Summary of ten commonly used quenching methods
There are ten commonly used quenching methods in the heat treatment process, namely, single medium (water, oil, air) quenching; dual medium quenching; martensite graded quenching; martensite graded quenching below the Ms point; bainite isothermal Quenching method; compound quenching method; pre-cooling isothermal quenching method; delayed cooling quenching method; quenching self-tempering method; spray quenching method, etc.
1. Single medium (water, oil, air) quenching
Single medium (water, oil, air) quenching: Quenching the workpiece that has been heated to the quenching temperature into a quenching medium to make it completely cooled. This is the simplest quenching method and is often used for carbon steel and alloy steel workpieces with simple shapes. The quenching medium is selected according to the heat transfer coefficient of the part, hardenability, size, shape, etc.
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2. Dual-medium quenching
Dual-medium quenching: The workpiece heated to the quenching temperature is first cooled in a quenching medium with strong cooling capacity to close to the Ms point, and then transferred to a slow-cooling quenching medium to cool to room temperature to achieve different quenching cooling temperature ranges, and has Ideal quenching cooling rate. It is used for complex shape parts or large workpieces made of high carbon steel and alloy steel, and carbon tool steel also adopts this method. Commonly used cooling media are water-oil, water-nitrate, water-air, oil-air, water is generally used as a quick cooling quenching medium, oil or air is used as a slow cooling quenching medium, and air is rarely used.
3. Martensite graded quenching
Martensitic 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, and kept for an appropriate time to treat the inside and outside of the steel. After the layer reaches the temperature of the medium, it is taken out and air-cooled, and the supercooled austenite is slowly transformed into a quenching process of martensite. It is 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
Martensitic graded quenching method below the Ms point: When the bath temperature is lower than the Ms of the workpiece steel and higher than Mf, the workpiece cools faster in the bath, and the same result as graded quenching can still be obtained when the size is larger. Commonly used for larger size low hardenability steel workpieces.
Five, bainite isothermal quenching method
Bainite isothermal quenching method: Quench the workpiece into the bath at the lower bainite temperature of the steel and isothermally, so that the lower bainite transformation occurs, and generally keep it in the bath for 30~60min. There are three main steps in the bainite austempering process: ① austenitizing treatment; ② cooling treatment after austenitizing; ③ bainite isothermal treatment; commonly used in small-sized parts of alloy steel, high carbon steel and ductile iron castings.
6. Composite quenching method
Composite quenching method: first rapidly cool the workpiece to below Ms to obtain martensite with a volume fraction of 10%~30%, and then isothermal in the lower bainite zone to obtain martensite and bainite structures for larger cross-section workpieces. Alloy tool steel workpieces.
Seven, pre-cooling isothermal quenching method
Pre-cooling isothermal quenching method: also known as elevated temperature 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 undergo isothermal transformation. It is suitable for steel parts with poor hardenability or workpieces with large dimensions that must be austempered.
8. Delayed cooling and quenching method
Delayed cooling and quenching method: The parts are pre-cooled in air, hot water, and salt bath to a temperature slightly higher than Ar3 or Ar1, and then single-medium quenching. It is often used for parts with complex shapes, different thicknesses and small deformation requirements.
Nine, quenching self-tempering method
Quenching and self-tempering method: heat all the workpiece to be processed, but only immerse the part that needs to be hardened (usually the working part) into the quenching liquid to cool when quenching, and take it out immediately in the air when the fire color of the unimmersed part disappears. Medium cooling quenching process. The quenching self-tempering method uses the heat that is not completely cooled through the core to transfer to the surface to temper the surface. Tools commonly used to withstand impacts such as chisels, punches, hammers, etc.
10. Spray quenching method
Spray quenching method: A quenching method in which water is sprayed onto the workpiece. The water flow can be large or small, depending on the required quenching depth. The spray quenching method does not form a vapor film on the surface of the workpiece, so that it can ensure a deeper hardened layer than the previous quenching in water. Mainly used for partial surface quenching
