Study on the microstructure and mechanical properties of Aermet 100 steel at the tempering temperature around 482 °c

The microstructure and mechanical properties of ultrahigh strength Aermet 100 steel tempered at five different temperatures in the 472-492 °C range have been studied. The results show that the mechanical properties of Aermet 100 steel are sensitive to the tempering temperature; especially at the temperature of 482 °C, in which significant changes can be observed. At 482 °C, the strength of Aermet 100 steel sharply decreases by 70-100 MPa when the tempering temperature increases by only 5 °C. This is mainly due to the growth of M₂C carbide at relatively high tempering temperature that can greatly decrease its interface strengthening effect. In addition to strength, the plasticity and fracture toughness of Aermet 100 steel obviously increase at 482 °C when compared with lower tempering temperatures. The rising amplitudes of elongation and fracture toughness can reach 1.7% and 30 MPa m^1/2 respectively. This is attributed to the reverted austenite film precipitated at 482 °C. For one respect, the existence of austenite film between lath martensite can reduce the local stress concentration and the chance of microcrack formation in Aermet 100 steel during deformation, which is responsible for the obvious increase of plasticity. For another respect, the excellent softness, toughness and stability of reverted austenite prompt the advancing crack to divert, branch and blunt or appear "zigzag" route, which results in an increase in the fracture toughness. Aermet 100 steel can achieve the optimum combination of strength, plasticity and toughness at the tempering temperature of 482 °C, which has the yield strength of 1845 MPa, the elongation of 12.9% and the fracture toughness of 113 MPa m^1/2.