EFFECTS OF TEMPERATURE AND ENVIRONMENT ON FATIGUE CRACK-GROWTH MECHANISMS IN A 9-PERCENT CR 1-PERCENT-MO STEEL

The environmental contribution of laboratory air to fatigue crack growth in a 9% Cr 1% Mo steel has been assessed by a comparison of crack propagation rates in air and vacuum over a range of temperatures (25-625-degrees-C). In the Paris regime, growth rates in air are generally higher than those in vacuum, where there is little variation of da/dN with temperature. In contrast, the enhancing effect of the air atmosphere on crack growth rates is strongly temperature dependent. A variety of environment-assisted crack growth mechanisms are found to be operative at different temperature ranges, and evidence of these is provided by both an analysis of activation energies and a fractographic investigation. The situation is different at lower stress intensities, where the threshold stress intensity range falls dramatically with increasing temperature in vacuum. and near-threshold growth rates at 525-degrees-C are higher in vacuum than in air. This effect is attributed to the occurrence of severe oxide-induced closure in air at elevated temperatures, where the crack is blocked with oxide at low stress intensities, reducing the crack driving force to a level below the intrinsic material threshold.