What Is Sintered Steel?

Sintered steel is one of the most important iron-based component materials in the powder metallurgy industry with the largest output and the widest application area. The annual demand has exceeded 100,000 tons. Sintering is a process of agglomerating particles in a powder aggregate through diffusion under high temperature heating. The manufacture of sintered steel parts is a contribution made by powder metallurgy.

The most commonly used method for the production of sintered steel products is metal injection molding. That is, very small steel powder is mixed with a certain proportion of plastic binder, and then injected into a mold to become a semi-finished product of any shape, such as gears, bearings or Case. This process is similar to plastic injection molding, but the injection temperature is slightly higher, but still below 200 ° C. The semi-finished product is then placed in a high-temperature furnace at a temperature of more than 1000 ° C, which burns out the plastic adhesive and leaves the steel behind.

The performance of sintered steel is similar to that of ordinary steel materials, which mainly depends on the density of the material, the content of combined carbon, the type and quantity of alloy elements, and the state of structure. The biggest difference between sintered steel and steel materials is that there are pores in the former structure, and the density can be arbitrarily adjusted within a certain range. Therefore, the material density is one of the important parameters affecting the performance of sintered steel. When selecting a sintered steel grade, the material density must first be determined. The powder metallurgy method can be used to manufacture sintered steel with various properties from low density to completely dense according to the requirements of use. Characteristics of heat treatment of sintered steel

(1) the effect of porosity on material properties

Powder metallurgy materials contain pores, making their density lower than the theoretical value, and changing the material's thermal conductivity, ferromagnetic,

Chemical composition, structure and properties of sintered steel Adding alloying elements to sintered steel can improve the metallographic structure and pore morphology of the material. Microstructure change law of sintered steel, basically following iron-carbon

Heat treatment of sintered steel, chemical heat treatment, steam treatment and electroplated sintered steel products, and overall heat treatment (quenched at austenitizing temperature and tempered at 200-280 ° C). The heat treatment of sintered steel is basically the same as that of smelted dense steel, but due to the existence of pores and the particularity of alloying, the heat treatment of iron-based structural materials has the following characteristics:

1) The density has a great influence on the properties of the material after heat treatment. The higher the density, the better the heat treatment effect and the more stable the performance.

2) Due to the effects of pores. The apparent hardness of the material is low. However, the micro hardness of the structure is not low. In order to reflect the hardness of the material itself, a load of 1.96N or less should be selected.

Process factors for sintered steel

When a variety of materials with different compositions and densities can meet the requirements for use, the material with the lowest density should be selected as much as possible. Because increasing the density will inevitably increase the pressing pressure, speed up the mold consumption and increase the process. Generally, when the density is less than 6.59 / cm ³ , it can be achieved by one pressing. When the density is 6.5 6.99 / cm ³ , iron powder with good compressibility needs to be selected. When the density is 6.9 7.49 / cm ³ , it is necessary to increase the steps of re-pressing and re-firing, hot re-pressing, and copper immersion, or adopting a warm-pressing process. When the density is greater than 7.49 / cm ³ , a hot forging process or an injection molding process is required. When a heat treatment process is required to improve the strength and hardness of the part, a material density of more than 6.69 / cm ³ should be selected to avoid excessive porosity and reduce thermal conductivity to affect the hardenability of the material.

Hardness characteristics of sintered steel

When measuring the apparent hardness of iron-based structural materials, the indenter acts on the composite of the metal matrix and pores of the material, and the apparent hardness value is lower than that of dense materials of the same composition and structure. However, the microhardness of the metal matrix may not be lower than that of the dense material (with the same composition), or it may be higher than that of the dense material (when the composition is different). Most structural materials that require hardness have low local compressive stress in use and are mainly required for wear resistance. Therefore, powder metallurgical materials with low apparent hardness often have better wear resistance than dense materials with high apparent hardness. It should be noted that the requirements for material hardness when selecting materials should be different from the requirements for dense materials in general, that is, when the same effect is used, powder metallurgical materials are allowed to have a lower apparent hardness. In addition, the hardness of the HV with a smaller load can better reflect the surface condition of the material than the hardness of the HRC. In addition, the non-uniformity of the density distribution and the imbalance of the various parts of the alloying effect increase the fluctuation range of the apparent hardness of the powder metallurgy material. When setting the hardness requirements, it is necessary to allow a wider hardness than the dense material. The range of fluctuation.

Density distribution of sintered steel parts

During the forming of powder metallurgy structural parts, due to factors such as mold wall friction and poor powder fluidity, the density unevenness of the product is caused, and the physical and mechanical properties of different parts of the part are different. For parts with strict performance requirements, the allowable difference in segmented density of different parts of the part must be specified, and the minimum allowable density for the main working parts of the part, such as the maximum stress, friction, and magnetically conductive parts. ^[2]