Hydrogen susceptibility in fatigue life has been examined using commercial JIS SCM420 steel. The effects of carbon content, austenitizing temperature and tempering temperature have been explored in comparison with JIS SCM435 steel that was given the standard heat treatment. Discussion has been made with the information of hydrogen thermal desorption spectroscopy (TDS) and appearance of carbide precipitation. When the tensile strength of SCM420 steel is adjusted to the normally heat treated SCM435 steel by combining higher austenitizing temperature and lower tempering temperature, fatigue life ratio that is the fatigue life of hydrogen pre-charged specimen divided by that of non-charged one stays almost unaffected. When tempering is further lowered to 448K, the tensile strength reaches almost 1500MPa, but the drop in fatigue life ratio is minute. One of the reasons is the suppressed phosphorus segregation along austenite grain boundary that is brought about by elevating the austenitizing temperature. The other is especially for 448K tempering. It is the reduced hydrogen segregation along prior austenite grain boundary, as the results of scarce carbide precipitation along it.
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