Effects of retained austenite on tension-compression fatigue properties of cold work tool steel

Effects of retained austenite (γ_R) on tension-compression fatigue properties of cold work tool steel have been investigated using modified SKD11 (Mod.SKD11), which contains 0.8C-8Cr-2Mo-0.5V Tension-compression fatigue specimens were tempered at 180°C, 470°C and 540°C after quenching from 1030°C. These conditions vary the amount and stability of γ_R with equal hardness of 60HRC. The amount of γ_R in these specimens tempered below 500°C was 15 vol%. The γ_R stability to stress induced transformation in specimen tempered at 470°C can be considered to increase than that of 180°C, because C content in γ_R for the specimen tempered at 470°C is higher than that of the specimen tempered at 180°C. Above 500°C, the γ_R in specimens was decomposed and secondary hardening occurred. Fatigue tests were done under fully reversed stress amplitude of 1000MPa. Each result was plotted on Weibull probability sheet. Fatigue lives of the specimens tempered at 470°C were longest, and decreased in the sequence 180°C→540°C. Fisheyes were observed on the fracture surface of each specimen. The diameter of fisheye for the specimens tempered at 180°C and 470°C was larger than that for the specimen tempered at 540°C. It seems that γ_R contributes to the increase of the resistance for crack propagation. As a result of measurement of γ_R volume fraction after fatigue test, it is found that γ_R in specimen tempered at 180°C is unstable and the volume fraction of γ_R decreased approximately 5%, while volume fraction of γ_R in specimen tempered at 470°C was unchanged. Thus, it is considered that the amount and stability of γ_R contribute to the increase of fatigue lives.