A temperature calibration method for a push boat type high temperature graphite tube furnace

Abstract

The invention discloses a temperature calibration method of a push-boat high-temperature graphite tube furnace, 1), using a WC-Co RTP mixture to prepare a standard pressed green sheet; 2), dewaxing the standard pressed green sheet and measuring the actual size of the standard pressed green sheet; 3), raising the temperature in the graphite tube furnace to the initial temperature, pushing the standard pressed green sheet from the furnace head to the furnace tail at a constant speed, and measuring the shrinkage, coercive force and specific saturation magnetization of the standard pressed green sheet after sintering; 4), raising the temperature in the graphite tube furnace at a certain gradient, and repeating step 3) for each increase in a gradient; 5), recording the shrinkage, coercive force and specific saturation magnetization data of the standard pressed green sheet obtained in the above step 4), drawing a relationship curve between temperature and shrinkage, and a relationship curve between the product ω of the coercive force and the specific saturation magnetization and the temperature, for temperature calibration. The present invention utilizes the product ω of the coercive force and the specific saturation magnetization to avoid the influence of the carbon atmosphere in the furnace on the magnetic properties of the standard pressed green sheet.

Description

Technical Field

[0001] The invention relates to the technical field of production of cemented carbide and powder metallurgy, and in particular to a temperature calibration method for a push-boat type high-temperature graphite tube furnace. Background Art

[0002] At present, the main equipment for producing tungsten carbide is a push-boat high-temperature graphite tube furnace, which is due to its high temperature, fast heating speed and simple structure. In the process of producing tungsten carbide, the carburization temperature is one of the most important process parameters. The carburization temperature is closely related to the grain size and grain distribution of tungsten carbide. This common graphite tube furnace temperature control adopts a closed-loop control temperature control system, which is composed of an infrared thermometer, a PID instrument, a power regulator, and a transformer.

[0003] Based on the above existing temperature control technology, the main problems are as follows: after the graphite tube furnace works at high temperature for a period of time, ash containing a small amount of impurities such as Si, Al, Fe, Ca, and Mg will be produced. Since one end of the temperature measurement channel is open and connected to the furnace tube, the ash is easy to interfere with the temperature measurement structure, resulting in deviations in the furnace tube temperature control. In addition, as the graphite tube furnace is used for a longer time, the graphite furnace tube will be worn out, and the temperature will be too high or too low. However, most of the existing temperature control technologies are single-point temperature control, which cannot reflect the local temperature being too high or too low. For example, CN112624117A discloses a method for preparing a special temperature measuring block for a high-temperature carbonization furnace and a method for calibrating the temperature thereof. Tungsten powder and carbon black are mixed to obtain a W+C mixture, which is pressed into a standard block and carbonized at different temperatures to obtain a temperature and shrinkage comparison chart for subsequent temperature calibration. However, the main defect of this invention is that the formability of the W+C mixture is relatively poor, which is not conducive to press molding, resulting in a large error in measuring the size of the standard block before shrinkage, and the standard block after sintering is prone to bending and deformation, resulting in errors in the calculation of the subsequent shrinkage rate. CN113218529A discloses a temperature measurement method based on cemented carbide, which uses cemented carbide samples with the same performance (magnetic saturation) and the same shape, and sinters them twice at different temperatures. The relationship between the coercive force and the temperature after the secondary sintering is used to perform temperature calibration. This invention is not suitable for a push-boat high-temperature graphite tube furnace. The carbon atmosphere in this graphite tube furnace is high, which will affect the magnetic saturation of the cemented carbide sample, and thus cannot play a role in temperature calibration. In summary, based on the existing technology, it is impossible to calibrate these temperature inaccuracies. Summary of the invention

[0004] In view of the shortcomings of the prior art, the purpose of the present invention is to provide a method for calibrating the furnace temperature of a push-boat type high-temperature graphite tube furnace by utilizing the shrinkage rate, coercive force and specific saturation magnetization of cemented carbide, which can reflect the overall temperature conditions in the furnace and the phenomenon of local temperature being too high or too low.

[0005] The technical solution adopted by the present invention to solve the technical problem is: a temperature calibration method for a push boat type high temperature graphite tube furnace, the calibration method mainly includes the following contents:

[0006] Step 1), using WC-Co RTP mixture as raw material to prepare a standard compact of a certain size;

[0007] Step 2), after dewaxing pretreatment of the standard compact, measuring the actual size of the standard compact;

[0008] Step 3), after the temperature in the graphite tube furnace is raised to the initial temperature, the standard green compact is pushed from the furnace head to the furnace tail at a constant pushing speed to ensure that the sintering time in the furnace is fixed, and the shrinkage rate, coercive force and specific saturation magnetization intensity of the standard green compact after sintering are measured;

[0009] Step 4), the temperature in the graphite tube furnace is increased at a certain gradient, and the operation in the above step 3) is repeated every time the gradient is increased;

[0010] Step 5), record the shrinkage rate, coercive force and specific saturation magnetization data of the standard pressed blank within a certain temperature range obtained in the above step 4), draw a curve of the relationship between temperature and shrinkage rate, and a curve of the relationship between the product of coercive force and specific saturation magnetization ω and temperature for temperature calibration.

[0011] Furthermore, in the step 3), the initial temperature in the graphite tube furnace is 1000° C., and the sintering time is 30 to 120 minutes.

[0012] Furthermore, in the step 4), the temperature gradient in the graphite tube furnace is 1 to 100°C.

[0013] Furthermore, in the step 5), the measured temperature range is 1000-1800°C.

[0014] Furthermore, the material of the tube furnace is not limited to graphite tube, and also includes a carbon tube furnace using resistance heating, an intermediate frequency furnace using intermediate frequency heating, and a molybdenum wire furnace using molybdenum wire heating.

[0015] Furthermore, the size of the standard compact is convenient for measurement, including a cube, a cuboid, and a cylinder.

[0016] WC-Co cemented carbide adopts liquid phase sintering. The pre-sintering and heating stage is solid phase sintering below the eutectic temperature. At 1200℃, liquid phase begins to form between Co particles and between Co and WC particles. WC begins to diffuse rapidly into Co, and the diffusion rate accelerates as the temperature rises. In the sintering and heat preservation stage, WC continues to dissolve into the liquid phase. In this stage, only the dissolution and reprecipitation of WC through the liquid phase occur. The WC grains gradually grow, and the composition and proportion of the two phases remain unchanged. When cooling after the insulation is completed, WC precipitates from the liquid phase, and the amount of liquid phase decreases. When it is below the eutectic temperature, secondary WC crystals continue to precipitate. It can be seen from the above that the shrinkage of WC-Co alloy is related to temperature. When the temperature is higher than 1200℃, the amount of liquid phase increases sharply, which accelerates the speed of WC particle rearrangement, dissolution and precipitation, thereby causing the alloy to shrink significantly. Therefore, the characteristic of the sensitivity of the shrinkage rate of WC-Co alloy to temperature can be used to calibrate the temperature of the graphite tube furnace.

[0017] Since the common push boat type high temperature graphite tube furnace is used to produce tungsten carbide, the furnace always maintains a high carbon atmosphere, and the concentration of carbon atmosphere varies between different furnaces, so that cemented carbide occasionally has different degrees of carburization during sintering in the furnace, which will increase the specific saturation magnetization intensity of cemented carbide samples. According to the formula of Swedish S.Sundin: Hc*Ms∝S wc / S Co , where Hc is the coercive force of the alloy, Ms is the saturation magnetization of the alloy, S wc / S Co is the interface area between WC and Co in the alloy. When the same WC-Co RTP mixture is used to make a standard compact, and sintered at the same graphite tube furnace temperature and sintering time, the grain growth of the alloy is the same, and the interface area between WC and Co is equal, that is, S wc / S Co is a constant. According to the above formula, Hc*Ms is a constant. Therefore, the product of the coercive force and the specific saturation magnetization of the WC-Co alloy, ω, can be used to avoid the influence of the carbon atmosphere in the furnace on the magnetic properties of the standard compact, and the relationship between ω and temperature can be used to calibrate the temperature of the graphite tube furnace.

[0018] The technical solution provided by the invention utilizes the shrinkage rate, coercive force and specific saturation magnetization of the WC-Co alloy, and combines the three properties to perform temperature calibration on the graphite tube furnace.

[0019] The beneficial effects of the present invention are as follows: compared with the prior art, the calibration method provided by the present invention has the following advantages:

[0020] 1) The raw material used is a WC-Co RTP mixture, which is mixed with paraffin as a molding agent and has good moldability; the temperature measurement sample only needs to prepare a standard compact of the same shape, which is easier to obtain than alloys with the same performance;

[0021] 2) Temperature calibration is performed by combining shrinkage rate with magnetic properties, using the performance of WC-Co cemented carbide sintering; the product of coercive force and specific saturation magnetization intensity ω can avoid the influence of carbon atmosphere in the furnace on the magnetic properties of standard compacts. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is a curve showing the relationship between the sintering temperature and the shrinkage rate of the standard green compact provided in the examples.

[0023] Figure 2 1 is a curve showing the relationship between the sintering temperature of the standard green compact provided in the examples and the product of the coercive force and the specific saturation magnetization ω. DETAILED DESCRIPTION

[0024] 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.

[0025] Example 1

[0026] (1) Using WC-Co RTP mixture as raw material, 20g was selected and pressed into a standard green compact of about 20mm*15mm*8mm;

[0027] (2) After dewaxing pretreatment of the standard pressed green sheet, the actual size of the sample block is measured;

[0028] (3) The graphite tube furnace is heated to 1050°C and kept warm for 30 minutes. The three pre-treated standard green sheets are placed side by side in the same graphite boat. The graphite boat is pushed from the furnace head to the furnace tail at a fixed pushing speed to ensure that the sintering temperature in the furnace is fixed at 65 minutes. The shrinkage rate, coercive force and specific saturation magnetization of the sintered standard green sheets are measured. The average value of the test results of the three samples is taken to draw a relationship curve.

[0029] (4) After pushing out, heat the graphite tube furnace to 1055°C, keep it warm for 30 minutes, and then proceed with the same steps as described in (3);

[0030] (5) Repeat the steps described in (4) with a gradient of 5°C until the temperature rises to 1180°C, and obtain a relationship curve between temperature and shrinkage rate, such as Figure 1 As shown, the dotted line in the figure is the fitted trend line.

[0031] Example 2

[0032] (1) Using WC-Co RTP mixture as raw material, 20g was selected and pressed into a standard green compact of about 20mm*15mm*8mm;

[0033] (2) After dewaxing pretreatment of the standard pressed green sheet, the actual size of the sample block is measured;

[0034] (3) The graphite tube furnace is heated to 1300°C and kept at this temperature for 30 minutes. The three pre-treated standard compacts are placed side by side in the same graphite boat. The graphite boat is pushed from the furnace head to the furnace tail at a fixed pushing speed to ensure that the sintering temperature in the furnace is fixed at 65 minutes. The shrinkage rate, coercive force and specific saturation magnetization of the sintered standard compact are measured. The average value of the test results of the three samples is taken to draw a relationship curve.

[0035] (4) After pushing out, heat the graphite tube furnace to 1305°C, keep it warm for 30 minutes, and then proceed with the same steps as described in (3);

[0036] (5) Repeat the steps described in (4) with a gradient of 5°C until the temperature rises to 1430°C, and obtain a curve of the relationship between the product of coercive force and specific saturation magnetization ω and temperature, as shown in Figure 2 As shown, the dotted line in the figure is the fitted trend line.

[0037] Using the standard curves obtained in Examples 1 and 2 above Figure 1 and Figure 2 As a standard, perform temperature calibration. The details are as follows:

[0038] The temperature of a push boat high temperature graphite tube furnace was raised to 1110℃, and 20g of WC-Co RTP mixture was selected to be pressed into a standard green sheet. The size of the standard green sheet after dewaxing was measured to be 20.05mm*15.04mm*7.76mm, and the size after sintering was 18.23mm*13.67mm*7.05mm. The shrinkage rate was calculated to be 9.10%. According to the relationship curve between temperature and shrinkage rate ( Figure 1 ), the corresponding temperature is 1120℃, and the temperature display of the furnace temperature needs to be compensated by +10℃.

[0039] The temperature of a push-boat high-temperature graphite tube furnace was raised to 1350℃, and 20g of WC-Co RTP mixture was selected to be pressed into a standard green sheet, which was then dewaxed and sintered. The Ms% after sintering was 93.3, the Hc was 111Oe, and the calculated ω was 10356. According to the relationship curve between the product of coercive force and specific saturation magnetization ω and temperature ( Figure 2 ), the corresponding temperature is 1350℃, then the furnace temperature is accurate and no calibration is required.

[0040] 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 temperature calibration method for a push boat type high temperature graphite tube furnace, characterized in that: The calibration method mainly includes the following contents: Step 1), using WC-Co RTP mixture as raw material to prepare a standard compact of a certain size; Step 2), after dewaxing pretreatment of the standard compact, measure the actual size of the standard compact; Step 3), after the temperature in the graphite tube furnace is raised to the initial temperature, the standard compact is pushed from the furnace head to the furnace tail at a constant pushing speed, and the shrinkage rate, coercive force Hc and specific saturation magnetization Ms of the sintered standard compact are measured; Step 4), the temperature in the graphite tube furnace is increased at a certain gradient, and the operation in the above step 3) is repeated every time the gradient is increased; Step 5), record the shrinkage rate, coercive force and saturation magnetization data of the standard compact within a certain temperature range obtained in the above step 4), draw a curve of the relationship between temperature and shrinkage rate, and a curve of the relationship between the product of coercive force and saturation magnetization and temperature for temperature calibration.

2. The temperature calibration method of a push boat type high temperature graphite tube furnace according to claim 1, characterized in that: In the step 3), the initial temperature in the graphite tube furnace is 1000° C., and the sintering time is 30 to 120 minutes.

3. The temperature calibration method of a push boat type high temperature graphite tube furnace according to claim 1, characterized in that: In the step 4), the temperature gradient in the graphite tube furnace is 1-100°C.

4. The temperature calibration method of a push boat type high temperature graphite tube furnace according to claim 1, characterized in that: In the step 5), the temperature range of the measurement is 1000-1800°C.

5. The temperature calibration method of a push boat type high temperature graphite tube furnace according to claim 1, characterized in that: The material of the tube furnace is not limited to graphite tube, and includes a carbon tube furnace using resistance heating, an intermediate frequency furnace using intermediate frequency heating, and a molybdenum wire furnace using molybdenum wire heating.

6. The temperature calibration method of a push boat type high temperature graphite tube furnace according to claim 1, characterized in that: The size of the standard compact is convenient for measurement, including a cube, a cuboid, and a cylinder.

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