Effects of the Addition of Alloying Elements on Hydrogen Diffusion and Hydrogen Embrittlement in Martensitic Steel

We investigated the effects of the substitutional alloying elements on the hydrogen diffusion and embrittlement properties of as-quenched and subsequently tempered martensitic steels containing Cr, Mo, Mn, and Ni. Hydrogen diffusion coefficients (Ds) of the as-quenched steels, measured via hydrogen permeation tests under cathodic hydrogen charging at 24°C, reduce as a function of the concentration of the added element. The reductions of D are higher for steels comprising Cr and Mo than those for steels containing Mn or Ni. Variation in D is explained based on the index Cr + Mo + 0.01 Mn + 0.1 Ni (at%) at the substitutional element concentration in the solid-solution state. Tempering heat treatments in the range from 200 to 600°C demonstrate small effects on D. Additionally, the formation of residual austenite reduces Ds of the steels comprising Mn or Ni. Regarding the resistance to hydrogen embrittlement, the results of the slow strain rate test under hydrogen charging imply that the addition of Cr or Mo slightly decreases the fracture stresses of the as-quenched and subsequently tempered steels at the same strength levels as that of the base steel. The detrimental effects of Cr or Mo addition are attributed to embrittlement along the grain boundaries or the increase in the concentration of absorbed hydrogen due to the decrease in D. Mn addition significantly decreases the fracture stress primarily owing to the grain boundary embrittlement. In contrast, Ni addition slightly affects the susceptibilities of the steels to hydrogen embrittlement.