Summary of Heat Treatment Process Knowledge of Powder Metallurgy

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1. 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. During the process of pressing and sintering powder metallurgy materials, the pores formed throughout the whole part, and the existence of pores affects the way of heat treatment. and effects.
There are several forms of heat treatment for powder metallurgy materials: quenching, chemical heat treatment, steam treatment and special heat treatment:

1.1 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; 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 are smaller microscopic regions. Inhomogeneity, therefore, the full austenitization time is 50% longer than the corresponding forging, and the full austenitization temperature will be higher and the time will be longer when alloying elements are added.

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. When it dissolves in austenite, it will increase the stability of supercooled austenite, ensure the austenite transformation during quenching, increase the surface hardness of the material after quenching, and also increase the quenching depth. In addition, powder metallurgy materials must 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 in air or oil at 175-250℃ for 0.5-1.0h.

1.2 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 a sufficient carbon concentration is obtained on the surface of the material, quenching and tempering are carried out, which will improve the surface hardness and quenching 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 should be used for protection. According to the pore characteristics of powder metallurgy materials, the heating and cooling rates are lower than those of dense materials, so the holding time should be prolonged and the heating temperature should be increased during heating.

Chemical heat treatment of powder metallurgy materials includes several forms such as carburizing, nitriding, sulphurizing and multi-component 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, when the material density is greater than 7g/cm3, the time is appropriately extended. The wear resistance of the material can be improved by chemical heat treatment. The uneven austenite carburizing process of powder metallurgy materials can make the carbon content of the surface of the treated material reach more than 2%, and the carbides are evenly distributed on the surface of the carburized layer. , can well improve the hardness and wear resistance.

1.3 Steam treatment

Steam treatment is to oxidize the surface of the material by heating steam to form an oxide film on the surface of the material, thereby improving the performance of the powder metallurgy material. Especially for the anticorrosion of the surface of powder metallurgy materials, its validity period is more effective than that of bluing treatment, and the hardness and wear resistance of the treated materials are significantly increased.

2. Analysis of Influencing Factors of Heat Treatment of Powder Metallurgy Materials

The pores generated in the sintering process of powder metallurgy materials are their inherent characteristics, which also have a great influence on the heat treatment, especially the relationship between the change of porosity and the heat treatment. In order to improve the compactness and grain size, the added alloying elements are also Has a certain effect on heat treatment:

2.1 The effect of pores on the heat treatment process

During the heat treatment of powder metallurgy materials, rapid cooling is used to inhibit the diffusion of austenite into other structures, thereby obtaining martensite, and the existence of pores has a great influence on the heat dissipation of the material. By the thermal conductivity formula:
Thermal conductivity = Theoretical thermal conductivity of metal × (1-2 × porosity)/100
It can be seen that the hardenability decreases with increasing porosity. On the other hand, porosity also affects the density of the material, and the effect on the surface hardness and hardening depth of the material after heat treatment is related to the effect of density, which reduces the surface hardness of the material. Moreover, because of the existence of pores, brine cannot be used as a medium during quenching, so as to avoid corrosion caused by residual salt, so the general heat treatment is carried out in a vacuum or gas medium.

2.2 The effect of porosity on the surface hardening depth during heat treatment

The heat treatment effect of powder metallurgy materials is related to the density, permeability (hardenability), thermal conductivity and electrical resistance of the material. Porosity is the biggest cause of these factors. When the porosity exceeds 8%, the gas will rapidly penetrate through the voids. In the case of carburizing hardening, increasing the carburizing depth reduces the effect of surface hardening. Moreover, if the infiltration rate of carburizing gas is too fast, soft spots will be generated during quenching, which will reduce the surface hardness and make the material brittle and deformed.

2.3 Effect of alloy content and type on powder metallurgy heat treatment

Common alloying elements are copper and nickel, and their content and type will affect the heat treatment effect. The depth of heat treatment hardening increases with the increase of copper content and carbon content and gradually decreases when it reaches a certain content; the stiffness of nickel alloy is greater than that of copper alloy, but the inhomogeneity of nickel content will lead to inhomogeneous austenite structure.

2.4 Influence of high temperature sintering

Although high temperature sintering can achieve the best alloying effect and promote densification, the difference in sintering temperature, especially when the temperature is lower, will lead to decreased heat treatment sensitivity (reduced alloy in solid solution) and mechanical properties. Therefore, using high-temperature sintering, supplemented by a sufficient reducing atmosphere, can obtain a better heat treatment effect.

The heat treatment process of powder metallurgy materials is a complex process, which is related to porosity, alloy type, alloy element content, and sintering temperature. Compared with dense materials, the internal uniformity is poor. In order to obtain higher hardenability In order to increase the full austenitization temperature and prolong the time, heterogeneous austenite carburization can obtain a high carbon concentration that is not limited by the austenite saturated carbon concentration. In addition, adding alloying elements can also improve the hardenability. Steam treatment can significantly improve its corrosion resistance and surface hardness.

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