Influence of Cathodic and Internal Trapped Hydrogen on Fatigue Crack Growth in a NiCrMoV Structural Steel

Fatigue crack propagation has been investigated in a NiCrMoV structural steel in air or in electrolytic hydrogen charging environments. The behavior of this steel containing internal trapped hydrogen absorbed during the steelmaking processes was also considered. Hydrogen, both internal and adsorbed by the environment, causes accelerated crack growth over the entire stress-intensity factor range. As the loading conditions are varied, two different damage mechanisms, triggered by hydrogen, are observed, and are separated by a transition zone where the fatigue crack growth rate is constant. The results of the fatigue tests and of a fractographic analysis suggest that the phenomenon is controlled by the stress distribution at the crack tip, and that a transition occurs when the cyclic plastic zone size at the crack tip is larger than the prior austenite grain size.