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

Abstract 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.