Effect of grain size on the susceptibility of martensitic steel to hydrogen-related failure

The effect of grain size on the susceptibility of martensitic steels to hydrogen-related failure has been examined with respect to the role of the hydrogen concentration and lattice defects acting as hydrogen trap sites. Plastic straining increased the hydrogen absorption capacity, but annealing at 250 degrees C after straining eliminated the increase, implying the point-defect-like nature of the increased number of trapping sites for hydrogen. Refinement of the prior austenite grain size, by means of repeated induction heating and quenching, increased the hydrogen absorption capacity in as-heat-treated samples as a consequence of the increased boundary area. Grain refinement reduced the susceptibility, as evaluated by means of a slow strain-rate tensile test of hydrogen-precharged specimens. This reduction of susceptibility is analysed to be associated with a smaller strain-induced increase in hydrogen absorption capacity, i.e. in the defect density. Fractographic features showed a decrease in ductile crack growth resistance in the presence of hydrogen, being less pronounced with grain refinement. The present results support the notion that the susceptibility of steels to hydrogen-related failure originates in the response of microstructures to plastic straining, primarily through the creation of vacancies.