For steel with high carbon content and good hardenability, low-level quenching medium (or quenching solution concentration) should be selected. On the premise of no quenching crack, the deeper depth of hardened layer can be obtained by using higher level quenchant (or quenchant concentration). Among the suitable grades, the varieties with stable performance, long service life, easy management and low price should be selected. Determine the applicable reference level range of each steel grade. For example, carbon structure is 70-110; low alloy carbon steel is 50-80; medium alloy steel with low carbon content is 30-70; medium carbon medium alloy steel is 10-30