Heat treatment - low pressure vacuum carburizing
At present, low pressure vacuum carburizing heat treatment has been gradually applied in the automobile industry of developed countries in the world. It will gradually replace conventional controlled atmosphere carburizing (or carburizing) heat treatment and become the main heat treatment production technology.
The principle and process control of conventional controlled atmosphere carburizing (or carburizing) heat treatment have been deeply studied and widely used at home and abroad through long-term development and laid a solid foundation for the development and innovation of automobile industry. However, with the gradual improvement of people's understanding of the environment and the general concern of sustainable development, "green" heat treatment has been put on the agenda. Relative to the conventional controlled atmosphere carburizing heat treatment, low pressure vacuum carburizing heat treatment process does not produce harmful gases such as CO2, SO2, and most of them adopt gas quenching technology and quenching gases can be recycled, even if the oil quenching technology, also with vacuum quenching oil, to avoid a large amount of gas and liquid pollution, has the characteristics of "green" environmental protection, thus the technology and equipment has been gradually get the attention of both at home and abroad.
On the other hand, compared with conventional controlled atmosphere carburizing (or carburizing) heat treatment, low pressure vacuum carburizing heat treatment, (1) can adopt high temperature treatment technology without producing harmful surface materials; (2) carburizing or carburizing is carried out under low pressure vacuum. The surface activity of the parts is high, the infiltration speed is significantly increased, and the process cycle is significantly reduced. According to the different infiltration layers, the process cycle can be reduced by 20-50% compared with the conventional controlled atmosphere carburizing heat treatment.
Due to low pressure vacuum carburizing heat treatment process, the parts has been in the vacuum state, there is no oxygen atmosphere, so that it can't be produced within the oxidation, surface microstructure, surface black group, etc., and won't produce parts surface alloying elements of dilution and the hardenability reduce problems such as surface, the surface hardness, surface residual stress level will be improved obviously, and thus can significantly reduce the surface of the parts early failure, improve the service life of parts.
If the low pressure vacuum carburizing heat treatment is combined with the high pressure gas quenching technology, and the process parameters such as quenching are adjusted appropriately to reduce the advantages of heat treatment deformation, the product precision can be improved.
At present, C3H8 (propane) and C2H2 (acetylene) are mainly used in low pressure vacuum carburizing heat treatment at home and abroad. According to the research data from abroad, the adoption of C2H2 can significantly improve the depth uniformity of infiltration layer at different positions of parts (curvature difference), and can well carburize the blind hole, especially the blind hole, which is a difficult problem in the process of controlled atmosphere heat treatment at present.
During the process of carburizing and diffusion pulse, the carbon concentration on the surface of parts changes
Schematic diagram of carburizing and diffusion pulse process
Low pressure vacuum carburizing heat treatment is a kind of heat treatment technology of environmental protection and energy saving. The main process parameters of low pressure vacuum carburizing are: carburizing temperature, the original carbon concentration of the carburized workpiece, the saturated carbon concentration of the surface after carburizing, the surface carbon concentration after diffusion, the final surface carbon concentration, the depth of carburizing layer and the enrichment rate of the medium on the workpiece surface. Low pressure vacuum carburizing heat treatment can improve the depth uniformity of the carburizing layer at different parts of the workpiece. These problems should be studied and discussed in production.