenitic Grain Size and Martensitic Hardness on Hydrogen Embrittlement of 4340 Steel

The effect of hydrogen on embrittlement of low alloy 4340 steel was studied using double-notched tensile samples that were electrochemically charged in-situ with hydrogen in a 1N H2SO4 + 5 mg/l As2O3 solution. The mechanical response of samples with prior austenitic grain sizes of 10, 40 and 100 pin with martensitic hardness of 40-52 HRC were examined after hydrogen charging times of 0-40 min. As expected, increases in hydrogen charging time and hardness resulted in decreased failure strains and decreased evidence of ductile fracture. Finally, increasing the size of the prior austenitic grains was found to lead to sample failure via brittle transgranular cleavage for the large-grained softer samples while brittle intergranular fracture dominated the large-grained hardest samples.