Effect of hydrogen atmosphere on crack initiation in heat-treated 3·5Ni–Cr–Mo–V steel

Abstract

Slow-strain-rate testing of both smooth and side-notched compact tension specimens of 3·5Ni–Cr–Mo–V steel, which had been precracked in fatigue after quenching and tempering, revealed a much lower stress-intensity factor for crack initiation in a gaseous hydrogen environment than was required in vacuum or in air. Although the mode of brittle failure depends on the temperature that has been employed for tempering, as does the KIc in vacuum, the stress-intensity factor for crack initiation shows very little change. Cracks initiated in a hydrogen environment are readily arrested if the hydrogen is removed, but propagation recommences when hydrogen is re-introduced at quite low pressures. The delay ofa few minutes observed before re-initiation at low hydrogen pressures probably has the same origin as the pressure dependence of both crack initiation and fracture mode that is observed at low hydrogen pressures, even when employing extremely slow strain rates.