Ten Common Quenching Methods

There are ten common quenching methods in heat treatment process, which are single medium (water, oil, air) quenching; Two medium quenching; Martensite step quenching; Martensite graded quenching method below Ms point; Bainite isothermal quenching method; Compound quenching method; Precooling and isothermal quenching method; Delayed cooling quenching method; Quenching and self tempering method; Spray quenching method, etc.

1、 Single medium (water, oil, air) quenching

Single medium (water, oil, air) quenching: Quench the workpiece that has been heated to the quenching temperature into a quenching medium to completely cool it. This is the simplest method of quenching and is commonly used for carbon steel and alloy steel workpieces with simple shapes. The quenching medium shall be selected according to the heat transfer coefficient, hardenability, size and shape of the part.

2、 Dual medium quenching

Two medium quenching: the workpiece heated to the quenching temperature is cooled to the point close to Ms in the quenching medium with strong cooling capacity, and then transferred to the slow cooling quenching medium to cool to room temperature, so as to reach different quenching cooling temperature ranges and have a relatively ideal quenching cooling rate. It is also used for large workpieces made of complex shapes or high carbon steel and alloy steel. Common cooling media include water oil, water nitrate, water air and oil air. Generally, water is used as fast cooling quenching medium, oil or air is used as slow cooling quenching medium, and air is less used.

3、 Martensite step hardening

Martensite staged quenching: the steel is austenitized, and then immersed into the liquid medium (salt bath or alkali bath) with a temperature slightly higher or lower than the upper martensite point of the steel, and kept 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, and the undercooled austenite is slowly transformed into martensite. It is generally used for small workpieces with complex shapes and strict deformation requirements. It is also commonly used for quenching high-speed steel and high alloy steel tools and dies.

4、 Martensite step quenching method below Ms point

Martensite graded quenching method below Ms point: when the bath temperature is lower than Ms of the steel used for the workpiece but higher than Mf, the workpiece cools faster in the bath, and the same results can still be obtained with graded quenching when the size is larger. It is commonly used for workpieces of large size and low hardenability steel.

5、 Bainite isothermal quenching

Bainite isothermal quenching method: the workpiece is quenched into a bath at the lower bainite temperature of the steel to isothermal, so that it can undergo lower bainite transformation. Generally, it is kept in the bath for 30~60min. There are three main steps in bainite isothermal quenching process: ① austenitizing; ② Cooling treatment after austenitizing; ③ Isothermal treatment of bainite; Commonly used in alloy steel, high carbon steel small size parts and ductile iron castings.

6、 Compound quenching method

Compound quenching method: firstly, the workpiece is quenched to less than Ms to obtain 10%~30% martensite by volume fraction, and then isothermal in the lower bainite zone to obtain martensite and bainite structures for workpieces with large cross section, which is commonly used for alloy tool steel workpieces.

7、 Precooling isothermal quenching

Precooling isothermal quenching method: also known as heating isothermal quenching, the parts are cooled in the bath with lower temperature (more than Ms), and then transferred to the bath with higher temperature to make the austenite undergo isothermal transformation. It is applicable to steel parts with poor hardenability or workpieces with large size that must be isothermal quenched.

8、 Delayed cooling quenching

Delayed cooling quenching method: the parts shall be precooled in air, hot water and salt bath to a temperature slightly higher than Ar3 or Ar1, and then subject to single medium quenching. It is often used for parts with complex shape and great thickness difference and small deformation.

9、 Quenching and self tempering method

Quenching and self tempering method: the quenching process that heats all the workpiece to be treated, but only immerses the part to be hardened (usually the working part) in the quenching liquid for cooling during quenching, and takes out the quenching process that is cooled in air immediately when the fire color of the part not immersed disappears. The quenching and self tempering method uses the heat not fully penetrated from the core to the surface to temper the surface. Tools commonly used to withstand impact, such as chisels, punches, hammers, etc.

10、 Jet hardening

Spray quenching method: the quenching method of spraying water to the workpiece. The water flow can be large or small, depending on the required quenching depth. The spray quenching method will not form a vapor film on the surface of the workpiece, so as to ensure that the hardened layer is deeper than that quenched in water. It is mainly used for local surface quenching.