Keywords: quenching, cooling, classification, method, double liquid quenching, graded, isothermal quenching,

quenching cooling classification method
　　quenching classification, which is most widely used in production practice, is divided according to different cooling methods. There are mainly single liquid quenching, double liquid quenching, graded quenching and isothermal quenching.

　　● single-liquid quenching is a quenching operation method in which austenitic chemical parts are immersed in a 1 quenching medium and cooled to room temperature. Single liquid quenching medium has water, brine, alkaline water, oil and special preparation of quenching agent, etc. In general, carbon steel quenching, alloy steel quenching oil.

The single-liquid quenching operation is simple, which is conducive to mechanization and automation. The disadvantage is that the cooling rate is limited by the cooling characteristics of the medium and affects the quality of quenching. Single liquid quenching for carbon steel is only applicable to the shape of a relatively simple workpiece.

　　● double-liquid quenching is to immerse austenitic chemical parts into a 1 medium with strong cooling capacity, take out the steel parts before reaching the temperature of the quenching medium, and immediately immerse them into another 1 medium with weak cooling capacity for cooling, such as water before oil, water before air, etc. The two-liquid quenching reduces the tendency of deformation and cracking, and the operation is not easy to master, which has certain limitations in application.

　　● martensite grading quenching is a quenching process in which austenitic chemical parts are first immersed in a liquid medium (salt bath or alkali bath) with a slightly higher temperature or slightly lower than the martensite point of steel for an appropriate time. after the inner and outer layers of the steel parts reach the medium temperature, they are taken out for air cooling to obtain martensite structure, also called grading quenching.
　　graded quenching can effectively reduce phase transformation stress and thermal stress, and reduce quenching deformation and cracking tendency due to air cooling after staying at graded temperature until the internal and external temperatures of the workpiece are consistent. Graded quenching is suitable for alloy steel and high alloy steel workpieces with high deformation requirements, and can also be used for carbon steel workpieces with small cross-sectional dimensions and complex shapes.

　　● bainite isothermal quenching is a quenching process in which steel parts are austenitized, rapidly cooled to the bainite transformation temperature range (260~400 ℃) and isothermal maintenance, so that austenite is transformed into bainite, sometimes also called isothermal quenching. The general holding time is 30~60min.

　　● compound quenching quench the workpiece to below Ms to obtain 10% ~ 20% martensite, and then isothermal in the lower bainite temperature zone. This cooling method allows the larger cross-section of the workpiece to obtain tissue M + B tissue. The martensite formed during the pre-quenching promotes the bainite transformation, which in turn tempers the martensite when isothermally. Composite quenching is used for alloy tool steel workpieces, which can avoid the first 1 of tempering brittleness and reduce the amount of residual austenite, that is, the tendency of deformation and cracking.

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