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Heat treatment process for powder metallurgy materials

Industry news
Release time:
2019-12-353 10:27
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Do you know the heat treatment process of powder metallurgy materials? Nowadays, powder metallurgy materials are more and more widely used.
Do you know the heat treatment process of powder metallurgy materials? Nowadays, powder metallurgy materials are more and more widely used. They have obvious advantages in replacing low density, low hardness and strength cast iron materials. The heat treatment of powder metallurgy materials includes quenching, chemical heat treatment, steam treatment and special heat treatment.
1, quenching heat treatment process
Powder metallurgy materials have a lower heat transfer rate than dense materials due to the existence of pores. Therefore, hardenability is relatively poor during quenching. In addition, during quenching, the sintered density of the powder material is proportional to the thermal conductivity of the material; because of the difference between the sintering process and the dense material, the internal uniformity of the powder metallurgical material is better than that of the dense material, but there are smaller microscopic areas. Non-uniformity, so the complete austenitizing time is 50% longer than the corresponding forging. When alloying elements are added, the complete austenitizing 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, and vanadium are usually added. Their role is the same as that in dense materials, and the crystals can be significantly refined. Grain, when dissolved in austenite, will increase the stability of supercooled austenite, ensure austenite transformation during quenching, increase the surface hardness of the material after quenching, and increase the hardening depth. In addition, the powder metallurgy material must be tempered after quenching. The temperature control of the tempering treatment has a great impact on the performance of the powder metallurgy material. Therefore, the tempering temperature should be determined according to the characteristics of different materials to reduce the impact of tempering brittleness. General materials can be tempered in air or oil at 175-250 ℃ for 0.5-1.0h.
2, 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 a 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 the powder metallurgy material, activated carbon atoms penetrate into the interior from the surface, 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 a higher carbon potential should be used for protection. According to the pore characteristics of powder metallurgy materials, its heating and cooling speed is lower than that of dense materials, so it is necessary to extend the holding time and increase the heating temperature during heating.
The chemical heat treatment of powder metallurgy materials includes several forms such as carburizing, nitriding, sulphurizing and multiple 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: when carburizing, appropriately extending the time when the material density is greater than 7g / cm3. The abrasion resistance of the material can be improved by chemical heat treatment, and the uneven austenite carburizing process of the powder metallurgy material can make the carbon content of the surface of the treated material's infiltration layer reach more than 2%, and the carbides are evenly distributed on the surface of the infiltration layer. , Can improve the hardness and wear resistance.
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