J. Phys. IV France
Volume 112, October 2003
Page(s) 403 - 406

J. Phys. IV France
112 (2003) 403
DOI: 10.1051/jp4:2003911

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

Y. Kimura, S. Takagi, T. Hara, S. Terasaki and K. Tsuzaki

National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan

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  $\mu$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  $\mu$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).

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