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Classification and selection of vacuum carburizing process

1.Direct quenching and low temperature tempering

Structure and performance characteristics: Can not refine the grain of steel. The quenching deformation of the workpiece is large, the residual austenite on the surface of the alloy steel carburized part is more, and the surface hardness is lower.

Scope of application: simple operation, low cost, used to process parts that are not subject to deformation and impact load, suitable for gas carburizing and liquid carburizing processes.

2.Precooling direct quenching, low temperature tempering quenching temperature 800-850℃

Microstructure and performance characteristics: It can reduce the quenching deformation of the workpiece, the amount of retained austenite in the infiltrated layer can also be slightly reduced, and the surface hardness is slightly increased, but the austenite grain does not change.

Scope of application: simple operation, small oxidation, decarburization and quenching deformation of the workpiece, widely used in various tools made of fine grain steel.

3.One-time heating and quenching, low temperature tempering, quenching temperature 820-850°C or 780-810°C

Microstructure and performance characteristics: If the strength of the core is high, quenching at 820-850°C is used. The core is low-carbon M, and the surface requires high hardness. The grain is refined by quenching at 780-810°C.

Scope of application: It is suitable for carbon steel and low alloy steel workpieces after solid carburizing, coarse-grained steel for gas and liquid carburizing, some workpieces that are not suitable for direct quenching after carburizing, and parts that need to be machined after carburizing.

4.Vacuum carburizing high temperature tempering, one heating quenching, low temperature tempering, vacuum quenching temperature 840-860℃

Microstructure and performance characteristics: High-temperature tempering decomposes M and residual A, and the carbon and alloy elements in the infiltrated layer are precipitated in the form of carbides, which is convenient for cutting and reduces residual A after quenching.

Scope of application: mainly used for carburizing workpieces of Cr-Ni alloy

5.Secondary quenching and low temperature tempering

Structure and performance characteristics: The first vacuum quenching (or normalizing) can eliminate the carburized layer network carbide and refine the core structure (850-870 ° C), the second vacuum quenching mainly improves the carburized layer structure, and the When the core performance is not high, it can be vacuum quenched between Ac1-Ac3 of the material, and when the core performance is high, it should be vacuum quenched above Ac3.

Scope of application: mainly used for important carburized parts with high requirements on mechanical properties, especially for coarse grain steel. However, after vacuum carburizing, it needs to be heated twice at high temperature, which will increase the deformation and oxidative decarburization of the workpiece, and the heat treatment process is more complicated.

6.Secondary quenching cold treatment and low temperature tempering

Microstructure and performance characteristics: Quenching at a temperature higher than Ac1 or Ac3 (core), the high-alloy surface layer has more residual A, and the cold treatment (-70°C/-80°C) promotes A transformation to improve surface hardness and wear resistance.

Scope of application: mainly used for high-alloy steel workpieces that are not machined after vacuum carburizing.

7.After vacuum carburizing, induction heating, quenching and low temperature tempering

Organization and performance characteristics: It can refine the infiltrated layer and the tissue near the infiltrated layer. The deformation of vacuum quenching is small, and the parts that are not allowed to be hardened do not need anti-seepage in advance.

Scope of application: various gears and shafts.

8.Carburizing process parameters of steel

The carburizing process parameters of steel mainly include the following three items:

1.The chemical composition of carburizing medium: carburizing medium can be divided into three categories: gas, liquid and solid.

2.Vacuum carburizing temperature: the commonly used temperature is 900-950°C. The higher the temperature, the faster the diffusion rate and the deeper the infiltration layer. However, if the temperature is too high, the austenite grains will grow, which will reduce the mechanical properties of the parts; increase the shape deformation of the workpiece and reduce the service life of the equipment.

3.Carburizing time: The commonly used approximate calculation of carburizing time mostly uses the Harris formula. In the formula: the depth is in mm; t time (h); T temperature (K).