Creep and Fatigue at Elevated Temperatures. High Temperature Low Cycle Fatigue of 21/4 Cr1Mo Steels: Role of Microstructure and Effect of Environment.

Abstract

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.