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

Abstract

To investigate the effect of the combined addition of V to Mo-added steel on the hydrogen trapping behavior, 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 for 0.5 to 10 hours. The thermal desorption analysis of hydrogen-charged specimens confirmed that Steel B showed a higher hydrogen trapping capacity than Steel A. According to thermodynamic equilibrium calculations, it was predicted that only M2C precipitated in Steel A and B after tempering at 873 K. However, according to TEM observation of specimens tempered for 4 hours, coarse M2C and fine MC precipitated in Steel A, whereas only fine MC precipitated in Steel B. Based on 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 MC 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 interface of MC 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 to be below; 1) The combined addition of V suppressed the precipitation of M2C and increased the amount of MC. 2) C vacancies in MC which act as hydrogen trapping sites increased by the partitioning of Mo into MC.