Abstract
Recent studies have shown that a high Mo content enables formation of Mo precipitates, and these precipitates act as hydrogen trap sites to mitigate hydrogen embrittlement. However, most of these studies did not consider the effects of other alloying elements or the different strengths of steel with various contents of Mo in the evaluation of hydrogen embrittlement. Moreover, the average amount and size of the formed Mo precipitates and their effect on hydrogen embrittlement have not been sufficiently studied. Therefore, in this study, the Mo content was varied, and the effect of the average amount and size of the formed Mo precipitates on hydrogen embrittlement was investigated. The steels used in this study were 0.25-2.0Mo mass% steel containing 0.35 mass% C and SCM435 steel as a reference steel. In order to exclude the effect of tensile strength, the tensile strengths of the sample steels except SCM435 were adjusted to be equal at around 1 000 MPa by controlling the heat treatment temperature, and the amount of Mo precipitates was quantitatively analyzed. To evaluate hydrogen embrittlement, continuous cathodic hydrogen charging slow strain rate tensile tests (SSRT) with a strain rate of 0.002 mm/s were conducted. Hydrogen analysis result showed that even though the 1.5Mo steel had the highest hydrogen concentration, the evaluation of hydrogen embrittlement showed that the 1.5Mo steel had the highest resistance to hydrogen embrittlement and displayed mainly quasi-cleavage (QC) fracture instead of intergranular (IG) fracture. The results of a precipitate analysis showed that the 1.5Mo steel also contained the largest amount of Mo precipitates smaller than 20 nm. This result indicates that these Mo precipitates play the role of a strong hydrogen trap, contributing to improved resistance to hydrogen embrittlement. Based on these results, the interaction of hydrogen between dislocations and precipitates during loading is considered. Due to the interaction between hydrogen and dislocations, in conventional steel containing no Mo precipitates smaller than 20 nm, the trapped hydrogen in dislocations is transported during loading by the HELP mechanism (HELP: Hydrogen Enhanced Localized Plasticity). The trapped hydrogen from the dislocations then accumulates near grain boundaries, resulting in IG fracture. In the 1.5Mo steel, it possible that during the action of the HELP mechanism, trapped hydrogen in dislocations is captured and re-trapped strongly by the Mo precipitates smaller than 20 nm, and this suppresses the accumulation of hydrogen near grain boundaries, resulting in QC fracture.