Summary of ten commonly used quenching methods There are ten commonly used quenching methods in heat treatment processes, namely single medium (water, oil, air) quenching; double medium quenching; martensite graded quenching; martensite graded quenching below Ms point; bainite isothermal quenching; composite quenching; pre-cooling isothermal quenching; delayed cooling quenching; quenching self-tempering; jet quenching, 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 quenching method, commonly used for carbon steel and alloy steel workpieces with simple shapes. The quenching medium is selected according to the heat transfer coefficient, hardenability, size, shape, etc. of the part.
2. Dual-medium quenching Dual-medium quenching: The workpiece heated to the quenching temperature is first cooled to a point close to the Ms point in a quenching medium with strong cooling capacity, and then transferred to a slow-cooling quenching medium to cool to room temperature, so as to achieve different quenching cooling temperature ranges and have a relatively ideal quenching cooling rate. It is used for large workpieces with complex shapes or made of high-carbon steel and alloy steel. Carbon tool steel also uses this method. Common cooling media include water-oil, water-nitrate, water-air, and oil-air. Generally, water is used as a fast-cooling quenching medium, and oil or air is used as a slow-cooling quenching medium. Air is rarely used. 3. Martensite graded quenching Martensite graded quenching: The steel is austenitized, and then immersed in a liquid medium (salt bath or alkali bath) with a temperature slightly higher or slightly lower than the upper martensitic point of the steel. It is kept for an appropriate time. After the inner and outer layers of the steel reach the medium temperature, it is taken out for air cooling. The supercooled austenite is slowly transformed into martensite. Quenching process. Generally used for small workpieces with complex shapes and strict deformation requirements. High-speed steel and high-alloy steel tools and molds are also often quenched by this method.
4. Martensite graded quenching method below the Ms point Martensite graded quenching method below the Ms point: When the bath temperature is lower than the Ms of the workpiece steel but higher than Mf, the workpiece cools faster in the bath, and the same results as graded quenching can be obtained when the size is larger. Commonly used for large-sized low-hardenability steel workpieces. 5. Bainite isothermal quenching method Bainite isothermal quenching method: Quench the workpiece into a bath at the lower bainite temperature of the steel to make it undergo a lower bainite transformation, and generally keep it warm in the bath for 30~60min. There are three main steps in the bainite isothermal quenching process: ① Austenitization treatment; ② Cooling treatment after austenitization; ③ Bainite isothermal treatment; Commonly used for small-sized parts of alloy steel, high-carbon steel and ductile iron parts.
6. Composite quenching method Composite quenching method: First, the workpiece is rapidly cooled to below Ms to obtain a martensite with a volume fraction of 10%~30%, and then isothermally quenched in the lower bainite zone to obtain martensite and bainite structures in the larger cross-section workpiece, which is often used for alloy tool steel workpieces.
7. Precooling isothermal quenching method Precooling isothermal quenching method: also known as heating isothermal quenching, the parts are first cooled in a bath with a lower temperature (greater than Ms), and then transferred to a bath with a higher temperature to make the austenite isothermally transformed. It is suitable for steel parts with poor hardenability or workpieces with large size that must be isothermally quenched. 8. Delayed cooling quenching method Delayed cooling quenching method: The parts are first precooled in air, hot water, and salt baths to a temperature slightly higher than Ar3 or Ar1, and then quenched in a single medium. It is often used for parts with complex shapes, large thickness differences in various parts, and small deformation requirements.
9. Quenching and self-tempering method Quenching and self-tempering method: All the workpieces to be processed are heated, but during quenching, only the part that needs to be hardened (usually the working part) is immersed in the quenching liquid for cooling. When the fire color of the unimmersed part disappears, it is immediately taken out and cooled in the air. The quenching and self-tempering method uses the heat that has not been completely cooled in the core to transfer to the surface to temper the surface. Tools commonly used to withstand impacts such as chisels, punches, hammers, etc. 10. Jet quenching method Jet quenching method: A quenching method that sprays water flow on the workpiece. The water flow can be large or small, depending on the required quenching depth. The jet quenching method will not form a steam film on the surface of the workpiece, so that a deeper hardened layer can be obtained than the previous quenching in water. Mainly used for local surface quenching.
