Abstract
To investigate the hydrogen trapping effect of the combined addition of V to Mo-added steel, 0.1C-2Mn-1.6Mo mass% steel (Steel A) and 0.1C-2Mn-1.6Mo-0.2V mass% steel (Steel B) were prepared, quenched, and tempered at 873 K. The hydrogen trapping effect was investigated by thermal desorption hydrogen analysis of hydrogen-charged specimens, and Steel B showed a higher hydrogen trapping capacity than Steel A. According to thermodynamic equilibrium calculations, hydrogen trapping site of Steel A and B after tempering were predicted as M2C carbides. However, according to TEM observation of these specimens, not only coarse M2C but fine MC carbides precipitated in Steel A, and only fine MC precipitated in Steel B. Chemical composition of these precipitates were investigated by the three-dimensional atom probe analysis. MC of both Steel A and B show a composition close to MC0.5, in which Mo is the primary element in metal sites. It was found that the carbon-site vacancy (C vacancy) ratio of MoC0.5 in the present work is higher than that of V4C3 (VC0.75). The hydrogen trapping capacity showed a good correlation with the product of the area of Fe–MC interface and the C vacancy ratio in MC. The reason of the higher hydrogen trapping capacity of Steel B than that of Steel A is considered as below. 1) The combined addition of V to Mo assisted the precipitation of MC instead of coarse M2C. 2) C vacancies in MC were increased by the partitioning of Mo into MC, and the vacancies acted as hydrogen trapping sites.
