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Quenching method

There are ten common quenching methods in heat treatment process, which are single medium (water, oil, air) quenching, double medium quenching, martensite stepped quenching, martensite stepped quenching below Ms point, bainite isothermal quenching, composite quenching, pre-cooling isothermal quenching, delayed cooling quenching, quenching self-tempering, spray quenching and so on.


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


2. Dual Dielectric Quenching
Dual-medium quenching: The workpiece heated to quenching temperature is first cooled to near Ms point in a quenching medium with strong cooling capacity, and then cooled to room temperature in a slow-cooling quenching medium to achieve different quenching cooling temperature ranges, with a relatively ideal quenching cooling rate. For large workpieces made of complex shape or high carbon steel or alloy steel, this method is also commonly used in carbon tool steels. Commonly used cooling media are water-oil, water-nitrate, water-air, oil-air. Generally, water is used as quick-cooling quenching medium, oil or air is used as slow-cooling quenching medium, and less air is used.


3. MARTENSITE STAGE QUENCHING
Martensite stepwise quenching: Austenitizing of steel, then immersing in liquid medium (salt bath or alkali bath) with a slightly higher temperature or a slightly lower upper martensite point of steel, keeping appropriate time, and taking out air cooling after the inner and outer layers of steel reach medium temperature, supercooled austenite slowly transforms into martensite quenching process. Generally used for small workpieces with complex shape and strict deformation requirements, high-speed steel and high-alloy steel tools and dies are also commonly quenched by this method.


4. MARTENSITE STAGE QUENCHING UNDER MS POINT
Martensite stepwise quenching below Ms point: When bath temperature is lower than MS of workpiece steel and higher than Mf, workpiece cooling in bath is faster, and the same result can be obtained when the size is larger. Usually used for large size low hardenability steel workpiece.


5. Isothermal Quenching of Bainite
Bainite isothermal quenching method: The workpiece is quenched into the bath at the lower bainite temperature of the steel, which causes the transformation of the lower bainite. It is generally kept in the bath for 30 to 60 minutes. The bainite isothermal quenching process consists of three main steps: austenitizing treatment, cooling treatment after austenitizing, bainite isothermal treatment, commonly used in alloy steel, high carbon steel small size parts and ductile iron castings.


6. Compound Quenching Method
Compound quenching method: First, the workpiece is quenched below Ms to obtain martensite with a volume fraction of 10%~30%, and then isothermal in the lower bainite region to obtain martensite and bainite structure for larger cross-section workpiece, which is often used in alloy tool steel workpiece.


7. Isothermal quenching with Precooling
Precooling isothermal quenching method: also known as heating isothermal quenching, parts are cooled in a bath with lower temperature (greater than Ms), and then transferred into a bath with higher temperature to make Austenite Isothermal transformation. It is suitable for steel parts with poor hardenability or those with large size which must be isothermally quenched.


8. Delayed Cooling and Quenching Method
Delayed cooling quenching method: parts are pre-cooled to a temperature slightly higher than Ar3 or Ar1 in air, hot water and salt bath, and then quenched by single medium. Usually used for parts with complex shape and large thickness difference and small deformation.


9. Quenching Self-tempering Method
Quenching and self-tempering: All the processed workpieces are heated, but only the hardened parts (usually working parts) are required to be immersed in the quenching fluid to cool during quenching, and the quenching process which is cooled in air is taken out immediately when the fire color of the immersed parts disappears. Quenching and self-tempering process uses the heat that is not fully cooled in the center to transfer to the surface, so that the surface tempers. Commonly used to withstand impact tools such as chips, punches, hammers, etc.


10. Jet Quenching Method
Jet 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 does not form a vapor film on the surface of the workpiece, which ensures that the hardened layer is deeper than that quenched in the past through water. Mainly used for local surface quenching.

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