Heat treatment process of powder metallurgy

The application of powder metallurgy materials in modern industry has become more and more extensive. In the application of high-density and high-precision complex parts that replace forged steel parts, rapid development has been achieved with the continuous progress of powder metallurgy technology. However, due to the difference in the subsequent treatment process, its physical and mechanical properties still have some defects. This article briefly elaborates and analyzes the heat treatment process of powder metallurgy materials, analyzes its influencing factors, and proposes strategies to improve the process.
 

 
I. Introduction
Powder metallurgy materials are more and more widely used in modern industry, especially in the automotive industry, daily necessities, machinery and equipment, etc., powder metallurgy materials have occupied a large proportion. They have obvious advantages in replacing low-density, low-hardness and strength cast iron materials, and are gradually popularized in the application of high-hardness, high-precision and strength precision and complex parts, thanks to the rapid development of powder metallurgy technology. The heat treatment process of fully dense steel has been successful, but the heat treatment of powder metallurgy materials, due to the difference in the physical properties of powder metallurgy materials and the difference in heat treatment process, there are still some defects. In the technical research of powder metallurgy materials by various foundry and smelting enterprises, hot forging, powder injection moulding, hot isostatic pressing, liquid phase sintering, combined sintering and other heat treatment and subsequent treatment processes, in the physical and mechanical properties of powder metallurgy materials defects During the improvement, certain effects have been achieved, the strength and wear resistance of powder metallurgy materials have been improved, and the application range of powder metallurgy has been greatly expanded.
2. 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. The pores formed during the pressing and sintering of powder metallurgy materials run through the entire part. The existence of pores affects the way of heat treatment. And effect.
 
The heat treatment of powder metallurgy materials has several forms: 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 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 structure uniformity of the powder metallurgy material is better than that of the dense material, but there is a smaller microscopic area. Inhomogeneity, therefore, the complete austenitization time is 50% longer than the corresponding forgings. When alloying elements are added, 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 alloying elements such as nickel, molybdenum, manganese, chromium, vanadium, etc. are usually added. Their role is the same as that in dense materials, which can significantly refine crystals. Granules, when dissolved in austenite, will increase the stability of the undercooled austenite, ensure the austenite transformation during quenching, increase the surface hardness of the material after quenching, and increase the quenching depth. In addition, the powder metallurgy materials must be tempered after quenching. The temperature control of the tempering treatment has a greater impact on the performance of the powder metallurgy materials. Therefore, the tempering temperature should be determined according to the characteristics of different materials to reduce the effect of temper 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 sufficient carbon concentration on the surface of the material, quenching and tempering treatment will increase 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 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 higher carbon potential should be used for protection. According to the pore characteristics of powder metallurgy materials, the heating and cooling rate is lower than that of dense materials, so the heat preservation time should be extended and the heating temperature should be increased when heating.
 
The chemical heat treatment of powder metallurgy materials includes several forms such as carburizing, nitriding, sulphurizing and multi-element 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, for example, when the material density is greater than 7g/cm3 during carburizing, the time should be appropriately extended. Chemical heat treatment can improve the wear resistance of the material. The uneven austenite carburizing process of powder metallurgy materials can make the carbon content of the surface of the treated material can reach more than 2%, and the carbides are evenly distributed on the surface of the carburized layer. , Can improve the hardness and wear resistance.
 
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 powder metallurgy materials. Especially for the anti-corrosion on the surface of powder metallurgy materials, the validity period is more obvious than that of bluing treatment, and the hardness and wear resistance of the treated materials are obviously increased.
 
4. Special heat treatment process
 
Special heat treatment processes are the product of technological development in recent years, including induction heating hardening, laser surface hardening, etc. Induction hardening is under the influence of high-frequency electromagnetic induction eddy current, the heating temperature increases rapidly, and it has a significant effect on the increase of surface hardness, but it is prone to soft spots. Generally, intermittent heating can be used to extend the austenitization time; laser surface hardening The process is to use laser as the heat source to quickly heat up and cool the metal surface, so that the substructure inside the austenite grains will not have time to recover and recrystallize to obtain an ultra-fine structure.
 
 

3. Analysis of influencing factors of heat treatment of powder metallurgy materials
 
The porosity generated during the sintering process of powder metallurgy materials is its inherent characteristic, and it also has a great impact on heat treatment, especially the relationship between the change of porosity and heat treatment. In order to improve the density and grain size, the added alloy elements are also Has a certain impact on heat treatment:
 
 
1. The effect of porosity on the heat treatment process
 
During the heat treatment of powder metallurgy materials, rapid cooling inhibits the diffusion and transformation of austenite into other structures, thereby obtaining martensite, and the existence of pores has a greater impact on the heat dissipation of the material. Through the thermal conductivity formula:
Thermal conductivity = theoretical thermal conductivity of metal × (1-2 × porosity) / 100
It can be seen that the hardenability decreases as the porosity increases. On the other hand, pores also affect 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 density, which reduces the surface hardness of the material. Moreover, because of the existence of pores, salt water cannot be used as the medium during quenching to avoid corrosion due to residual salt. Therefore, heat treatment is generally carried out in a vacuum or gas medium.
 
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. The porosity is the biggest cause of these factors. When the porosity exceeds 8%, the gas will quickly penetrate through the voids. In the case of carburizing hardening, increasing the carburizing depth will reduce the effect of surface hardening. Moreover, if the carburizing gas infiltration rate is too fast, soft spots will be produced during quenching, reducing the surface hardness, and making the material brittle and deformed.
 
3. The influence of alloy content and type on powder metallurgy heat treatment
 
Common alloy elements are copper and nickel, their content and type will affect the heat treatment effect. The depth of heat treatment hardening gradually increases with the increase of copper content and carbon content, and then gradually decreases when it reaches a certain content; the stiffness of nickel alloy is greater than that of copper alloy, but the unevenness of nickel content will cause uneven austenite structure.
 
4. The influence of high temperature sintering
 
Although high-temperature sintering can obtain the best alloying effect and promote densification, the difference in sintering temperature, especially when the temperature is lower, will cause the sensitivity of heat treatment to decrease (the alloy in the solid solution is reduced) and the mechanical properties are decreased. Therefore, the use of high temperature sintering, assisted by a sufficient reducing atmosphere, can obtain a better heat treatment effect.
 
 
 
Four, conclusion
 
The heat treatment process of powder metallurgy materials is a complicated process, which is related to porosity, alloy type, alloy element content, and sintering temperature. Compared with dense materials, the internal uniformity is poor, and it is necessary to obtain higher hardening In order to increase the complete austenitization temperature and extend the time, uneven 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 hardenability. Steam treatment can significantly improve its corrosion resistance and surface hardness.