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What are the types of surface quenching heat treatment processes

What are the types of surface quenching heat treatment processes?

This article introduces you to the knowledge of surface quenching heat treatment. If you want to learn steel casting and processing, you can first learn about the theoretical knowledge related to quenching. Let's learn about heat treatment quenching process type and surface quenching heat treatment.
 
Surface hardening heat treatment introduction:


The workpiece is rapidly heated by different heat sources, and when the surface temperature of the part reaches above the critical point (when the core temperature of the workpiece is below the critical point), it is rapidly cooled, so that the surface of the workpiece is hardened and the core remains intact. organization. Industrial applications are more inductive heating and flame heating surface quenching.
 

Quenching heating temperature:


The quenching heating temperature is mainly determined by the phase transition point of the steel. For the sub-eutectic steel, the quenching heating temperature is generally Ac3+ (30-50 °C), and the hypereutectoid steel is Ac1+ (30-50 °C). The reason for this is that, for the hypoeutectic steel, if the heating temperature is lower than Ac3, the heating state is composed of austenite and ferrite two phases, and the ferrite is preserved after quenching and cooling, so that the hardness of the parts after quenching Uneven, reduced strength and hardness. The purpose of 30-50 °C higher than the Ac3 point is to ensure that the core of the workpiece can reach the temperature above Ac3 in the specified heating time. The ferrite can be completely dissolved in the austenite, and the austenite composition is relatively uniform. The body grains are not coarse. For hypereutectoid steel, when the quenching heating temperature is between Ac1 and Ac3, the heating state is fine austenite grains and undissolved carbides, and after quenching, cryptocrystalline martensite and uniformly distributed spherical carbon are obtained. This kind of tissue not only has high strength and hardness, high wear resistance, but also good toughness. If the quenching heating temperature is too high, the carbides are dissolved, the austenite grains grow, and after the quenching, flake martensite (crystallized martensite) is obtained, the microcracks increase, the brittleness increases, and the quenching cracking tendency increases. Big. Due to the dissolution of the carbide, the carbon content in the austenite increases, the amount of retained austenite increases after quenching, and the hardness and wear resistance of the steel decrease. The purpose of higher than Ac1 point 30-50 °C is similar to that of hypoeutectoid steel, in order to ensure that the temperature of each part in the workpiece is higher than Ac1.

 


What are the types of heat treatment quenching processes?

(1) Liquid quenching

Haituo electromechanical single-liquid quenching is a quenching operation method in which an austenitic workpiece is rapidly immersed in a quenching medium and cooled to room temperature. The selection of the single-liquid quenching cooling medium is based on the fact that the cooling rate of the workpiece in the medium must be greater than the critical cooling rate of the steel of the workpiece, and the workpiece should not be quenched. Single-liquid quenching medium is water, brine, alkaline water, oil and some specially formulated water-based quenching agents.

(2) Two-liquid quenching

In order to overcome the shortcomings of single-liquid quenching, the quenching and cooling of the workpiece is as close as possible to the ideal situation, and two media with different cooling capacities can be used together, that is, the heated workpiece is first quenched into the first medium with high cooling capacity. Allow it to cool slightly above the Ms temperature (about 300 ° C), then immediately transfer to a second medium with less cooling capacity and cool to room temperature. This quenching cooling method is called two-liquid quenching.

For some workpieces, in order to further slow down the cooling rate below Ms, Haituo Electromechanical can also use water quenching air cooling or oil quenching air cooling, and air can also be treated as a cooling medium.

(3) Classification quenching (martensitic quenching)

The cooling method is characterized in that the workpiece is first immersed in a bath having a temperature slightly higher than Ms, and is kept in the bath until the surface of the workpiece and the center are cooled to the bath temperature, and then the air cooling is taken out. The bath temperature is generally Ms + (10 ~ 20) ° C. The components of the medium in the bath are a salt bath, an alkali bath, and a neutral salt bath.

(4) Pre-cooling and quenching

After quenching and heating, the workpiece is not immediately immersed in the cooling medium, but is cooled in the air for a short time. When the workpiece is lowered to a certain temperature, it is immersed in the cooling medium. This quenching method is called pre-cooling or delayed quenching. .

The key to pre-cooling quenching is to control the pre-cooling time, and the pre-cooling time is short; the long time may reduce the quenching hardness of the workpiece (non-martensitic transformation occurs). Due to the different materials of the workpiece, the shape and size vary widely, and it is also affected by the temperature of the furnace and the ambient temperature. Therefore, it is difficult to accurately calculate the pre-cooling time, mainly by the skill and experience of the operator.

(5) Local quenching

Some workpieces have high hardness and some other parts have no hardness requirements or require low hardness. In this case, a partial quenching method, that is, a method of quenching only a part of the workpiece, can be generally employed. There are two main forms of local quenching, local heating local cooling and integral heating local cooling. The former is mainly applicable to the workpiece when the salt bath furnace is heated, and the latter box furnace and salt bath furnace can be used.

(6) Cold treatment

The cold treatment is a quenching operation in which the quenched steel is continuously cooled to a temperature below room temperature, and the retained austenite which has not been transformed at room temperature continues to be transformed into martensite.

For some parts with high dimensional stability, the retained austenite of the quenched structure must be reduced to the lowest temperature, and the shape and dimensional changes do not exceed the accuracy requirements. The purpose of cold treatment is here. The cold treatment temperature is mainly determined by the Ms point of the steel, combined with the technical requirements of the parts and the process equipment conditions. The cold treatment must be carried out immediately after the quenched workpiece is cooled to room temperature, otherwise it will affect its effect. The cold treatment of small and medium-sized parts is generally kept for 1~3h, and should be slowly heated in the air after treatment. When the workpiece is raised to room temperature, it is tempered immediately, which can effectively avoid cracking of the workpiece.


The above is the introduction of surface hardening heat treatment and the related types of heat treatment and quenching technology shared by everyone. If you want to understand the quenching process, you must first understand the basics of these principles.
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