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Effect of vacuum sintering process on the properties of cemented carbide

Cemented carbide refers to the use of high hardness, high elastic modulus and refractory metal carbides (such as TiC, WC, etc.) as the matrix, and then using transition metals (usually Fe, Ni, Co, etc.) as the binder , a multi-phase composite material prepared by powder metallurgy. The composite material has various advantages, such as high hardness, good wear resistance, good thermal stability, etc. Therefore, among today’s tool materials, high temperature resistant materials, wear resistant materials and corrosion resistant materials have gained the most Widely used, known as the “teeth of industry”. The ultra-fine WC-Co cemented carbide has a smaller grain size. Compared with ordinary cemented carbide with the same composition, its Rockwell hardness HRA is 1.5~2MPa higher. At room temperature, its resistance to The flexural strength is 700-1500MPa higher. The high-temperature hardness is much better than that of ordinary alloys, so it can shine and be widely used in difficult-to-machine materials and even high-tech fields.

High-hardness carbide (HRA 86 ~ 93) has high strength, good wear resistance and toughness, high temperature resistance, corrosion resistance and other excellent physical and chemical properties. It can not only be used in various hardware tools, cutting tool materials, precision bearings, Wear-resistant parts and cylinder liners are also widely used in mining and geological drilling operations with mechanical crushing tools, including hammers, drill bits, scrapers, knives, drill bits, ball tooth plates, glasses and blow hammers. Therefore, it is a key component of mine tunnel engineering in the world. It plays one of the vital materials in oil exploration, geological exploration and coal mining.

Sintering process: The ultra-fine cemented carbide mixture has a large specific surface area and high activity, and the grains tend to grow during the sintering process. In order to suppress the growth of WC grains, the sintering temperature is lower than that of traditional cemented carbide. At the same time, its density is small and its porosity makes it difficult to eliminate pores in the alloy during the sintering process, and the overall properties of this alloy are very sensitive. If you want to reduce the porosity, you must use a more advanced sintering method, that is, low-pressure hot isostatic pressing sintering (often called sintering process, sintering pressure) is a widely used production practice for ultra-fine cemented carbide. The step of low-pressure sintering means that during sintering, Ar gas is introduced and the gas pressure is increased to 5-6MPa. Under such high pressure conditions, the liquid phase flow becomes easier, thus making the distribution of the cobalt phase more uniform. , reducing the pores in it and effectively eliminating cobalt pools, thereby improving the strength of its alloy. Repeated sintering tests were conducted on the sintering process to study ultra-fine cemented carbide. Comparison of the physical properties of ultra-fine cemented carbide vacuum sintering, pressure sintering and hot isostatic pressing sintering and performance, statistical results through 109 material tests , after pressure sintering, the flexural strength is increased by about 18% on average. According to the statistical results of 106 tests, after hot isostatic pressing, the strength is increased by 23% on average.

Vacuum sintering

The method is to place the green compact in a vacuum sintering furnace. The first is sintering under vacuum conditions, after the furnace cavity temperature reaches. After the holding time is continuously extended. The shrinkage rate of the specimen is greatly reduced. This shows that the sample has basically completed shrinkage in the vacuum sintering state. Applying 3-6MPa pressure in argon or nitrogen can cause the sample to shrink significantly. Therefore, the ultimate pressure sintered body of the sample plays an important role in improving the structure and eliminating residual pores.

Vacuum sintering furnace

Vacuum furnaces are capable of performing heat treatment, sintering and chemical reactions on materials under vacuum conditions. They are divided into two types: vertical and horizontal. The heating chamber is usually composed of multiple layers of insulated carbon felt. The heating components are mainly graphite materials. The maximum working temperature can even reach 2200°C. If a three-stage high vacuum pump unit is used, its ultimate cold vacuum degree will be even better. Reach 10^-3Pa. When tungsten and molybdenum materials are used as heat shields and heating elements, the vacuum degree can reach 10^-5Pa.

This experimental device is a furnace produced by SIMUWU, with an effective working size of 300×300mm, using molybdenum plates as heat shields, an ultimate vacuum of 6.7×10^-5Pa, a heating power of 40kW, and a maximum design temperature of 2400°C.

Vacuum sintering process of WC-Co cemented carbide

Vacuum sintering is to mix the powder with a binder, press it into blocks, and then sinter it in a vacuum furnace. The vacuum sintering process of WC-Co cemented carbide is roughly as follows:

(1) Pre-sintering, pre-sinter the WC-Co powder at 350℃~800℃;

(2) WC-Co powder sintering stage. Traditional sintering consists of two stages. One is solid-phase sintering with a sintering temperature of 800°C, which is the eutectic temperature. The other is the existence of the liquid phase after liquid-phase sintering. By referring to the literature, 1420°C was selected as the sintering temperature of WC-Co in this experiment.