The application of powder metallurgy materials in modern industry has become more and more extensive. In the application of high-density and high-precision complex parts to replace forged steel parts, rapid development has been achieved with the continuous progress of powder metallurgy technology. However, due to the difference in subsequent treatment processes, there are still some defects in its physical and mechanical properties. This article briefly describes and analyzes the heat treatment process of powder metallurgy materials, analyzes its influencing factors, and proposes strategies to improve the process.
Powder metallurgy materials are used more and more widely in modern industry, especially in the automotive industry, daily necessities, machinery and equipment, etc., powder metallurgy materials have occupied a large proportion. They have obvious advantages in replacing low-density, low-hardness and strength cast iron materials, and are gradually popularized in the application of high-hardness, high-precision and strength precision and complex parts, thanks to the rapid development of powder metallurgy technology. The heat treatment process of fully dense steel has been successful, but the heat treatment of powder metallurgy materials still has some defects due to the difference in the physical properties of powder metallurgy materials and the difference in heat treatment process. In the technical research of powder metallurgy materials by various foundry and smelting enterprises, hot forging, powder injection molding, hot isostatic pressing, liquid phase sintering, combined sintering and other heat treatment and subsequent treatment processes, in the physical and mechanical properties of powder metallurgy materials defects During the improvement, certain effects have been achieved, the strength and wear resistance of powder metallurgy materials have been improved, and the application range of powder metallurgy has been greatly expanded.
Heat treatment process of powder metallurgy materials
The heat treatment of powder metallurgy materials should be determined according to their chemical composition and grain size. The existence of pores is an important factor. The pores formed during the pressing and sintering of powder metallurgy materials run through the entire part. The existence of pores affects the way of heat treatment. And effect.
The heat treatment of powder metallurgy materials has several forms: quenching, chemical heat treatment, steam treatment and special heat treatment:
Quenching heat treatment process
Due to the existence of pores, the heat transfer rate of powder metallurgy materials is lower than that of dense materials, so the hardenability is relatively poor during quenching. In addition, during quenching, the sintered density of the powder material is directly proportional to the thermal conductivity of the material; because of the difference between the sintering process and the dense material, the internal structure uniformity of the powder metallurgy material is better than that of the dense material, but there is a smaller microscopic area. Non-uniformity, so the complete austenitization time is 50% longer than the corresponding forgings. When alloying elements are added, the complete austenitization temperature will be higher and the time will be longer.
In the heat treatment of powder metallurgy materials, in order to improve the hardenability, some alloying elements such as nickel, molybdenum, manganese, chromium, vanadium, etc. are usually added. Granules, when dissolved in austenite, will increase the stability of the undercooled austenite, ensure the austenite transformation during quenching, increase the surface hardness of the material after quenching, and increase the quenching depth. In addition, the powder metallurgy materials must be tempered after quenching. The temperature control of the tempering treatment has a great influence on the performance of the powder metallurgy materials. Therefore, the tempering temperature should be determined according to the characteristics of different materials to reduce the influence of temper brittleness. General materials can be tempered in air or oil at 175-250℃ for 0.5-1.0h.
Chemical heat treatment process
Chemical heat treatment generally includes three basic processes of decomposition, absorption, and diffusion. For example, the reaction of carburizing heat treatment is as follows:
2CO≒[C]+CO2 (exothermic reaction)
CH4≒[C]+2H2 (endothermic reaction)
After the carbon is decomposed, it is absorbed by the metal surface and gradually diffuses into the interior. After obtaining sufficient carbon concentration on the surface of the material, quenching and tempering treatment will increase the surface hardness and hardening depth of the powder metallurgy material. Due to the existence of pores in powder metallurgy materials, activated carbon atoms penetrate into the interior from the surface to complete the process of chemical heat treatment. However, the higher the material density, the weaker the porosity effect and the less obvious the effect of chemical heat treatment. Therefore, a reducing atmosphere with a higher carbon potential must be used for protection. According to the pore characteristics of powder metallurgy materials, the heating and cooling rate is lower than that of dense materials, so the heat preservation time should be prolonged and the heating temperature should be increased when heating.
The chemical heat treatment of powder metallurgy materials includes several forms such as carburizing, nitriding, sulphurizing and multi-element co-infiltration. In chemical heat treatment, the hardening depth is mainly related to the density of the material. Therefore, corresponding measures can be taken in the heat treatment process, such as: during carburizing, the time should be appropriately extended when the material density is greater than 7g/cm3. Chemical heat treatment can improve the wear resistance of the material. The uneven austenite carburizing process of powder metallurgy materials makes the carbon content of the surface of the treated material can reach more than 2%, and the carbides are evenly distributed on the surface of the carburized layer. , It can improve the hardness and wear resistance.