Enhanced susceptibility to delayed fracture in pre-fatigued martensitic steel

Interaction between fatigue damage and hydrogen is the concern of the present study. The susceptibility of a high-strength martensitic steel to delayed fracture has been examined using as-heat-treated and pre-fatigued specimens. The pre-fatigued specimens showed a tendency to fail earlier, but annealing the pre-fatigued specimens at 200 °C recovered nearly all of the delayed fracture life. Production of point defects during fatigue was detected by means of hydrogen thermal desorption analysis (TDA), using hydrogen as a probe of defects. Hydrogen absorption capacity increased in fatigued specimens, but it was reduced to the level of the as-heat-treated specimens when fatigued specimens were annealed at 200 °C, implying that increased hydrogen-trapping defects, presumably vacancies, were produced during fatigue. Hydrogen TDA peak profiles showed alterations that imply agglomeration of vacancies associated with an increase in fatigue cycles. The involvement of vacancies created during fatigue in the enhanced delayed fracture is consistent with a model that proposes strain-induced vacancies and their agglomeration are the primary mechanism of hydrogen-related failure.