Summary of ten common quenching methods

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Summary of Ten Common Quenching Methods: There are ten common quenching methods used in heat treatment processes, namely quenching in single media (water, oil, air)； double-medium quenching ； Martensitic graded quenching ； Martensitic step quenching below Ms point ； Bainite isothermal quenching method ； Composite quenching method ； Pre-cooling isothermal quenching method ； Delayed cooling quenching method ； Quenching and self-tempering method ； Jet quenching method, etc. I. Quenching in a single medium (water, oil, air): Quenching in a single medium (water, oil, air) involves immersing a workpiece that has been heated to the quenching temperature into a quenching medium to cool it completely. This is the simplest quenching method, commonly used for carbon steel and alloy steel workpieces with simple shapes. The quenching medium is selected based on factors such as the heat transfer coefficient of the part, its hardenability, size, and shape. II. Dual-medium quenching: In dual-medium quenching, the workpiece heated to the quenching temperature is first cooled in a quenching medium with high cooling capacity until it reaches a temperature close to the Ms point, and then transferred to a quenching medium with slower cooling rate to be cooled to room temperature. This approach allows for different quenching temperature ranges and yields a relatively ideal quenching cooling rate. This method is used for large workpieces with complex shapes or those made of high-carbon steel and alloy steel; carbon tool steel is also often processed in this way. Common cooling media include water-oil, water-nitrate, water-air, and oil-air. Water is generally used as a medium for rapid cooling quenching, while oil or air is used for slow cooling quenching; air is less frequently used. III. Martensitic Classification quenching: Martensitic graded quenching involves austenitizing the steel, then immersing it in a liquid medium (salt bath or alkali bath) at a temperature slightly higher or lower than the steel’s upper martensite transformation temperature. The part is held in this medium for an appropriate period of time; once both the inner and outer layers of the steel have reached the temperature of the medium, it is removed and cooled air-dry. This is a quenching process in which the supercooled austenite gradually transforms into martensite. It is generally used for small workpieces with complex shapes and strict deformation requirements; high-speed steel and high-alloy steel tools and dies are also commonly quenched using this method. IV. Martensitic staged quenching below the Ms point: When the temperature of the quenching bath is below the Ms point but above the Mf point of the steel used for the workpiece, the workpiece cools rapidly in this bath; even for larger workpieces, the same results as those obtained through staged quenching can be achieved. It is commonly used for large-sized low-hardenability steel workpieces. V. Isothermal quenching of bainite: In this method, the workpiece is placed in a bath at a temperature suitable for the formation of lower bainite, where it undergoes such transformation; generally, it is held in the bath for 30 to 60 minutes. The isothermal quenching process for bainite consists of three main steps: ① Austenitization ； ②Cooling treatment after austenitization ； ③Bainite isothermal treatment ； It is commonly used for small-sized parts made of alloy steel and high-carbon steel, as well as ductile iron parts. VI. Composite Quenching Method: In this method, the workpiece is first rapidly cooled below the Ms temperature to obtain a martensite content of 10%~30%; thereafter, it is held at a temperature in the lower bainite region, so that workpieces with larger cross-sections develop both martensite and bainite structures. This method is commonly used for alloy tool steel workpieces. VII. Pre-cooling isothermal quenching method: Also known as temperature-raising isothermal quenching, in this method the part is first cooled in a bath at a lower temperature (above Ms), and then transferred to a bath at a higher temperature to allow for isothermal transformation of the austenite. Suitable for steel parts with poor hardenability or large workpieces that require isothermal quenching. VIII. Delayed cooling quenching method: In this method, the part is first pre-cooled in air, hot water, or a salt bath to a temperature slightly above Ar3 or Ar1, and then quenched using a single medium. It is commonly used for parts with complex shapes, large variations in thickness across different areas, and low deformation requirements. IX. Quenching and self-tempering method: This quenching process involves heating the entire workpiece to be treated, but during quenching, only the parts that need to be hardened (usually the working surfaces) are immersed in the quenching liquid for cooling. As soon as the red color in the unimmersed parts disappears, the workpiece is removed and cooled in the air. The quenching and self-tempering method utilizes the heat from the core, where cooling has not been complete, to be transferred to the surface, thereby tempering the surface. Commonly used in tools that withstand impact, such as chisels, punches, hammers, etc. X. Jet quenching method: A quenching technique that involves spraying water flow onto the workpiece; the intensity of this water flow can be adjusted, depending on the desired depth of quenching. The jet quenching method does not form a vapor film on the surface of the workpiece, which allows for a harder hardened layer to be achieved compared to quenching in water. It is mainly used for local surface quenching.