What are the common quenching methods and what are the principles for selecting different quenching methods?

Single liquid quenching - a process of cooling to the bottom in a quenching medium, where the stress and thermal stress of the structure are relatively high and the quenching deformation is large. 

Double liquid quenching - Objective: To rapidly cool between 650 ℃ and Ms to achieve V>Vc, and slowly cool below Ms to reduce tissue stress. Carbon steel: water first, oil later. Alloy steel: oil before air, 

Graded quenching - the process of taking out the workpiece and holding it at a certain temperature to make the temperature inside and outside the workpiece consistent, and then air cooling. Graded quenching is a process where the M-phase changes and the internal stress is small during air cooling. 

Isothermal quenching refers to the process of isothermal transformation in the bainite temperature range, resulting in a decrease in internal stress and minimal deformation. The principle of selecting quenching methods should consider both meeting performance requirements and minimizing quenching stress to avoid quenching deformation and cracking,