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Heat Treatment Process of Powder Metallurgy Materials

Release time:

2024-02-04 09:25

Powder metallurgy materials are now more and more widely used, they have obvious advantages in replacing low density, low hardness and strength of cast iron materials. The heat treatment of powder metallurgy materials has several forms of quenching, chemical heat treatment, steam treatment and special heat treatment:

1. 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 sintering density of the powder material is proportional to the thermal conductivity of the material. Due to the difference between the sintering process and the dense material, the internal microstructure uniformity of the powder metallurgy material is better than that of the dense material, but there is less unevenness in the microscopic region. Therefore, the complete austenitizing time is 50% longer than that of 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, vanadium, etc. are usually added. Their function mechanism is the same as that in dense materials, which can obviously refine the grain. When it is dissolved in austenite, it will increase the stability of undercooled austenite, ensure the austenite transformation during quenching, increase the surface hardness of the material after quenching, and increase the hardening depth. In addition, the powder metallurgy materials should be tempered after quenching. The temperature control of tempering treatment has a great influence 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. General materials can be tempered for 0.5-1.0h in air or oil at 175-250 ℃.

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 the decomposition of carbon is absorbed by the metal surface and gradually diffused to the interior, the surface of the material to obtain sufficient carbon concentration and then quenching and tempering treatment, will improve the surface hardness and hardening depth of powder metallurgy materials. Due to the existence of pores in powder metallurgy materials, activated carbon atoms penetrate into the interior from the surface, completing the process of chemical heat treatment. However, the higher the density of the material, 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 to protect it. According to the pore characteristics of powder metallurgy materials, the heating and cooling speed is lower than that of dense materials, so the heating time should be extended and the heating temperature should be increased.

The chemical heat treatment of powder metallurgy materials includes several forms such as carburizing, nitriding, sulfurizing and multi-co-penetration, and 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: carburizing, when the material density is greater than 7g/cm3, the appropriate extension of time. The wear resistance of the material can be improved by chemical heat treatment, and the uneven austenite carburizing process of powder metallurgy materials can make the carbon content of the treated material seepage layer surface reach more than 2%, and the carbide is evenly distributed on the surface of the seepage layer, which can improve the hardness and wear resistance.

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