Vacuum carburizing

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Carburizing refers to the process of making carbon atoms penetrate into the surface layer of steel. After carburizing, the parts of low carbon steel have the surface layer of high carbon steel. After quenching and tempering, the carburized parts have high surface hardness, high wear resistance and fatigue strength, and maintain the strength, toughness and plasticity of low carbon steel after quenching in the center, so that the workpiece can bear the alternating load of high strength and frequency.

Carburizing includes three basic processes: decomposition, adsorption and diffusion. According to different carburizing methods, it can be divided into atmosphere carburizing, solid carburizing, liquid carburizing and vacuum carburizing.

At present, traditional atmosphere carburizing is widely used. Solid carburizing and liquid carburizing are gradually replaced by many factors, such as production efficiency, working conditions, environmental protection requirements and so on. As a widely used carburizing method, traditional atmosphere carburizing plays an important role in improving the performance of common material parts, but it also exposes many problems in the actual production process, such as internal oxidation of workpieces, large exhaust emissions, long carburizing cycle, easy oxidation and decarburization of workpieces, high alloy and stainless steel, etc.

Compared with traditional atmosphere carburizing, vacuum carburizing can reduce the cost of treatment, eliminate environmental pollution, and obtain excellent surface state and carburizing layer uniformity of workpieces. Vacuum carburizing also has the advantages of small quenching deformation, high carburizing efficiency and avoiding grain boundary oxidation. The vacuum carburizing furnace has the advantages of all equipment of the vacuum quenching furnace. In terms of technology, it can not only be used for vacuum carburizing + quenching, but also for die quenching, annealing, solid solution and brazing treatment of some parts. Based on one equipment, diversified production of heat treatment process is realized. At the same time, the vacuum carburizing furnace has incomparable advantages in realizing carburizing of high alloy steel and stainless steel, improving working conditions and product quality.

The following is an example of the application of vacuum carburizing
We have studied the data and characteristics of vacuum carburizing process of 20CrMnMo heavy duty gear with large diameter, and applied it to the actual production with good results. The gear has been in normal operation for several years and is in good condition.

1、 Low pressure vacuum carburizing and quenching parts and technical requirements
Material: 20CrMnMo
Gear parameters and requirements of vacuum carburizing and quenching process

2、 Vacuum carburizing process plan and production equipment
The production equipment is a double chamber vacuum carburizing oil quenching furnace produced by Simuwu, with an effective working area of 1500 * 1500 * 800 (mm).

3、 Experimental process and result analysis of deep vacuum carburizing
Process Description:
1. Preheat once at 650 ℃: evenly heat the gear and reduce the internal thermal stress of the workpiece;
2. After carburizing, the gas cooling normalizing was carried out under 1barn2 pressure: because of the obvious tendency of carburizing grain growth for a long time, the possible network carbide in carburizing layer was eliminated;
3. High temperature tempering at 680 ℃: the carbide containing Cr is precipitated in the carburized layer, the net carbide in the carburized layer is further eliminated, the carbide is spheroidized and the stress is eliminated.
4. In the experiment, four samples with tooth shape were placed on the circumference of tooth top circle, and four samples were analyzed after carburizing and quenching.

4、 Result analysis
1. The surface of workpiece and sample is bright, silver white, and the appearance is very good.
2. In the deep vacuum carburizing process of 20CrMnMo steel, when the proper carburizing ratio is selected, the carburizing layer error is very small, the number of granular carbide at the sharp corner of carburizing layer is small, there is no network carbide, and the carbide grade is 2-3.
3. The simulated surface carbon concentration of 20CrMnMo steel in deep vacuum carburizing (4.4mm) is 0.035% lower than the actual carbon concentration, and the average value of the simulated carburizing layer depth is 0.22mm lower than the actual carburizing layer depth, with small error.
4. When the carburizing ratio of 20CrMnMo steel heavy duty gear is 1:20, after deep vacuum carburizing and quenching treatment, the surface carbon concentration is 0.86%, the depth of carburizing layer is 4.4mm, and the carbide grade is 1-2.
5. When the total holding time is 28.75h (carburizing + diffusion), the appropriate carburizing ratio is selected, and the total carburizing layer and metallographic structure fully meet the simulation value and technical requirements.