Quenching Methods

Ten common quenching methods are used in heat treatment processes: single-medium (water, oil, air) quenching, dual-medium quenching, martensite step quenching, martensite step quenching below the Ms point, bainite austempering, combined quenching, pre-cooling austempering, delayed cooling quenching, quenching and self-tempering, and jet quenching.

1.Single-medium (water, oil, air) quenching

A workpiece heated to the quenching temperature is quenched in a quenching medium and allowed to cool completely. This is the simplest quenching method and is commonly used for simple carbon steel and alloy steel workpieces. The quenching medium is selected based on the part's heat transfer coefficient, hardenability, size, and shape.

2.Dual-medium quenching

The workpiece, heated to the quenching temperature, is first cooled in a high-cooling quenching medium to near the Ms point. It is then transferred to a slower-cooling quenching medium and cooled to room temperature. This allows for different quenching temperature ranges and a relatively ideal cooling rate. This method is used for complex shapes or large workpieces made of high-carbon steel or alloy steel. Carbon tool steel is also commonly used with this method. Common cooling media include water-oil, water-nitrate, water-air, and oil-air. Water is generally used for rapid quenching, while oil or air is used for slow quenching. Air is less commonly used.

3.Martensite Step Quenching

After the steel is austenitized, it is then immersed in a liquid medium (a salt bath or alkaline bath) at a temperature slightly above or below the upper martensitic point of the steel. The temperature is maintained for an appropriate period of time until both the inner and outer layers of the steel have reached the medium temperature. The steel is then removed and air-cooled, allowing the supercooled austenite to slowly transform into martensite. This quenching process 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 molds.

4.Martensite Step Quenching Below the Ms Point

When the bath temperature is below the Ms of the workpiece steel but above the Mf, the workpiece cools faster in this bath. Even with larger sizes, the same results as step quenching can be achieved. This method is often used for larger workpieces made of low-hardenability steel.

5. Bainite Austempering Method

The workpiece is quenched in a bath at the lower bainite temperature of the steel and held there for 30-60 minutes to undergo a lower bainite transformation. The bainite austempering process consists of three main steps:
① austenitization;

② cooling after austenitization;

③ bainite isothermal treatment.

This method is commonly used for small-sized parts made of alloy steels and high-carbon steels, as well as ductile iron castings.

6. Combined Quenching Method

The workpiece is first rapidly cooled to below Ms to obtain a martensite volume fraction of 10%-30%. The workpiece is then isothermally treated in the lower bainite region to achieve a martensite and bainite structure in larger cross-sections. This method is commonly used for alloy tool steel workpieces.

7. Pre-Cold Austempering Method

Also known as rising temperature austempering, the part is first cooled in a bath at a lower temperature (greater than Ms), then transferred to a bath at a higher temperature to undergo an isothermal transformation of the austenite. Suitable for steel parts with poor hardenability or large workpieces that require austempering.

8.Delayed Cooling Quenching Method

The part is pre-cooled in air, hot water, or a salt bath to a temperature slightly above Ar3 or Ar1, then quenched in a single medium. This method is often used for parts with complex shapes, large thickness differences, and where minimal deformation is required.

9.Quenching and Self-Tempering Method

The entire workpiece is heated, but during quenching, only the portion to be hardened (usually the working area) is immersed in the quenching liquid to cool. As soon as the unimmersed portion disappears from the heat, it is immediately removed and cooled in air. The quenching and self-tempering method utilizes heat from the core that has not yet fully cooled to transfer to the surface, tempering it. It is commonly used for impact-bearing tools such as chisels, punches, and hammers.

10.Jet Quenching Method

A quenching method in which a jet of water is sprayed onto the workpiece. The water flow can be large or small, depending on the desired quenching depth. The spray quenching method does not form a steam film on the workpiece surface, which ensures that a deeper hardened layer can be obtained than the traditional water quenching method. It is mainly used for local surface quenching.