Heat treatment of powder metallurgy

The application of powder metallurgy materials in modern industry is becoming more and more widespread. With the continuous progress of powder metallurgy technology, the application of iron powder for water atomization electrode has been rapidly developed by using high-density and high-precision complex parts to replace forged steel parts. However, due to the differences in subsequent treatment processes, there are still some defects in its physical and mechanical properties. The heat treatment process of powder metallurgy materials is briefly introduced and analyzed, the influencing factors are analyzed, and improvement measures are proposed.

Powder metallurgy materials are widely used in modern industry, especially in the automotive industry, home appliances, machinery and equipment, etc., powder metallurgy materials account for a large proportion. It has obvious advantages in replacing low density, low hardness and low strength cast iron materials. With the rapid development of powder metallurgy technology, it has gradually been popularized in the application of high-hardness, high-precision, high-strength precision and complex parts. The heat treatment process of fully dense steel has been successful, but due to the differences in the physical properties and heat treatment processes of powder metallurgy materials, there are still some defects in the heat treatment of powder metallurgy materials. In the technical research of powder metallurgy materials, heat treatment such as hot forging, powder injection molding, hot isostatic pressing, liquid phase sintering, and composite sintering and subsequent treatment processes have achieved certain effects in improving the physical and mechanical properties of powder metallurgy felt . The strength and wear resistance of powder metallurgy materials are improved, and the powder metallurgy industry is greatly expanded. Scope of application.

two. Heat treatment process for powder metallurgy materials

The heat treatment of powder metallurgy materials should be determined according to its chemical composition and particle size. The presence of pores is an important factor. The pores formed during the pressing and sintering of powder metallurgy materials penetrate the entire part. The presence of pores affects the way and effect of heat treatment.

The heat treatment of powder metallurgy materials includes quenching, chemical heat treatment, steam treatment and special heat treatment.

1. Quenching heat treatment process

Due to the existence of voids, the heat transfer rate of powder metallurgy materials is lower than that of dense materials, so the hardenability of powder metallurgy materials is poor during the quenching process. In addition, the sintered density and thermal conductivity of the powder material are proportional during quenching. Due to 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. During quenching, the internal uniformity of powder metallurgy materials is better than that of dense materials. The time of complete austenitization is 50% longer than that of the corresponding forging. When alloying elements are added, complete austenitization occurs. The temperature will be higher and the time will be longer.

In order to improve the hardenability of powder metallurgy materials during heat treatment, alloy elements such as nickel, molybdenum, manganese, chromium, and vanadium are usually added. It functions the same as the compact material. They can significantly refine grain size. When they are dissolved in austenite, the stability of supercooled austenite will be improved, the transformation of austenite during the quenching process will be guaranteed, and the surface hardness of the quenched material will be improved. As the depth of hardening increases, the depth of hardening increases. In addition, powder metallurgy materials are tempered after quenching. Tempering temperature control 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 effect of tempering brittleness. General materials can be tempered in air or oil at 175-250 ℃ for 0.5-1.0 h.

2. Chemical heat treatment process

Chemical heat treatment generally includes three basic processes of decomposition, absorption, and diffusion. For example, the reactions of carburizing heat treatment are 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, sulfurizing 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.

3.Steam treatment

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

4. Special heat treatment process

The special heat treatment process is the product of scientific and technological development in recent years, including induction heating quenching, laser surface hardening, etc. Induction heating quenching is under the influence of high-frequency electromagnetic induction eddy current, and the heating temperature increases quickly, which has a significant effect on the increase of surface hardness, but soft spots are easy to occur. Generally, intermittent heating method can be used to extend the austenizing time; laser surface hardening The process uses a laser as a heat source to rapidly heat up and cool the metal surface, so that the substructure inside the austenite grains is too late to recover and recrystallize to obtain an ultrafine junction.