Effect of Alloying Elements and Heating Parameters on the Hardness of Quenched and Tempered Steel 42CrMo after Induction Hardening

The effect of alloying elements, heating rate and quenching temperature on the hardness of steel 42CrMo after induction hardening were investigated by thermal simulation tests, hardness tests, optical microscopy (OM) and scanning electronic microscopy (SEM). The steel No.2 was obtained by adding 1.49% Ni and reducing 0.32% Cr in steel No.1. Experimental results show that the hardness of steel No.1 decreases while that of steel No.2 slightly increases under the quenched temperature of 900 °C when the heating rate increases from 100 °C/s to 300 °C/s. When the heating rate is 300 °C/s, the hardness of steel No.1 increases sharply with the elevating of the quenched temperature. However, the hardness of steel No.2 increases gently with the quenched temperature increasing below 900 °C but keeps constant above 900 °C. It is interesting that the hardness of steel No.2 is obviously higher than that of steel No.1 under the same heating process. This phenomenon can be attributed to the following aspects: firstly, the Ac1 and Ac3 temperatures can largely decrease and the dissolution of carbide proceeds further; secondly, the addition of alloying element and the dissolution of carbide in the austenite can improve the hardenability of steel and thus increase the volume percentage of martensite in the quenched microstructure; in addition, the quenched microstructure of steel No.2 is finer than that of steel No.1 which undoubtedly helps the increase of hardness. In consideration the disadvantage of higher quenched temperature and heating rate, it is comprehensive to determine the quenched temperature as 900 °C and heating rate as 100 °C/s in order to increase the hardness of steel No.2.