There are ten quenching methods in the heat treatment process, which are:
- single-medium (water, oil, air) quenching;
- interrupted quenching;
- martempering below MS point;
- isothermal quenching of bainite;
- compound quenching;
- precooled isothermal quenching;
- delayed cooling quenching;
- quenching self tempering;
- jet quenching;
In this process, the workpiece heated to the quenching temperature is quenched into a quenching medium to make it cool completely.
This is the simplest quenching method, which is commonly used in the shape of simple carbon steel and alloy steel workpiece.
The quenching medium is selected according to the heat transfer coefficient, hardenability, size and shape of the parts.
Fig. 1 Single medium (water, oil, air) quenching
2. Interrupted quenching
The workpiece heated to the quenching temperature is cooled to the point close to MS in the quenching medium with strong cooling capacity.
Then it is cooled to room temperature in slow cooling quenching medium to achieve different quenching temperature range and 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.
The common cooling media are water-oil, water-nitrate, water-air and oil-air.
Generally, water is used as quick cooling quenching medium, oil or air is used as slow cooling quenching medium, and air is less used.
The steel is austenitized, and then immersed in the liquid medium (salt bath or alkali bath) with the temperature slightly higher or lower than the upper martensite point of the steel for a proper time.
After the inner and outer layers of the steel reach the medium temperature, the steel is taken out for air cooling, and the undercooled austenite slowly transforms into martensite.
It is 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. Graded martensitic quenching method below Ms point
When the bath temperature is lower than MS and higher than MF, the workpiece cools faster in the bath, and the same results can be obtained when the bath size is larger.
It is often used in large size low hardenability steel workpiece.
5. Isothermal quenching of bainite
The workpiece is quenched into the bath with lower bainite temperature for isothermal treatment, so that the lower bainite transformation occurs. Generally, the workpiece is kept in the bath for 30 ~ 60min.
Isothermal quenching of bainite process consists of three steps:
① Austenitizing treatment;
② Cooling treatment after austenitizing;
③ Bainite austempering;
It is commonly used in alloy steel, high carbon steel small size parts and ductile iron castings.
6. Compound quenching
Martensite with the volume fraction of 10% ~ 30% is obtained by quenching the workpiece below MS, and then the martensite and bainite are obtained by isothermal treatment in the lower bainite region, which is commonly used in alloy tool steel workpiece.
7. Precooled isothermal quenching
It is also called step-up austempering.
First, the parts are cooled in the bath with lower temperature (higher than MS), and then transferred to the bath with higher temperature to make austenite isothermal transformation.
It is suitable for steel parts with poor hardenability or large size and workpieces that must be austempered.
8. Pelayed cooling quenching
The parts are pre-cooled to a temperature slightly higher than Ar3 or Ar1 in air, hot water and salt bath, and then single medium quenching is carried out.
It is often used for parts with complex shape, wide thickness difference and small deformation.
9. Quenching self tempering
It is a quenching process in which all the workpieces to be treated are heated, but only the parts to be hardened (usually the working parts) are immersed in the quenching liquid for cooling during quenching.
When the fire color of the part not immersed disappears, the quenching process is immediately taken out for cooling in the air.
The quenching and self tempering method transfers the heat from the center to the surface to temper the surface.
It is often used to bear the impact of tools such as chisels, punches, hammers and so on.
10. Jet quenching
The quenching method of spraying water to the workpiece, the water flow can be large or small, depending on the required quenching depth.
The steam film will not be formed on the surface of the workpiece by jet quenching, which can ensure a deeper hardened layer than that quenched in normal water.
It is mainly used for local surface quenching.
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