Structure, Phase Composition, and Mechanical Properties of a High Strength Steel with Transition Carbide η-Fe2C

The effects of quenching and tempering on the structure, phase composition, and mechanical properties of the Fe-0.34C high strength steel with 1.77 wt % Si are studied. The tempering at temperatures up to 500 degrees C barely has an effect on the structural characteristics of packet martensite formed during quenching. The precipitation of intermediate (transition) eta-carbide particles at tempering temperatures in the range of 200-400 degrees C leads to increases in the yield strength to 1490 MPa and in the impact toughness to 35 J/cm(2). The determined temperature of the ductile-brittle transition of the steel tempered at 200 degrees C is about -50 degrees C. A decrease in the impact toughness and a decrease in the proportion of ductile fracture with a decrease in the test temperature are accompanied by a transition from transgranular to intergranular fracture. The precipitation of cementite particles along the boundaries of the laths and blocks is observed after tempering at 500 degrees C. This leads to a decrease in the yield strength, while the impact toughness of the steel remains unchanged.