Abstract: Effectiveness of adding Mo, Mo+B and Mo+Ni to improve the hardenability of a medium carbon Cr-Mn-Si steel was studied by means of traditional end quenching test method.The results show that the granular bainite phase transformation tendency of the medium carbon Cr-Mn-Si steels with adding Mo, Mo+B and Mo+Ni respectively is large, and the air-cooled end of Jominy end-quenching specimens forms granular bainite. Therefore, the quantitative hardenability information such as ideal critical diameter (D_I) cannot be obtained from the end quenching curve in accordance with SAE J406 standard.However, the correspondence between the hardness of the end quenching curve and the microstructure proves that the addition of Mo, Mo+B and Mo+Ni in turn reduces the tendency of granular bainite transformation, increases martensite forming ability and improves hardenability.Although adding Mo+B can inhibit granular bainite transformation and improve hardenability effectively, it is susceptible to the influence of smelting residual Al and N in steel. While adding Mo+Ni has more advantages in improving hardenability, and it is not affected by smelting residual Al and N in steel. Therefore, adding Mo+Ni becomes the optimal alloying direction to improve the hardenability of medium carbon Cr-Mn-Si steel.

Key words: medium carbon Cr-Mn-Si steel, end quenching, hardness, granular bainite, hardenability