Now take No.45 steel with φ12.5mm×60mm as an example, wC is 0.46%, heating temperature is 840℃, and the quenching results in different media are discussed.

(1) Water quenching
At the end of quenching at 840℃, the transformation of pearlite and ferrite is inhibited due to the rapid cooling rate (about 540℃/s), and the undercooled austenite transforms into martensite at 300℃. The properties of this structure are: Rm≥1100MPa, ReL≥720MPa, A≥7%, Z≥15%, hardness 52 - 60HRC.

(2) Oil quenching
After austenitizing at 840℃, it is quenched into N32 oil immediately. Due to the slow cooling rate (about 240℃/s), the undercooled austenite does not transform at the beginning. Until 450℃, part of the austenite transforms into bainite. When it is cooled to about 300℃, the decomposition of austenite stops. When it is cooled further, the rest of the undercooled austenite transforms into martensite. The microstructure at room temperature is less than 20% bainite + 80% martensite + a small amount of retained austenite.

(3) Air cooling
The microstructure of undercooled austenite is 30% ferrite + 70% pearlite at room temperature, and the hardness is 220 - 250HBW.