Fatigue behavior of 1500 MPa bainite/martensite duplex-phase high strength steel

The fatigue properties of C–Si–Mn–Cr low alloy steel can be improved at the tensile strength of 1500 MPa or higher level by the following heat treatment: austenitizing at 900 °C for 20 min followed by air cooling and then tempering at 280 and 370 °C for 2 h. The steel had a duplex-phase of carbide-free bainite and martensite containing retained austenite of 8 vol% and higher (designated as CFB/M steel). The results obtained for the CFB/M steel were compared with those of the fully martensitic steel which was oil-quenched and then tempered at 280 and 370 °C for 2 h after austenitization at 900 °C for 20 min (designated as FM steel). The fatigue strength was determined by a method of axial fatigue test, and the fatigue crack propagation was studied by using compact-tension specimens. Compared to the FM steel, the CFB/M steel increased the fatigue strength and fatigue crack threshold (ΔKth), and then lowered crack propagation rate. In particular, the CFB/M steel had a better combination of strength, toughness and fatigue properties when tempered at 370 °C for 2 h.