Method for improving steel CCT curve testing efficiency by transforming sample size

Abstract

The invention discloses a method for improving the steel CCT curve test efficiency by transforming the sample size, which specifically comprises the following steps: 1) machining a sample: machining the sample to be tested into phi 3 * 10 mm, and forming a conical small hole with the bottom surface diameter of 1.5 mm and the height of 2 mm at one end of the sample; 2) welding: welding the platinum-platinum-rhodium ends of the thermocouple together through instantaneous ignition of a carbon rod to form a round head shape; welding a thermocouple round head at the bottom end of the conical small hole at one end of the sample; 3) loading a sample; 4) setting a test program; 5) data processing; (6) observing a metallographic structure; and 7) drawing a CCT curve. According to the method for improving the steel CCT curve testing efficiency by transforming the sample size, the CCT curve testing sample is transformed, the welding success rate is increased, and therefore the CCT curve testing efficiency is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of metal material testing, and particularly relates to a method for improving the testing efficiency of the CCT curve of steel by modifying the specimen size. Background Art

[0002] The CCT curve, namely the continuous cooling transformation curve, is a curve that describes the phase transformation law of steel materials during continuous cooling. By analyzing the CCT curve, the phase transformation temperature, phase transformation time, and phase transformation products of steel materials at different cooling rates can be determined, thereby providing a basis for formulating a reasonable heat treatment process. In particular, the CCT curve provides necessary technical data for the development of new steel grades, guides the development of new processes, and promotes the innovation and improvement of the production process of steel materials by studying the CCT curves under different compositions and process conditions.

[0003] Generally, the testing of the CCT curve requires equipment such as a thermal simulation testing machine, a phase transformation instrument, a high-precision heat treatment furnace, a temperature controller, etc. The phase transformation instrument can be used to test the static CCT curve, that is, the specimen is measured without deformation, which is suitable for studying the phase transformation law of steel during continuous cooling. When the phase transformation instrument tests the specimen, the specimen needs to be welded to platinum-platinum rhodium (positive-negative) thermocouples with a diameter of 0.2 mm respectively, and signals such as the temperature, expansion, and contraction of the specimen are transmitted and fed back to the recording system through the thermocouples. However, when welding the thermocouples to the specimen, the low welding success rate leads to a reduction in the testing efficiency of the CCT curve. Especially when welding specimens with poor welding performance, multiple weldings are required, which also increases the heat loss of the precious metal thermocouples. Therefore, it is necessary to innovate in the specimen processing method and modify the specimen to make it easy to weld. Summary of the Invention

[0004] The purpose of the present invention is to provide a method for improving the testing efficiency of the CCT curve of steel by modifying the specimen size, modifying the specimen for testing the CCT curve, improving the welding success rate, and thus improving the testing efficiency of the CCT curve.

[0005] To solve the above technical problems, the present invention adopts the following technical solutions:

[0006] A method for improving the testing efficiency of the CCT curve of steel by modifying the specimen size according to the present invention specifically includes the following steps:

[0007] 1) Machining the specimen: The specimen to be tested is machined into Φ3*10 mm, and a conical small hole with a bottom diameter of 1.5 mm and a height of 2 mm is opened at one end of the specimen;

[0008] 2) Welding: The two ends of the platinum-platinum rhodium of the thermocouple are instantaneously welded together by a carbon rod to form a round head shape; then the round head of the thermocouple is welded to the bottom end of the conical small hole at one end of the specimen;

[0009] 3) Sample loading: Put the assembled thermocouple and the specimen into the sample chamber of the phase transformation instrument together and evacuate it.

[0010] 4) Setting the test procedure: Set the thermal simulation tests at different cooling rates according to the requirements of CCT curve testing.

[0011] 5) Data processing: Calculate the parameters of the phase transformation start temperature and end temperature at different cooling rates.

[0012] 6) Microstructure observation: Prepare the specimens obtained at different cooling rates for metallographic examination, polishing, and etching, and observe the microstructure using a metallographic microscope.

[0013] 7) Drawing the CCT curve: Connect the phase transformation start temperature and end temperature respectively to obtain different tissue regions, and finally draw the complete CCT curve of the test specimen.

[0014] Furthermore, in step 4), it includes at least the parameters of heating temperature, heating rate, holding time, and cooling rate.

[0015] Furthermore, in step 7), it also includes: Combining with the metallographic microstructure, it is necessary to supplement and encrypt the cooling rate test to accurately calculate the phase transformation start temperature and end temperature at different cooling rates.

[0016] Furthermore, set the test procedure according to the following requirements: Set the austenitizing temperature to 900 °C, hold for 10 minutes, and cool to room temperature at cooling rates of 0.5 °C / S, 1 °C / S, 5 °C / S, 10 °C / S, and 30 °C / S respectively, and record the temperature-expansion change curve.

[0017] Furthermore, use the tangent method on the temperature-expansion change curve to calculate the phase transformation start temperature and end temperature at different cooling rates.

[0018] Furthermore, combining with the metallographic microstructure, supplement the cooling rate test at 3 °C / S and 50 °C / S to accurately calculate the phase transformation start temperature and end temperature at different cooling rates.

[0019] It also provides the application of the method for improving the test efficiency of the steel CCT curve by modifying the specimen size in improving the test efficiency of the steel CCT curve.

[0020] Compared with the prior art, the beneficial technical effects of the present invention:

[0021] The method of the present invention is demonstrated by experiments. The optimal scheme is that the diameter of the conical small hole of the specimen is 1.5 mm. The improved specimen size is easy to weld, has good stability, reduces the loss of precious metal thermocouples, and improves the test efficiency of the steel CCT curve. Description of the Drawings

[0022] The present invention will be further described below in conjunction with the accompanying drawings.

[0023] Figure 1 It is a schematic diagram of the specimen size (the size change of the present invention is reflected in ΦA).

[0024] Figure 2 It is the measured temperature-expansion curve at 1 °C / s.

[0025] Figure 3 It is the CCT curve graph measured in the example. Specific embodiments

[0026] Example:

[0027] For seamless steel pipes of the Cr-Mo-V system produced by a certain steel mill, according to their chemical composition characteristics, the austenitizing temperature is set at 900 °C, held for 10 minutes, cooled to room temperature at different cooling rates respectively, and the phase transformation points are measured to obtain the CCT curve. Specifically, it includes the following steps:

[0028] 1) Machining the specimen: The specimen to be tested is machined into Φ3*10mm, and a conical small hole with a bottom diameter of 1.5mm and a height of 2mm is opened at one end of the specimen.

[0029] 2) Welding: The two ends of the thermocouple platinum-platinum rhodium (positive-negative) are instantaneously welded together into a round head shape through a carbon rod. Then the round head of the thermocouple is welded to the bottom end of the conical small hole at one end of the specimen.

[0030] 3) Loading the specimen: The combined thermocouple and the specimen are placed together in the sample chamber of the phase transformation instrument, and the air is evacuated.

[0031] 4) Setting the test procedure: The austenitizing temperature is set at 900 °C, held for 10 minutes, cooled to room temperature at cooling rates of 0.5 °C / S, 1 °C / S, 5 °C / S, 10 °C / S, and 30 °C / S respectively, and the temperature-expansion change curve is recorded.

[0032] 5) Data processing: Using the tangent method on the temperature-expansion change curve, calculate the phase transformation start temperature and end temperature at different cooling rates, as Figure 2 shown.

[0033] 6) Metallographic structure observation: The specimens obtained at different cooling rates are subjected to metallographic sample preparation, polishing, and etching, and the microstructures are observed using a metallographic microscope.

[0034] 7) CCT curve plotting: Combining with the metallographic structure, supplement the cooling rate tests at 3°C / S and 50°C / S, and accurately calculate the phase transformation start temperature and end temperature at different cooling rates. Connect the phase transformation start temperature and end temperature respectively to obtain different tissue regions, and finally draw the complete CCT curve of the test sample, as Figure 3 shown.

[0035] The embodiments described above are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for improving the efficiency of steel CCT curve testing by modifying the sample size, characterized in that: The specific steps include: 1) Machined specimen: The specimen to be tested is machined into Φ3*10mm, and a conical hole with a bottom diameter of 1.5mm and a height of 2mm is opened at one end of the specimen; 2) Welding: The two ends of the thermocouple platinum-platinum-rhodium are welded together by instantaneous ignition of a carbon rod to form a round head shape; then the round head of the thermocouple is welded to the bottom of the conical hole at one end of the sample; 3) Sample loading: Place the assembled thermocouple and sample into the sample chamber of the phase change instrument and evacuate the chamber; 4) Set up the test procedure: according to the CCT curve test requirements, set up thermal simulation tests at different cooling rates; 5) Data processing: Calculate the parameters of the phase change start temperature and end temperature at different cooling rates; 6) Metallographic structure observation: The samples obtained at different cooling rates were subjected to metallographic sample preparation, polishing, and corrosion, and the microstructure was observed using a metallographic microscope; 7) CCT curve drawing: Connect the phase change start temperature and end temperature together to obtain different tissue regions, and finally draw the complete CCT curve of the test sample.

2. The method for improving the efficiency of steel CCT curve testing by modifying the sample size according to claim 1 is characterized in that: The step 4) at least includes parameters of heating temperature, heating rate, holding time and cooling rate.

3. The method for improving the efficiency of steel CCT curve testing by modifying the sample size according to claim 1, characterized in that: The step 7) also includes: combining the metallographic structure, if necessary, supplementing the cooling rate test to accurately calculate the phase transformation start temperature and end temperature at different cooling rates.

4. The method for improving the efficiency of steel CCT curve testing by modifying the sample size according to claim 1, characterized in that: The test procedure was set up according to the following requirements: the austenitizing temperature was set to 900°C, kept at this temperature for 10 minutes, cooled to room temperature at cooling rates of 0.5°C / S, 1°C / S, 5°C / S, 10°C / S, and 30°C / S, respectively, and the temperature-expansion change curve was recorded.

5. The method for improving the efficiency of steel CCT curve testing by modifying the sample size according to claim 4 is characterized in that: The tangent method is used on the temperature-expansion change curve to calculate the phase change start temperature and end temperature at different cooling rates.

6. The method for improving the efficiency of steel CCT curve testing by modifying the sample size according to claim 4 is characterized in that: Combined with the metallographic structure, 3℃ / S and 50℃ / S cooling rate tests were performed to accurately calculate the starting and ending temperatures of the phase transformation at different cooling rates.

7. Application of the method for improving the CCT curve testing efficiency of steel by modifying the sample size according to any one of claims 1 to 6 to improving the CCT curve testing efficiency of steel.

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