AbstractThe crack propagation behaviour of secondary hardened alloy steels having various Ni contents and a 18%Ni maraging steel was studied using modified compact tension specimens under 98–784 kPa hydrogen gas pressure pH2. The effect of Ni content and retained austenite was examined by comparing oil quenched specimens with those cooled to liquid nitrogen temperature. It was estimated from the effect of pH2on the crack propagation rate da/dt that the permeation of hydrogen from the crack tip surface decreased with increasing Ni content in the order 6 or 9, 13, then 18%Ni. It was also estimated that the hydrogen induced embrittlement of grain boundaries decreased with increasing Ni content in the order 6, 9, then 13%Ni and that the embrittlement was greater for the steel containing 18%Ni than for the steel containing 13%Ni. The effect of retained austenite was expected to suppress not the permeation of hydrogen, but the embrittlement of grain boundaries.MST/757