Low cycle fatigue of Eurofer 97

Abstract

We have investigated the low cycle fatigue and creep-fatigue properties of Eurofer 97 and observed the associated microstructural changes. The as received structure is composed of equiaxed subgrains and a few martensite laths with a high dislocation density. Fatigue tests have been carried out in air or in high vacuum, from room temperature to 550 °C, under total strain control. It has been found that the influence of the test temperature on the fatigue endurance is not significant. The softening behaviour as a function of the imposed strain amplitude and temperature has been analysed in detail. The softening rate is independent of the imposed strain but strongly enhanced at the highest test temperature. Creep-fatigue tests were run, imposing a 500 s dwell at the maximum tensile strain of the loading cycle, at a total strain range of 0.5%, 0.8% and 1.4%, and at 150, 300 and 550 °C. The influence of the hold time is important only at the highest test temperature, under low applied strains. It was found that at the beginning of life, at the highest temperature, the softening rate with hold times is much stronger as compared to the softening rate without hold times. The amount of stress relaxed during the dwell is independent of the applied strain, at the end of life. The effect of fatigue with and without hold times up to medium temperatures on the microstructure was to lower the dislocation density and to decompose the laths and large grains into a homogeneous structure of submicron grains. At the highest test temperature, an increase of the subgrain size and carbide coarsening were observed.