Effect of molybdenum and cooling rate on microstructure and properties of medium carbon non-quenched and tempered steel

Continuous cooling transformation for two kinds of medium carbon non-quenched and tempered steels with Mo and without Mo was studied by use of thermal dilatometer. Microstructure and mechanical properties of these two steels at different cooling rates were analyzed. The room temperature tensile strength and the impact energy of the tested steels were measured and the fractography was carried out under SEM. The results show that Mo can obviously improve the stability of undercooled austenite, postpone the transformation of proeutectoid ferrite and pearlite and promote the formation of acicular ferrite. For the No.2 steel with Mo, when the cooling rates range from 1 ℃/s to 3 ℃/s, the microstructure is composed of a lot of acicular ferrites. Furthermore, with addition of 0.089wt% Mo, the air cooled microstructure is refined and the toughness is thereby enhanced by about 45%.