Hydrogen pressure and crack tip stress effects on slow crack growth thresholds in an iron-based superalloy

Abstract

Summary IN903 slow crack growth thresholds decreased as hydrogen pressure increased and reached a lower limiting value of 30 MPa-m1/2 at a pressure of approximately 100 MPa. This transition to lower limiting threshold value behavior corresponded to a transition from slip band to intergranular failure. The slow crack growth threshold values were consistently lower than fracture toughness values and were associated with different fracture modes at similar hydrogen concentrations. The hydrogen concentrations were calculated from gas phase equilibrium expressions assuming no stress field interactions near the crack tip. Based on the assumption that the interaction between hydrogen and crack tip stress fields is as strong in this fcc superalloy as in bcc and hcp alloys, recalculation of the hydrogen concentrations at threshold showed that the hydrogen concentrations at threshold were significantly higher than calculated without consideration of stress effects. The recalculated crack tip hydrogen concentrations were also much higher than the concentrations in hydrogen charged samples. Most importantly, threshold and fracture toughness failure modes correlated with hydrogen concentration. From the relationship between void growth to failure at matrix carbides during slip band fracture and hydrogen concentration, it was shown that the near crack tip hydrogen concentration at threshold during slow crack growth in 20.7 MPa hydrogen gas was approximately 6000 appm. This hydrogen concentration corresponded to a partial molar volume of hydrogen equal to 2.0±0.3 cm3/mol. The surong interaction between hydrogen and near crack tip stress fields was responsible for the greater susceptibility of IN903 to slow crack growth in hydrogen gas than indicated by fracture toughness values. As a consequence, rapid tests such as fracture toughness and tensile tests for determination of hydrogeninduced crack growth susceptibility cannot be used to indicate crack growth susceptibility in hydrogen gas unless the effects of stress and rate of redistribution on near crack tip hydrogen concentration are considered.