Environmental crack growth in 0.5CrMoV steel during isothermal high strain fatigue and temperature cycling

Abstract

Crack growth rates, measured during push-pull, high-strain fatigue of lightly-notched 0.5CrMoV turbine casing steel specimens at 550 °C, were determined in air, steam, and under vacuum. With a dwell at peak tension, propagation rates decrease in the order air, steam, vacuum, except at low strains where rates in steam are less than under vacuum. When the dwell occurs in the middle of the cycle at zero mean strain, i.e., at a starting stress less than at peak strain, crack growth rates in air and under vacuum are lower than in the first case because of a smaller contribution from oxidation and creep. When the temperature is varied in the range 200–550 °C so that maximum temperature occurs at peak compression (thermo-mechanical cycling), crack growth rates are less than isothermal values at 550 °C for the same plastic strain range. The reduced crack growth rates observed in steam are due to very stable oxide behaviour in the crack, and estimates of endurance from integrated growth rates thus show that the air data give a lower limit in lifetime prediction.