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Study on the Process of Vacuum Low Pressure Carburizing

The product quality of vacuum carburizing is better than that of controlled atmosphere furnace carburizing. For gear carburizing that needs to control quenching deformation, after carburizing in controlled atmosphere furnace, it is necessary to use a quenching press to perform quenching and correction at the same time. The quality sometimes reaches It does not meet the technical requirements, and it is difficult to ensure the uniformity of the carburized layer and carbon concentration distribution. Internal oxidation often occurs, resulting in the appearance of non-horse black structures, and productivity is sometimes affected, so the quality requirements are relatively high. The carburization of small and medium-sized gears and gear shafts in components such as motorcycles, automobile reducers, and automobile engines has gradually transitioned from controlled atmosphere carburizing to vacuum carburizing in qualified enterprises. For the carburization of workpieces such as gears and gear shafts with higher quality requirements, low-pressure vacuum carburizing and high-pressure gas quenching technologies can be used.

Advantages of vacuum carburizing:
No pollution;
faster carbon transfer;
Achieve high-temperature carburizing;
Reduce heat treatment deformation of parts;
Carburizing uniformity, suitable for parts with complex shapes;
Surface clean, no intergranular oxidation;
No post-cleaning required;
Save and efficient;
Can achieve small load or large batch production;

Vacuum carburizing applications
carburizing pressure
During controlled atmosphere carburizing, the absolute pressure of carburizing is 1002-1003mbar, while during vacuum carburizing, the absolute pressure of carburizing is less than or equal to 30mbar. It not only indicates the vacuum state in the furnace, but more importantly, it is related to the carburizing temperature, Time and carburizing gas flow together directly or indirectly affect the depth of the carburized layer and the carbon concentration on the surface of the workpiece.
Research shows that low-pressure vacuum carburizing pressure is closely related to carburizing temperature, carburizing gas flow and pumping speed of the vacuum pump unit. The carburizing pressure is proportional to the carburizing temperature and carburizing gas flow, and is directly proportional to the pumping speed of the vacuum pump unit. Quick inverse ratio. When selecting the carburizing gas flow rate, the furnace loading capacity is mainly considered, because the carburizing gas flow rate is proportional to the total surface area of the carburizing workpiece.
Generally, an increase in carburizing pressure means an increase in carburizing gas flow and an increase in carbon supply capacity. Although the reduction of carburizing pressure will reduce the carbon supply capacity, it will increase the vacuum in the furnace, reduce the surface pressure of the workpiece, and increase the gaps in the crystal structure of the metal workpiece, resulting in an increase in the adsorption capacity of the workpiece to activated carbon atoms. Therefore, the appropriate carburizing pressure should be selected when performing low-pressure vacuum carburizing. Experience shows that the pressure should be controlled within the range of 3-25mbar.

Low pressure carburizing process
The carburizing atmosphere (C2H2) passed into the low-pressure vacuum carburizing furnace forms C+H2 after cracking in the furnace, so that the “carbon” in the heated carburizing chamber is in a saturated state, and the carbon enrichment rate F (mg/h·cm2 ) to express. When the surface area of the workpiece is less than its critical value and the flow rate of C2H2 is constant, the F value is constant; and when the flow rate of C2H2 is greater than its critical value and the surface area of the workpiece is constant, the F value is also a constant value. Therefore, the carburizing process can be controlled by four parameters: temperature, time, flow and pressure of C2H2 and N2. During the carburization and diffusion process, the pressure is maintained between 70 and 2000Pa. Low-pressure carburizing consists of a process in which carburizing gas and neutral gas are alternately introduced. After each carburizing, the “carbon” on the surface of the workpiece will diffuse into the interior of the workpiece.
In each carburizing and diffusion cycle, a time is required to switch from the carburizing atmosphere to the diffusion atmosphere. Depending on the temperature, the cracking of the atmosphere, the characteristics of the gas expansion and the capacity of the vacuum pump, this time is only 5 seconds. According to the requirements of the carburizing layer of the workpiece, the computer simulation system will calculate the time and number of cycles of the carburizing and diffusion process. Since the maximum temperature of the heated carburizing chamber can reach 1100°C, even a carburizing temperature of 980°C will not affect the service life of the heating element and insulation layer. Similarly, the working temperature of 1100°C can realize vacuum quenching treatment of tools and molds.
Before computer simulation calculation, the characteristics and initial parameters of the workpiece material need to be input. After completing the above operations, the computer starts to simulate and calculate the number of cycles of (carburizing + diffusion), the time of the final diffusion period, the total processing time, the final surface carbon concentration and the final carburized layer depth. The error between the computer simulation and the actual measurement of the workpiece is less than 5%.

Advantages of low pressure carburizing technology
(1) Better quality There is no oxygen in the heat treatment process, so there is no intergranular oxidation and no plaque; due to the use of gas quenching, the deformation of the workpiece is smaller; the workpiece after heat treatment is bright and silvery white; due to precise control of the four main parameters (carburizing temperature, time, gas flow, pressure), therefore, the production repeatability is very good; using a computer monitoring system, precise quality control can be achieved.
(2) Better flexibility can adjust the productivity from time to time according to the productivity of the machine tool, that is, achieving “synchronous” production; the modular design of the low-pressure vacuum carburizing furnace, different heating carburizing chambers can simultaneously process workpieces with different carburizing layer requirements ; The working temperature is 800~1100℃, which can not only carburize but also perform vacuum quenching treatment of tool and die steel; it is easy to shut down the furnace on weekends, and the additional cost is very low.
(3) Environmentally friendly and safe low-pressure vacuum carburizing furnace uses vacuum technology, no carbon black, no fire curtain door, no smoke, no oil vapor, no risk of fire. The low-pressure vacuum carburizing furnace is a cold and clean equipment. Therefore, it can be used together with the machine tool shop.
(4) Better productivity and less labor consumption, because the entire operation process is controlled by a computer and completed automatically; less energy consumption; greatly reduced atmosphere consumption (compared to traditional controlled atmosphere multi-purpose furnaces) ; The carburizing time is shortened; the low-pressure vacuum carburizing furnace has a very compact design, so it occupies a small area; no other auxiliary equipment is required, such as straightening machines, pressure quenching equipment, etc.
Compared with low-pressure vacuum carburizing and controlled atmosphere carburizing, it has incomparable advantages in terms of the structure and performance of the workpiece after carburization, process flexibility, production cost and environmental protection, and will definitely have broad prospects. Application prospects and rapid development.