High Temperature Low Cycle Fatigue of 2 1/4Cr1Mo Steels: Role of Microstructure and Effect of Environment

The paper aims to analyse the high temperature low cycle fatigue behaviour of 2.25Cr1Mo steels that have been exposed in service at 565°C for about 110000h and 160000h. The volumic fraction of ferrite and bainite and the effect of the in-service long term ageing are analysed. Under strain controlled tests at 565°C in air, cyclic hardening is promoted by a large volumic fraction of ferrite. With a volumic fraction of 35% ferrite, a softening is favoured but the intensity and the rate are controlled by the state of bainite. Long term aged bainite promotes a softening of the steel. However, fatigue resistance did not appear to depend strongly on the microstructure because of the marked oxidation. Tests carried out in vacuum at a reduced pressure of 10-4Pa result in much longer fatigue lives than in air. At low strain amplitude, the fatigue resistance in vacuum is increased with a 65% bainite volumic fraction.