Heat treatment process of powder metallurgy materials

Do you know about the heat treatment process of powder metallurgy materials? Nowadays, powder metallurgy materials are being applied more and more widely, and they have obvious advantages in replacing low-density, low hardness, and low strength cast iron materials. There are several forms of heat treatment for powder metallurgy materials, including 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 powder materials is directly proportional to their thermal conductivity; Due to the difference between sintering process and dense materials, powder metallurgy materials have better internal microstructure uniformity than dense materials, but there are smaller micro area inhomogeneity. Therefore, the complete austenitization time is 50% longer than the corresponding forgings. 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 hardenability, some alloying elements such as nickel, molybdenum, manganese, chromium, vanadium, etc. are usually added. Their mechanism of action is the same as that in dense materials, which can significantly refine the grain size. When they dissolve in austenite, it will increase the stability of undercooled austenite, ensure the transformation of austenite during quenching, and increase the surface hardness and hardening depth of the quenched material. In addition, powder metallurgy materials need to undergo 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 according to the characteristics of different materials to reduce the influence 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 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 inward. After obtaining sufficient carbon concentration on the surface of the material, quenching and tempering treatment are carried out to improve the surface hardness and quenching depth of powder metallurgy materials. Due to the presence of pores in powder metallurgy materials, activated carbon atoms infiltrate 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 high 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 the insulation time should be extended and the heating temperature should be increased during heating. The chemical heat treatment of powder metallurgy materials includes several forms such as carburizing, nitriding, sulfurization, and multi-element co infiltration. In 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 carburizing. The wear resistance of materials can be improved through chemical heat treatment. The uneven austenite carburizing process of powder metallurgy materials can increase the carbon content on the surface of the treated material's infiltration layer to over 2%. Carbides are evenly distributed on the surface of the infiltration layer, which can effectively improve hardness and wear resistance. Powder metallurgy materials 3, steam treatment Steam treatment is the process of heating steam to oxidize the surface of materials, forming an oxide film on the surface of the material, thereby improving the performance of powder metallurgy materials. Especially for the surface anti-corrosion of powder metallurgy materials, its effective period is more significant than that of bluing treatment, and the hardness and wear resistance of the treated material are significantly increased. 4. Special heat treatment process is a product of technological development in recent years, including induction heating quenching, laser surface hardening, etc. Induction heating quenching is a process in which the heating temperature is rapidly increased under the influence of high-frequency electromagnetic induction eddy currents, resulting in a significant increase in surface hardness. However, soft spots are prone to occur, and intermittent heating can generally be used to extend the austenitization time; Laser surface hardening process uses laser as a heat source to rapidly heat and cool the metal surface, preventing the substructure inside the austenite grains from recovering and recrystallizing in time to obtain ultrafine structure. The above is an introduction to the heat treatment process of powder metallurgy materials. The heat treatment of powder metallurgy materials should be determined based on their chemical composition and grain size, and the presence of pores is an important factor. During the pressing and sintering process of powder metallurgy materials, pores are formed throughout the entire part, and the presence of pores affects the way and effect of heat treatment.