Heat treatment process of powder metallurgy materials

Heat treatment process of powder metallurgy materials
Do you understand the heat treatment process of powder metallurgy materials? Nowadays, powder metallurgy materials are increasingly widely used, and they have obvious advantages in replacing cast iron materials with low density, low hardness, and strength. The heat treatment of powder metallurgy materials includes several forms: quenching, chemical heat treatment, steam treatment, and special heat treatment:
powder metallurgy
1. Quenching heat treatment process
Due to the presence of pores, powder metallurgy materials have lower heat transfer rates than dense materials, resulting in relatively poor hardenability during quenching. In addition, during quenching, the sintering density of the powder material is directly proportional to the thermal conductivity of the material; Due to the difference between sintering process and dense materials, the internal structure uniformity of powder metallurgy materials is better than that of dense materials. However, there is a small unevenness in the micro area, so the complete austenitization time is 50% longer than the corresponding forging. When adding alloy elements, 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 alloy elements such as nickel, molybdenum, manganese, chromium, vanadium, etc. are usually added. Their action mechanism is the same as in dense materials, which can significantly refine the grain. When they dissolve in austenite, they will increase the stability of undercooled austenite, ensure the transformation of austenite during quenching, increase the surface hardness of the material after quenching, and also increase the quenching depth. In addition, powder metallurgy materials require tempering treatment after quenching. The temperature control of tempering treatment has a significant impact on the performance of powder metallurgy materials. Therefore, the tempering temperature should be determined based on the characteristics of different materials to reduce the impact of tempering brittleness. Generally, materials can be tempered in air or oil at 175-250 ℃ for 0.5-1.0 hours.
2. Chemical heat treatment process
Chemical heat treatment generally includes three basic processes: 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 carbon decomposition, it is absorbed by the metal surface and gradually diffuses internally. After obtaining sufficient carbon concentration on the surface of the material, quenching and tempering treatment can improve the surface hardness and quenching depth of powder metallurgy materials. Due to the presence of pores in powder metallurgy materials, activated carbon atoms penetrate from the surface into the interior, completing the process of chemical heat treatment. However, the higher the material density, the weaker the pore effect, and the less obvious the effect of chemical heat treatment. Therefore, a reducing atmosphere with higher carbon potential should be used for protection. According to the pore characteristics of powder metallurgy materials, their heating and cooling rates are lower than those of dense materials, so it is necessary to extend the insulation time and increase the heating temperature during heating.
The chemical heat treatment of powder metallurgy materials includes several forms, such as carburization, nitriding, sulfurization, and multicomponent co infiltration. In the chemical heat treatment, the quenching depth is mainly related to the density of the material. Therefore, corresponding measures can be taken in the heat treatment process, such as extending the time appropriately when the material density is greater than 7g/cm3 during carburization. Chemical heat treatment can improve the wear resistance of materials. The uneven austenite carburization process of powder metallurgy materials can achieve a carbon content of over 2% on the surface of the treated material's carburized layer. Carbides are evenly distributed on the surface of the carburized layer, which can effectively improve hardness and wear resistance.
Powder Metallurgical Materials
3. Steam treatment
Steam treatment is the process of heating steam to oxidize the surface of a material, forming an oxide film on the surface, thereby improving the performance of powder metallurgy materials. Especially for the surface anti-corrosion of powder metallurgy materials, the validity period is significantly better than that of blueing treatment, and the hardness and wear resistance of the treated material are significantly increased.
4. Special heat treatment process
Special heat treatment technology is a product of technological development in recent years, including induction heating quenching, laser surface hardening, and so on. Induction heating quenching is under the influence of high-frequency electromagnetic induction eddy currents, which increases the heating temperature quickly and has a significant effect on the increase of surface hardness. However, soft spots are prone to occur, and intermittent heating can generally be used to extend the austenitizing time; Laser surface hardening process uses laser as a heat source to rapidly heat up and cool the metal surface, making it difficult for the substructure inside the austenite grains to recover and recrystallize, resulting in ultrafine structure.