Method for regulating and controlling carbon distribution in hard alloy sintering process

Abstract

The invention discloses a method for regulating and controlling carbon distribution in a hard alloy sintering process, which mainly comprises the following steps of: 1) filling argon into a sintering furnace, controlling the flow of the argon to be 30-100L / min, and regulating the pressure in the furnace to be 100mbar-900mbar, so that micro negative pressure is formed in the sintering furnace; (2) when the temperature in the furnace rises to 300 DEG C, the volatilization temperature of paraffin under the micro-negative pressure condition is reached; setting upper and lower limits of pressure in the furnace, and controlling the pressure in the furnace to oscillate and change between the upper and lower limits through pressure feedback; and (3) the temperature in the furnace is gradually increased to 600 DEG C in a stepped heating and heat preservation mode, and paraffin contained in the hard alloy product is evenly removed. According to the method for regulating and controlling the carbon distribution in the hard alloy sintering process, the carbon content uniformity of a hard alloy product can be remarkably improved, and the use performance stability of the hard alloy is ensured; the problems that in an existing paraffin removal process, removal is not uniform, and the fluctuation of the carbon amount of a product is large are solved.

Description

Technical Field

[0001] The invention relates to the technical field of cemented carbide sintering, and in particular to a method for regulating carbon distribution in a cemented carbide sintering process. Background Art

[0002] In the field of cemented carbide, carbon content is one of the decisive factors affecting the performance of cemented carbide. Generally, for cemented carbide components of a specific formula, two-phase (WC phase and Co phase) cemented carbide can be obtained within a certain range of carbon content. When the carbon content is higher than the highest value in the two-phase range, a graphite phase will eventually precipitate in the cemented carbide after sintering. When the carbon content is lower than the lowest value in the two-phase range, a decarburized phase will eventually be produced in the cemented carbide after sintering. Both of the above situations will seriously deteriorate the performance of the cemented carbide. Even if the carbon content can be controlled within the two-phase range, from the perspective of two-phase control, they are all qualified products, but the performance of cemented carbide with low carbon content and high carbon content will be quite different, resulting in unstable product performance. Therefore, how to accurately control the carbon content in the cemented carbide preparation process is one of the key points and difficulties.

[0003] Usually, the carbon control of cemented carbide mainly consists of two aspects: initial ball milling carbon mixing in the batching stage and carbon control in the sintering process. Carbon control in the sintering process is one of the technical difficulties. This is because the preparation process of cemented carbide usually involves the process of removing the molding agent (such as paraffin or PEG). Whether the molding agent is completely removed, whether the vapor volatilized by the molding agent during the removal process is evenly distributed in the furnace chamber, whether the carbon residue formed by the cracking of the molding agent is evenly distributed in the furnace chamber, and whether the reduction reaction between carbon and oxygen in the cemented carbide blank is complete. The above factors will affect the carbon content in the products in different sintering areas in the final sintering furnace, and thus affect the performance stability of the products.

[0004] Usually, there are two main methods for removing the molding agent: hydrogen positive pressure or inert gas negative pressure removal. Patent CN101343697 discloses a hydrogen positive pressure dewaxing method: during the dewaxing process, the furnace is controlled under a slightly positive pressure hydrogen atmosphere, and the temperature is alternately raised and kept warm to 450°C in a step-by-step manner to gradually remove the volatile paraffin vapor. This process has high removal efficiency and good carbon control accuracy, but it requires the use of hydrogen, which places relatively high requirements on safety measures in the production workshop; in addition, the steam needs to be burned at the steam outlet, which not only wastes energy, but also causes certain pollution to the environment.

[0005] Patent CN1597192A discloses a negative pressure dewaxing method, in which a certain amount of argon is filled into the furnace as a carrier gas, and a mechanical pump is used to control the vacuum degree in the furnace at about -98Kpa, gradually and in stages raising the temperature in the furnace from room temperature to 600°C. Usually, a dewaxing pipe is provided at the bottom of the sintering furnace, which directly passes into the inner tank of the sintering furnace. The gaseous paraffin enters the collector through the dewaxing pipe and is condensed into a liquid or solid state to separate from the carrier gas, and the paraffin is removed and collected. The negative pressure dewaxing process has low safety requirements, but the heat transfer mechanism under negative pressure conditions is mainly radiation heat exchange, and the heat transfer efficiency is low, so the dewaxing efficiency is slow. At the same time, due to the influence of the external vacuum system, the carrier gas cannot evenly purge the material, but is quickly pumped away by the vacuum pump after entering the inner tank. It is inevitable that some areas in the furnace cannot be purged by the carrier gas, resulting in the accumulation of paraffin vapor, which leads to a serious increase in the carbon content of the product in some local locations; at the same time, the dewaxing process is also accompanied by deoxidation: that is, the oxides in the cemented carbide compacts are reduced by carbon; usually the oxygen content of each compact in the whole furnace product is different, and the amount of carbon required for reduction is also different. The above factors will cause large fluctuations in the carbon content of different parts of the whole furnace product after sintering, affecting the stability of material performance. Summary of the invention

[0006] In view of the deficiencies in the prior art, the purpose of the present invention is to provide a method for regulating the carbon distribution in the sintering process of cemented carbide. The method of the present invention can significantly improve the uniformity of the carbon content of cemented carbide products, ensure the stability of the performance of cemented carbide, and solve the problems of uneven removal and large fluctuations in the carbon content of products in the existing paraffin removal process.

[0007] The technical solution adopted by the present invention to solve the technical problem is: a method for regulating the carbon distribution in the sintering process of cemented carbide, the method mainly comprising the following steps: Step 1), filling the sintering furnace with argon gas, controlling the argon gas flow rate to 30-100 L / min, adjusting the pressure in the furnace to 100 mbar-900 mbar, so that a slight negative pressure is formed in the sintering furnace; Step 2), when the temperature in the furnace rises to 300°C, reaching the volatilization temperature of paraffin under slightly negative pressure conditions; set the upper and lower limits of the furnace pressure, and control the furnace pressure to oscillate between the upper and lower limits through pressure feedback; Step 3) Using a step-by-step heating and heat preservation method, the temperature in the furnace is gradually raised to 600°C to evenly remove the paraffin contained in the cemented carbide product.

[0008] Furthermore, in step 1), the pressure in the furnace is 600-900 mbar; and the argon flow rate is 45-100 L / min.

[0009] Furthermore, in step 2), the upper and lower limits of the pressure in the furnace are set, the upper limit of the pressure is 10-50 mbar higher than the set pressure, and the lower limit of the pressure is 10-50 mbar lower than the set pressure. Furthermore, in step 2), the oscillating change of pressure is regulated and controlled by the stop valve and pressure sensor on the dewaxing pipeline. The pressure sensor feeds back the detection result to the control device, and the control device controls the opening or closing of the stop valve according to the set pressure to achieve the oscillating change of the pressure in the furnace.

[0010] Furthermore, when the pressure in the furnace is higher than the set upper pressure limit, the stop valve is fully opened to pump out the paraffin vapor; then when the pressure drops below the set lower pressure limit, the stop valve is closed; at this time, the volatilized paraffin vapor is evenly mixed with the argon carrier gas in the furnace, and after a period of time, the pressure in the furnace exceeds the upper limit again, and the stop valve is reopened to pump it out; this is repeated until the paraffin of the products at different positions in the sintering furnace is synchronously removed.

[0011] The beneficial effects of the present invention are as follows: compared with the prior art, the method for regulating carbon distribution in the cemented carbide sintering process provided by the present invention has the following advantages: In the control method of the present invention, by controlling the furnace pressure during the dewaxing process of the cemented carbide product to be slightly negative pressure, it is beneficial to convective heat transfer, promote the temperature uniformity in the furnace, and make the paraffin of the products at different positions in the furnace be removed synchronously; combined with the oscillation change of the pressure in the furnace, after the paraffin vapor evaporates from the surface of the material, it is evenly mixed with the Ar gas carrier gas, and as the temperature rises, the paraffin vapor gradually cracks, and the carbon oxygen reaction occurs synchronously in the products at different positions in the furnace first, so that the oxygen content in the pressed blank is removed thoroughly first; thereafter, the products at different positions in the furnace are uniformly carburized; after a period of time, the pressure reaches the set upper limit, the valve is opened, and the paraffin vapor, the carbon oxygen reaction product and the carrier gas are pumped away by the vacuum system, and then the pressure drops to exceed the set lower limit, the valve is closed, and the above process is repeated to achieve the uniform, efficient and synchronous removal of the paraffin of the products at different positions in the furnace. The control method of the present invention can avoid the situation of paraffin vapor gathering in the furnace, significantly improve the uniformity of the carbon content in the cemented carbide product, carburize the cemented carbide product in the local area of ​​the furnace, and improve the product performance stability. DETAILED DESCRIPTION

[0012] The present invention is further described below by specific examples, but these examples are only used to illustrate the present invention and are not used to limit the scope of the present invention.

[0013] Example 1 is a 0.8um~10%Co submicron cemented carbide rod with a rod diameter of 4mm; Example 2 is a 0.4um~6%Co ultrafine low-cobalt cemented carbide with a rod diameter of 3.2mm. The two examples use the same implementation steps, as described below, and the different process control parameters used in the two examples are shown in Appendix 1.

[0014] A conventional pressurized sintering integrated furnace in the field of cemented carbide sintering is used, and the furnace is ready for sintering according to the conventional process flow. First, the sintering furnace and the dewaxing pipeline are evacuated, and a vacuum leak detection is performed using the pressure rise method. After the leak detection is qualified, argon gas is filled into the furnace, and the argon gas flow rate is fixed at 45~100SLM (liters / minute). At the same time, the opening and closing degree of the dewaxing pipeline pressure regulating valve is adjusted to gradually increase the pressure in the furnace and stabilize at 600~900mbar. This pressure is slightly negative relative to the standard atmospheric pressure, which is conducive to convective heat transfer in the furnace, thereby improving the heat transfer efficiency of the material and reducing the temperature equalization time.

[0015] Then the temperature in the furnace is stepped up and kept warm to 600℃ in several stages. When the furnace temperature gradually increases from room temperature to 300℃ and reaches the volatilization temperature of paraffin under slightly negative pressure, paraffin begins to volatilize from the surface of the material. At this time, the upper and lower limits of the furnace pressure are set. At the same time, the dewaxing pipeline is also equipped with a pressure control stop valve. When the pressure in the furnace is higher than the set upper pressure limit, the stop valve is fully opened to extract the paraffin vapor; then the pressure drops to below the set lower pressure limit, and the stop valve is closed. Then, when the volatilized paraffin vapor is evenly mixed with the argon carrier gas in the furnace, after a period of time, the pressure in the furnace exceeds the upper limit again, and the stop valve is reopened to extract it.

[0016] During the entire heating and insulation process, the pressure is set to a slightly negative pressure, and through pressure oscillation, the paraffin in the products at different positions in the furnace is uniformly volatilized in the form of "steam", and after being relatively evenly mixed in the furnace, it is discharged from the furnace through the dewaxing pipe along with the argon flow, thereby achieving the reduction of the combined oxygen of the products at different positions in the furnace, the uniform and synchronous removal of the paraffin molding agent, and the uniform carbon increase of the paraffin vapor on the products at different positions in the furnace.

[0017] After the wax forming agent is removed, the temperature is raised for vacuum sintering. Argon is filled in the later stage of vacuum sintering for argon pressure sintering. After the argon pressure sintering is completed, it is naturally cooled and the product is taken out of the furnace after cooling. The above vacuum sintering, pressure sintering, cooling and furnace removal processes are similar to the currently known technologies.

[0018] The carbon content of cemented carbide has a good correspondence with its saturation magnetization. For cemented carbide products of a fixed grade, relative magnetic saturation is usually used to characterize its carbon content. The smaller the fluctuation of relative magnetic saturation, the smaller the fluctuation of carbon content of the alloy product.

[0019] The 0.8um-10wt.%Co submicron cemented carbide prepared in the above Example 1 was taken from products at different positions of the sintering furnace for magnetic property uniformity detection, and the relative magnetic saturation fluctuation value was 3% (maximum value - minimum value); for the 0.4um-6wt.%Co ultrafine low-cobalt cemented carbide prepared in Example 2, the products at different positions of the sintering furnace were taken for magnetic property uniformity detection, and the relative magnetic saturation fluctuation value was 6% (maximum value - minimum value). Appendix 1: Comparison of different process control parameters used in two embodiments

[0020] The above implementation modes are only used to illustrate the present invention, but not to limit the present invention. Ordinary technicians in the relevant technical field can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, all equivalent technical solutions also belong to the scope of the present invention. The patent protection scope of the present invention should be defined by the claims.

Claims

1. A method for regulating carbon distribution in a cemented carbide sintering process, characterized in that: The method mainly comprises the following steps: Step 1), filling the sintering furnace with argon gas, controlling the argon gas flow rate to 30-100 L / min, adjusting the pressure in the furnace to 100 mbar-900 mbar, so that a slight negative pressure is formed in the sintering furnace; Step 2), when the temperature in the furnace rises to 300°C, reaching the volatilization temperature of paraffin under slightly negative pressure conditions; set the upper and lower limits of the furnace pressure, and control the furnace pressure to oscillate between the upper and lower limits through pressure feedback; Step 3) Using a step-by-step heating and heat preservation method, the temperature in the furnace is gradually raised to 600°C to evenly remove the paraffin contained in the cemented carbide product.

2. A method for regulating carbon distribution in a cemented carbide sintering process according to claim 1, characterized in that: In step 1), the pressure in the furnace is 600-900 mbar; and the argon flow rate is 45-100 L / min.

3. The method for regulating carbon distribution in a cemented carbide sintering process according to claim 1, characterized in that: In step 2), the upper and lower limits of the pressure in the furnace are set, the upper limit of the pressure is 10~50mbar higher than the set pressure, and the lower limit of the pressure is 10~50mbar lower than the set pressure.

4. The method for regulating carbon distribution in a cemented carbide sintering process according to claim 1, characterized in that: In step 2), the oscillating change of pressure is regulated and controlled by the stop valve and pressure sensor on the dewaxing pipeline. The pressure sensor feeds back the detection result to the control device, and the control device controls the opening or closing of the stop valve according to the set pressure to achieve the oscillating change of the pressure in the furnace.

5. A method for regulating carbon distribution in a cemented carbide sintering process as claimed in claim 4, characterized in that: When the pressure in the furnace is higher than the set upper pressure limit, the stop valve is fully opened to pump out the paraffin vapor; then when the pressure drops below the set lower pressure limit, the stop valve is closed; at this time, the volatilized paraffin vapor is evenly mixed with the argon carrier gas in the furnace, and after a period of time, the pressure in the furnace exceeds the upper limit again, and the stop valve is reopened to pump it out; this is repeated until the paraffin vapor of the products at different positions in the sintering furnace is synchronously removed.

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