Effect of undercooled austenite cooling rate on the low cycle fatigue properties of an austempering bainitic steel

Carbide-free bainitic microstructures with different morphologies were obtained by designing different cooling rates in medium-carbon bainitic steel, and the effect of undercooled austenite cooling rate on the low cycle fatigue (LCF) properties of austempering bainitic steels was systematically investigated. The results show that with the reduction of the cooling rate, the bainitic ferrite laths are coarsened, the content of retained austenite is reduced, and the proportion of filmy retained austenite is increased. The samples with a cooling rate of 30 degrees C/s at low strain amplitude possessed higher fatigue life, while the samples with a cooling rate of 0.3 degrees C/s at high strain amplitude exhibited higher fatigue life. This is because the phase transformation induced plasticity (TRIP) effect at low strain amplitude improves the samples' plastic deformation resistance. In contrast, the brittle martensite produced by the TRIP effect at high strain amplitude is more likely to provide a crack propagation path.