Low cycle fatigue behaviour of a Cr–Mo–V matrix-type high-speed steel used for cold forging

Abstract

The low cycle fatigue (LCF) behaviour of a Cr–Mo–V matrix-type high-speed steel (matrix-type HSS), which was used as cold forging tool material, was investigated. Fatigue tests were conducted under strain-controlled cyclic loading conditions with different axial strain amplitude levels at room temperature. The tests indicated that hysteresis loops formed when the total strain amplitude was 1.5%, 1.0% and 0.75%. At total strain amplitudes that were less than 0.7%, there was no cyclic plastic deformation or hysteresis loop observed. The material fatigue behaviour was analysed specifically at the above three strain levels to investigate the cyclic plastic deformation of the material because the low cycle fatigue behaviour was of principal interest in this study. At all selected total strain amplitudes, the cyclic stress responses showed cyclic softening, and reached a “quasi-stabilised” softening state before the material finally ruptured from fatigue. The cyclic strain–stress relationship and the strain–life relationship were obtained through the test results, and related material parameters were calculated. Fractographic and metallographic analyses were also conducted in this study. The metallographic structure of the steel before and after heat treatment was examined. Two types of carbide phases were found in the steel. The fatigue damage process through the crack initiation and propagation was explained by observing the features of the fractures.