Hydrogen-induced delayed fracture of a martensitic steel with fine prior-austenite grain size

The influence of the grain size of the prior-austenite on the hydrogen-induced delayed fracture property was investigated for a medium carbon tempered martensitic steel (JIS-SCM440) with a tensile strength of 1400MPa. Thermomechanical treatment, which consists of a warm working at 873K by multi-pass bar rolling of tempered martensite and subsequent rapid austenitizing at 1093K for 1s, resulted in the refinement of the prior-austenite grain to 3 μm. The hydrogen embrittlement susceptibility was examined using a conventional creep test machine under a constant load for hydrogen pre-charged notched specimens with a stress concentration factor of 4.9. The critical diffusible hydrogen content, below which the specimen never fracture, was approximately 0.24 mass ppm for the fine-grained specimen under an applied stress of 0.9TS, two times higher than that of the conventional QT sample (prior-austenite grain size; 17 μm). The immersion test in 0.1M HCl water solution (pH=1.0) at 298K showed that the intruded hydrogen content (HE) at 100h was 0.14 mass ppm for the fine-grained sample which was almost the same as that of the conventional QT sample (0.17 mass ppm).