Carbide Precipitation and Hydrogen Trapping Behavior in Mo and V Added Tempered Martensitic Steel

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 M 2 C carbides. However, according to TEM observation of these specimens, not only coarse M 2 C 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 MC 0.5 , in which Mo is the primary element in metal sites. It was found that the carbon-site vacancy (C vacancy) ratio of MoC 0.5 in the present work is higher than that of V 4 C 3 (VC 0.75 ). 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 M 2 C. 2) C vacancies in MC were increased by the partitioning of Mo into MC, and the vacancies acted as hydrogen trapping sites.