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

Abstract
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 140MPa. 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 ls, resulted in the refinement of the prior-austenite grain to 3  m. The hydrogen embrittlement suscepûbüity was examined using a conventional creep test machine under a constant bad for hydrogen pre-charged notched specimens with a stress concentration factor of 4.9. The critical dffusile hydrogen content, below which the specimen never fracture, was approximately 024 mass ppm for the fine-grained specimen under an apphed stress of 0.9TS, two times higher than that of the conventional QT sampi (prior-austenite grain size; 17  m). The immersion test in O. 1M HCI 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).