A thermal simulation method to obtain accurate austenite grain size

Abstract

The present invention discloses a thermal simulation method for obtaining accurate austenite grain size, the method is to conduct heat treatment test on MMS-200 testing machine, in the heating stage, heat preservation stage and cooling stage of the sample, relying on the cooperation of the instrument's own cooling system and pneumatic system, by manually adjusting the gear of the movable shaft cylinder, effectively solve the bulging phenomenon in the middle of the sample caused by excessively high heating temperature or too long heat preservation time. Different from the current stage, which only relies on the computer to input parameters through the system operation interface to conduct experiments, the bulging phenomenon in the middle of the sample is ignored. In contrast, the present invention can effectively avoid the sample from falling during the heating process; avoid the bulging phenomenon in the middle of the sample due to high-temperature heating and long-term heat preservation, so as to obtain accurate austenite grain size.

Description

Technical Field

[0001] The invention belongs to the technical field of physical simulation test of hot processing of metal materials, and relates to a method for preventing a sample from bulging in the middle during a heat preservation stage when a heat treatment test is carried out on a thermal simulation MMS-200 test machine. Background Art

[0002] Heat treatment tests are carried out using a thermal simulation test machine, usually by heating the steel to A c1 Temperature above A c3 Austenitization is carried out below the temperature. A c1 is the temperature at which pearlite transforms into austenite when heated, A c3 It is the temperature at which ferrite transforms into austenite during heating. Then, during cooling, austenite transforms into ferrite, pearlite, bainite and martensite. In actual production, the heating temperature of steel in the heating furnace is above one thousand degrees. After austenitization, the austenite grains of steel become larger, which has an important influence on subsequent rolling and structural transformation. Therefore, studying the austenite grain growth tendency of steel during high temperature heating and long-term heat preservation can provide an important basis for formulating rolling process and heat treatment process.

[0003] In actual operation, in order to make the steel austenitized more fully, a higher heating temperature and a longer holding time are usually set. When the test enters the holding stage, bulging is likely to occur in the middle of the sample. In this case, the measured austenite grain size is inaccurate, causing difficulties for the experiment.

[0004] Therefore, it is urgent to explore a method to avoid bulging of samples during the insulation process, so as to obtain accurate austenite grain size for subsequent research. Summary of the invention

[0005] The purpose of the present invention is to provide a thermal simulation method for obtaining accurate austenite grain size. When a sample is subjected to a heat treatment test on a thermal simulation MMS-200 testing machine, during the heat preservation process, bulging in the middle of the sample due to excessively high heating temperature or excessively long heat preservation time can be avoided, thereby obtaining accurate austenite grain size and performing austenite grain size rating.

[0006] The technical solution of the present invention:

[0007] A thermal simulation method for obtaining accurate austenite grain size comprises the following steps:

[0008] (1) The sample is processed into a cylindrical sample with a specification of Φ8×15mm, and the surface roughness of the sample is Ra≤3.2um;

[0009] (2) Use sandpaper to polish the middle of the sample to make it present a metallic luster, and use a spot welder to weld the positive and negative thermocouple wires to the 1 / 2 position of the sample; select a suitable thermocouple according to the peak temperature. When the peak temperature is ≤1250℃, use a nickel-chromium-nickel-aluminum thermocouple, and select the thermocouple type TK1; when the peak temperature is greater than 1250℃, use a platinum-rhodium-platinum thermocouple, and select the thermocouple type TK4;

[0010] (3) Before installing the sample, start the air compressor, then open the operating box, adjust the cylinder gear of the fixed axis to the "compression" gear, press the button to move the fixed axis to the compressed limit position, and use the movable axis cylinder to adjust the position of the movable axis so that the distance between the left and right clamps is slightly smaller than the length of the sample; when installing the sample, adjust the cylinder gear of the movable axis to the "compression" gear, press the button so that both ends of the sample can be firmly clamped by the clamp; after the sample is installed, close the operating box;

[0011] (4) Setting test conditions: Heat the sample to 1150°C, 1200°C, and 1250°C at a rate of 5°C / s, keep the temperature for 10 min and 30 min respectively, and then quench;

[0012] (5) Start the main system, turn on the circulating water pump and vacuum pump, evacuate the air for one or two minutes, then turn off the vacuum pump, fill with protective gas argon, and start the test;

[0013] (6) During the experiment, observe the middle position of the sample: when the test enters the heating stage, adjust the cylinder gear of the movable axis to the "compression" gear; when the test enters the insulation stage, adjust the cylinder gear of the movable axis from the "compression" gear to the "no force" gear; when the test enters the cooling stage, adjust the cylinder gear of the movable axis from the "no force" gear to the "compression" gear;

[0014] (7) Observe the austenite grain morphology of the thermal simulation specimen and rank the austenite grain size.

[0015] Beneficial effects of the present invention: The present invention conducts heat treatment tests on an MMS-200 testing machine. During the heating stage, insulation stage, and cooling stage of the sample, the instrument's own cooling system and pneumatic system cooperate with each other, and the gear position of the movable shaft cylinder is manually adjusted to effectively solve the bulging phenomenon in the middle of the sample caused by excessively high heating temperature or too long insulation time. This is different from the current practice of only relying on a computer to input parameters through the system operation interface to conduct experiments, ignoring the bulging phenomenon in the middle of the sample. In contrast, the present invention can effectively prevent the sample from falling during the heating process; avoid the bulging phenomenon in the middle of the sample due to high-temperature heating and long-term insulation, thereby obtaining an accurate austenite grain size. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1The austenite grain morphology obtained by sampling at 1 / 2 of the plate thickness and subjecting it to different heating temperatures and holding times.

[0017] Figure 2 The austenite grain morphology obtained by sampling at 1 / 4 of the plate thickness and subjecting it to different heating temperatures and holding times. DETAILED DESCRIPTION

[0018] The following is further described with reference to the embodiments. Example

[0019] The MMS-200 thermal simulation test machine was used to conduct heat treatment experiments to study the austenite grain growth tendency of XG2311 steel. This experiment used the MMS-200 thermal simulation test machine produced by Northeastern University. The main structure is mainly composed of two beams, a main hydraulic cylinder, a hydraulic motor, a movable shaft, a fixed shaft, an operating box, a positioning beam and other components. Among them, the movable shaft and the fixed shaft are installed on both sides of the operating box. The movable shaft is connected to the tension sensor and a fixture. The fixed shaft is connected to another fixture through screws, and the specimen is installed between the two fixtures. The key process steps of the experiment are as follows:

[0020] (1) The sample is processed into a cylindrical sample with a specification of Φ8×15mm, and the surface roughness of the sample is Ra≤3.2um;

[0021] (2) Use sandpaper to polish the middle of the sample to make it metallic, use a spot welder to weld the nickel-chromium-nickel-aluminum thermocouple wire to the 1 / 2 position of the sample, and connect the positive and negative thermocouple wires to the positive and negative poles of the TK1 thermocouple;

[0022] (3) Before installing the sample, start the air compressor, then open the operating box, adjust the cylinder gear of the fixed axis to the "compression" gear, press the button to move the fixed axis to the compressed limit position, and use the movable axis cylinder to adjust the position of the movable axis so that the distance between the left and right clamps is slightly smaller than the length of the sample; when installing the sample, adjust the cylinder gear of the movable axis to the "compression" gear, press the button so that both ends of the sample can be firmly clamped by the clamp; after the sample is installed, close the operating box;

[0023] (4) The specific test process is as follows: the sample is heated to 1150℃, 1200℃, and 1250℃ at a rate of 5℃ / s, kept at this temperature for 10min and 30min respectively, and then quenched;

[0024] (5) Start the main system, turn on the circulating water pump and vacuum pump, evacuate the air for one or two minutes, then turn off the vacuum pump, fill with protective gas argon, and start the test;

[0025] (6) Click the "Heating" button and the "Program Run" button to start the experiment. When the sample is heated, adjust the cylinder gear of the movable axis to the "Compression" gear; when the sample is kept warm, adjust the cylinder gear of the movable axis from the "Compression" gear to the "No Force" gear; when the sample is cooled, adjust the cylinder gear of the movable axis from the "No Force" gear to the "Compression" gear;

[0026] During the sample insulation stage, observe the middle of the sample. If a bulge (called "bulge") appears in the middle of the sample, it is caused by too high a heating temperature or too long a heat preservation time. You need to manually adjust the cylinder gear of the moving axis from the "compression" gear to the "no force" gear so that the sample is not clamped by the fixture and there will be no bulge, thereby obtaining an accurate austenite grain size.

[0027] (7) The heat-treated specimens were cut in half, mounted, ground, and polished, and then corroded with a supersaturated picric acid solution. The austenite grain morphology was then observed using a LEICA DMi8 metallographic microscope. Finally, the intercept method was used to measure the average intercept of the austenite grains and grade the austenite grain size.

[0028] The austenite grain size of the samples at 1 / 2 and 1 / 4 of the plate thickness is shown in Table 1.

[0029] Table 1 Austenite grain size of samples at 1 / 2 and 1 / 4 of plate thickness

[0030] .

[0031] Table 1 shows that: 1) The growth of austenite is affected by the heating temperature and the holding time; the higher the heating temperature and the longer the holding time, the larger the austenite grain size; 2) Under the same conditions, the austenite grain size at the 1 / 2 position is slightly larger than that at the 1 / 4 position; 3) When the heating temperature is 1200℃ and the holding time is 30min, the growth trend of austenite begins to become obvious.

[0032] Figure 1 Display: The accurate austenite grain size is obtained after the sample at 1 / 2 thickness is heated to 1150℃, 1200℃, and 1250℃ and kept warm for 10min and 30min respectively.

[0033] Figure 2 Display: The accurate austenite grain size is obtained when the sample at 1 / 4 of the plate thickness is heated to 1150℃, 1200℃, and 1250℃ and kept warm for 10min and 30min respectively.

Claims

1. A thermal simulation method for obtaining accurate austenite grain size, characterized in that The following steps are involved: (1) The sample is processed into a cylindrical sample with a specification of Φ8×15mm, and the surface roughness of the sample is Ra≤3.2um; (2) Use sandpaper to polish the middle of the sample to make it present a metallic luster, and use a spot welder to weld the positive and negative thermocouple wires to the 1 / 2 position of the sample; select a suitable thermocouple according to the peak temperature. When the peak temperature is ≤1250℃, use a nickel-chromium-nickel-aluminum thermocouple, and select the thermocouple type TK1; when the peak temperature is greater than 1250℃, use a platinum-rhodium-platinum thermocouple, and select the thermocouple type TK4; (3) Before installing the sample, start the air compressor, then open the operating box, adjust the cylinder gear of the fixed axis to the "compression" gear, press the button to move the fixed axis to the compressed limit position, and use the movable axis cylinder to adjust the position of the movable axis so that the distance between the left and right clamps is slightly smaller than the length of the sample; when installing the sample, adjust the cylinder gear of the movable axis to the "compression" gear, press the button so that both ends of the sample can be firmly clamped by the clamp; After the specimen is installed, close the operating box; (4) Setting test conditions: Heat the sample to 1000-1250°C at a rate of 5°C / s, keep it at that temperature for 10-30 min, and then quench it; (5) Start the main system, turn on the circulating water pump and vacuum pump, evacuate the air for one or two minutes, then turn off the vacuum pump, fill with protective gas argon, and start the test; (6) During the experiment, observe the middle position of the sample: when the test enters the heating stage, adjust the gear position of the moving axis cylinder to the "compression" gear position; When the test enters the heat preservation stage, the cylinder block of the movable shaft is adjusted from the "compression" gear position to the "no force" gear position; when the test enters the cooling stage, the cylinder block of the movable shaft is adjusted from the "no force" gear position to the "compression" gear position; (7) Observe the austenite grain morphology of the thermal simulation specimen and rank the austenite grain size.

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