Heat treatment process of powder metallurgy

author: YITECH

2025-02-22

Powder metallurgy materials are increasingly used in modern industry. In the application of high-density and high-precision complex parts replacing forged steel parts, they have also achieved rapid development with the continuous advancement

Powder metallurgy1 materials are increasingly used in modern industry. In the application of high-density and high-precision complex parts replacing forged steel parts, they have also achieved rapid development with the continuous advancement of powder metallurgy technology. However, due to the differences in subsequent processing technology, there are still some defects in its physical and mechanical properties. This article briefly explains and analyzes the heat treatment process of powder metallurgy materials, analyzes its influencing factors, and proposes strategies to improve the process.

01 Introduction

Powder metallurgy materials are increasingly used in modern industry, especially in the automotive industry, daily necessities, mechanical equipment, etc., where powder metallurgy materials have occupied a large proportion. They have obvious advantages in replacing low-density, low-hardness and low-strength cast iron materials, and are gradually being promoted in the application of high-hardness, high-precision and high-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 still has some defects due to the differences in the physical properties of powder metallurgy materials and the differences in heat treatment processes. In the technical research of powder metallurgy materials, various casting and smelting enterprises have achieved certain results in improving the physical and mechanical properties of powder metallurgy materials, such as hot forging, powder injection molding, hot isostatic pressing, liquid phase sintering, and combined sintering. Heat treatment and subsequent treatment processes have improved the strength and wear resistance of powder metallurgy materials, which will greatly expand the application scope of powder metallurgy.

02 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 pressing and sintering process of powder metallurgy materials, the pores formed run through the entire part. The existence of pores affects the method 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 presence of pores, the heat transfer rate of powder metallurgy materials is lower than that of dense materials. Therefore, during quenching, the hardenability is relatively poor. In addition, during quenching, the sintering density of the powder material is proportional to the thermal conductivity of the material; because of the difference in sintering process and dense materials, the internal structure uniformity of powder metallurgy materials is better than that of dense materials, but there is a small micro-area inhomogeneity, so the complete austenitization time is 50% longer than the corresponding forgings. When adding alloy elements, 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 alloy elements such as nickel, molybdenum, manganese, chromium, vanadium, etc. are usually added. Their role is the same as that in dense materials. They can significantly refine the grains. When they are dissolved in austenite, they will increase the stability of supercooled austenite, ensure the transformation of austenite during quenching, increase the surface hardness of the material after quenching, and increase the hardening depth. In addition, powder metallurgy materials must be tempered after quenching. The temperature control of tempering 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.

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 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 the powder metallurgy material. Due to the pores of the powder metallurgy material, the activated carbon atoms penetrate from the surface to the interior to complete the chemical heat treatment process. 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 porosity characteristics of powder metallurgy materials, their heating and cooling rates are lower than those of dense materials, so the insulation time should be extended and the heating temperature should be increased during heating.

The chemical heat treatment of powder metallurgy materials includes several forms such as carburizing, nitriding, sulfurizing and multi-element co-diffusion. 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, the time should be appropriately extended when the material density is greater than 7g/cm3. 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 treated material surface reach more than 2%, and the carbides are evenly distributed on the surface of the carburized layer, which can well 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 surface corrosion protection of powder metallurgy materials, its effective period is more obvious than that of bluing treatment, and the hardness and wear resistance of the treated materials are significantly increased.

4. Special heat treatment process

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 induced eddy current, the heating temperature rises quickly, and it has a significant effect on the increase of surface hardness, but it is easy to have soft spots. Generally, intermittent heating method can be used to extend the austenitization time; laser surface hardening process uses laser as heat source to quickly heat up and cool the metal surface, so that the substructure inside the austenite grain has no time to recover and recrystallize to obtain ultrafine structure.

03 Analysis of factors affecting heat treatment of powder metallurgy materials

The pores generated during the sintering process of powder metallurgy materials are their inherent characteristics, which also have 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 alloying elements also have a certain impact on the heat treatment:

1. The impact of pores on the heat treatment process

During the heat treatment of powder metallurgy materials, the austenite diffusion is inhibited by rapid cooling to transform into other tissues, thereby obtaining martensite, and the presence of pores has a great 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 hardenability decreases with the increase of porosity. 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 influence of density, which reduces the surface hardness of the material. Moreover, due to the presence of pores, salt water cannot be used as a medium during quenching to avoid corrosion caused by salt residue. Therefore, general heat treatment is carried out in a vacuum or gas medium.

2. The influence 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 quickly penetrate through the gaps. When carburizing and hardening are carried out, the carburizing depth is increased and the surface hardening effect will be reduced. Moreover, if the carburizing gas infiltrates too quickly, soft spots will be generated during quenching, reducing the surface hardness and causing the material to become brittle and deform.

3. The influence of alloy content and type on powder metallurgy heat treatment

Copper and nickel are common alloy elements, and their content and type will affect the heat treatment effect. The heat treatment hardening depth

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 lead to uneven austenite structure.

4. Effect of high temperature sintering

Although high temperature sintering can achieve the best alloying effect and promote densification, different sintering temperatures, especially at low temperatures, will lead to a decrease in heat treatment sensitivity (reduction of alloy in solid solution) and a decrease in mechanical properties. Therefore, high temperature sintering, assisted by a sufficient reducing atmosphere, can achieve better heat treatment effects.

04 Conclusion

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. To obtain higher hardenability, the complete austenitization temperature must be increased and the time must be extended. Uneven austenite carburization can obtain a high carbon concentration that is not limited by the saturated carbon concentration of austenite. In addition, the addition of alloying elements can also improve hardenability. Steam treatment can significantly improve its corrosion resistance and surface hardness.

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