Method for characterizing phase transition temperature of carbon steel heat treatment using in-situ thermoelectric potential measurement technology

Abstract

The present invention relates to the technical field of material analysis, and in particular to a method for characterizing the phase change temperature of carbon steel heat treatment by using an in-situ thermoelectric potential measurement technology, comprising the following steps: S1. Copper wires are respectively welded at both ends of a carbon steel sample (1), and the copper wires at both ends of the carbon steel sample (1) are connected to a digital DC potentiometer (2) to form a loop; calibrated thermocouples (3) are respectively connected at both ends of the carbon steel sample (1); S2. The carbon steel sample (1) is heat treated, and a computer obtains temperature difference ΔT, temperature difference thermoelectric potential ΔV data, and furnace temperature T data in real time; S3. The thermoelectric power S of the carbon steel sample (1) relative to the reference electrode copper wire is calculated in real time, and a curve of the thermoelectric power S of the carbon steel sample (1) changing with temperature T is established. The method has high testing accuracy, is sensitive to slight changes in the material phase structure, is convenient and fast in data processing, and is less affected by the shape and size of the sample.

Description

Technical Field

[0001] The invention relates to the technical field of material analysis, and in particular to a method for characterizing the phase transition temperature of carbon steel heat treatment by adopting an in-situ thermoelectric potential measurement technology. Background Art

[0002] Metal heat treatment obtains different microstructures and phases by controlling different phase changes of metals. Different heat treatment processes can obtain different microstructures and properties of materials. Therefore, the properties of metal materials such as steel can be changed by controlling their phase changes through heat treatment. However, the phase change point temperature of the material is not constant, and different temperature change rates directly affect the phase change point temperature of the material. Therefore, the accurate measurement of the phase change point temperature under different heat treatment processes is one of the key technologies for preparing high-performance materials.

[0003] In recent years, some scholars have proposed a new heat treatment technology, namely "precision controlled heat treatment". Through this special heat treatment technology, gradient materials with different surfaces and cores can be prepared. For example, in the journals "Super long-range diffusion of carbon during proeutectoid ferrite transformation [J]. Journal of Central South University, 2019, 26 (3).", "Study on local ferrite phase transformation of hypoeutectoid steel [D]. Northeastern University, 2019" and patents CN2018116229555 and CN2018116230016, it is disclosed that a surface ferrite layer is obtained on the surface of low-carbon steel (C ≤ 0.15wt%) through a special controlled cooling heat treatment, and its core is still a mixed structure of ferrite and pearlite (or bainite), that is, the sample undergoes a surface (local) ferrite phase transformation. This special controlled cooling heat treatment method obtains a surface structure different from the core by precisely controlling the cooling rate of the heat treatment, so that the material obtains a better combination of strength and bending performance and improves its corrosion resistance. Therefore, the research and development of new technologies and methods for characterizing the phase change of precision-controlled heat treatment has broad application prospects. At present, the main research methods for analyzing phase change are: traditional metallographic observation method; modern detection methods such as high-temperature metallographic method; X-ray diffraction method, thermal expansion method and differential thermal analysis method. The traditional metallographic observation method requires observation of the metallographic structure after the heat treatment, and it is impossible to characterize the phase change process online and continuously; the high-temperature metallographic method has certain limitations and is only suitable for observing the organization with large volume changes between the new phase and the parent phase during the transformation process. The high-temperature X-ray diffraction method analyzes the phase change of the material by comparing and analyzing the changes in the diffraction peaks at different temperatures. However, since the intensity of the diffraction peak is proportional to the relative content of the phase, the phase change temperature measured by the high-temperature X-ray diffraction method has a large deviation from the actual one. The thermal expansion method studies the phase change process inside the material by measuring the volume effect accompanied by the phase change. However, when the measured phase change has no obvious volume effect, if the phase change rate is small, it will affect the accuracy of the test. Differential thermal analysis detects phase change based on the latent heat released during the phase change process of the material. It is not very suitable for phase change processes with small latent heat and slow transformation rate. At the same time, this method is also affected by the sample size. The phase change temperature measured by differential thermal analysis will be different from the actual temperature. Therefore, for the determination of phase change during precision heat treatment, especially the determination of phase change point temperature at different temperature change rates, there is still a lack of a phase change detection and characterization method that can achieve online in-situ precise quantification and is not affected by sample size. Summary of the invention

[0004] The purpose of the present invention is to address the limitations of existing methods and utilize in-situ thermoelectric potential measurement technology to collect online thermoelectric power versus temperature curves of different phases to characterize the phase change during the precision controlled heat treatment of carbon steel.

[0005] The technical problem to be solved by the present invention is to provide a method for characterizing the phase transition temperature of carbon steel heat treatment using in-situ thermoelectric potential measurement technology. The method solves the defect that the measurement method in the prior art is not sensitive enough under the conditions of small phase transition amount or slow phase transition rate, which causes a certain error in the measured phase transition point temperature. The method has high testing accuracy and is sensitive to slight changes in the material phase structure. The data processing is convenient and fast and is less affected by the shape and size of the sample.

[0006] In order to solve the above technical problems, the present invention provides a method for characterizing the phase transition temperature of carbon steel heat treatment by using in-situ thermoelectric potential measurement technology, comprising the following steps:

[0007] S1. Copper wires are welded at both ends of the carbon steel sample, and the copper wires at both ends of the carbon steel sample are connected to a digital DC potentiometer to form a loop; calibrated thermocouples are provided at both ends of the carbon steel sample, and the thermocouples are connected to a furnace temperature tester respectively; the digital DC potentiometer and the furnace temperature tester are connected to a computer;

[0008] S2. Place the carbon steel sample in a tubular furnace with a temperature sensor for heat treatment. The furnace temperature tester collects temperature data at both ends of the carbon steel sample in real time and sends the temperature data to a computer. The computer calculates the temperature difference ΔT at both ends of the carbon steel sample in real time. The digital DC potentiometer collects temperature difference thermoelectric potential ΔV data at both ends of the carbon steel sample in real time and sends the temperature difference thermoelectric potential ΔV data to the computer. The temperature sensor collects temperature T data of the tubular furnace in real time and sends the temperature T data to the computer.

[0009] S3. Calculate the thermoelectric power S of the carbon steel sample relative to the reference electrode copper wire using the following formula: S = ΔV / ΔT; take temperature T as the horizontal coordinate and thermoelectric power S as the vertical coordinate to establish a curve of the thermoelectric power S of the carbon steel sample changing with temperature T.

[0010] After obtaining the curve of thermoelectric power S versus temperature T, the curve can be analyzed. Since thermoelectric power S is a structure-sensitive physical quantity, it is an important parameter to characterize the changes in the internal structure of metal materials. At a certain temperature, slight changes in its structure, such as the decomposition of solid solution, the transformation of phase structure, the precipitation and dissolution of carbides, and the change of grain size, can cause significant changes in thermoelectric power. Therefore, the temperature T corresponding to the inflection point of the curve is the phase transition point temperature.

[0011] The above method has the following beneficial effects:

[0012] 1. The present application characterizes the phase change under the above-mentioned special heat treatment conditions by online in-situ tracking and measuring the changes in thermoelectric power of different phases during the heat treatment of carbon steel. The in-situ tracking of the present application is that the temperature difference ΔT and the temperature difference thermoelectric potential ΔV at both ends of the carbon steel sample 1 are directly measured. The measurement position is directly the sample itself with high accuracy, thereby accurately and quantitatively evaluating the phase change point temperature at different temperature change rates.

[0013] 2. This application applies thermoelectric potential measurement technology to the analysis and research of phase changes during the precise heat treatment of metal materials. This method has high measurement accuracy and is sensitive to changes in the organizational structure of metal materials.

[0014] 3. This application collects thermoelectric potential signals of different phases in the precision-controlled heat treatment process. This physical quantity is sensitive to slight changes in the organizational structure of metal materials, and the collection process is convenient and fast. It does not require high preparation of the samples to be tested and is not affected by the shape and size of the samples.

[0015] 4. The present application proposes that information on changes in the organizational structure of metal materials can be obtained by measuring changes in the thermoelectric potential of the metal material. This method has high measurement accuracy and high repeatability, and can achieve in-situ detection and continuous monitoring of the phase change process. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] Figure 1 Schematic diagram of the experimental device structure in the method of characterizing the phase transformation temperature of carbon steel heat treatment using in-situ thermoelectric potential measurement technology.

[0017] Figure 2 This is the curve of the thermoelectric power S of the carbon steel sample 1 to be tested changing with temperature at a cooling rate of 0.02°C / s in Example 1.

[0018] Figure 3 This is the cross-sectional metallographic structure of the carbon steel sample 1 to be tested at a cooling rate of 0.02°C / s in Example 1.

[0019] Figure 4 This is the curve of the thermoelectric power S of the carbon steel sample 1 to be tested changing with temperature at a cooling rate of 0.05°C / s in Example 2.

[0020] in:

[0021] 1. Carbon steel test specimen; 2. Digital DC potentiometer; 3. Thermocouple; 4. Furnace temperature tester; 5. Computer; 6. Tube furnace. DETAILED DESCRIPTION

[0022] The invention will be further described in detail below in conjunction with the accompanying drawings and specific embodiments.

[0023] The inventive concepts of the present disclosure will be described below using the terms commonly used by those skilled in the art to convey the essence of their work to other persons skilled in the art. However, these inventive concepts can be embodied in many different forms and should not be considered to be limited to the embodiments described herein. These embodiments are provided to make the present disclosure more detailed and complete, and to fully convey the scope of its inclusion to those skilled in the art. It should also be noted that these embodiments are not mutually exclusive. Components, steps or elements from one embodiment may be assumed to exist or be used in another embodiment. Without departing from the scope of the embodiments of the present disclosure, various alternative and / or equivalent implementations may be used to replace the specific embodiments shown and described. This application is intended to cover any modifications or variations of the embodiments discussed herein. It is obvious to those skilled in the art that only some of the described aspects can be used to practice alternative embodiments. For the purpose of illustration, specific numbers, materials and configurations are described in the embodiments herein, however, those skilled in the art may also practice alternative embodiments without these specific details. In other cases, well-known features may be omitted or simplified so as not to make the illustrative embodiments difficult to understand.

[0024] Furthermore, the terms “first”, “second”, and “third” are used for descriptive purposes only and should not be understood as indicating or implying relative importance.

[0025] In the description of the present invention, it should be noted that, unless otherwise clearly specified and limited, the terms "provided with", "installed", "connected", and "connected" should be understood in a broad sense, for example, it can be a fixed connection, a detachable connection, or an integral connection; it can be a mechanical connection or an electrical connection; it can be a direct connection, or it can be indirectly connected through an intermediate medium, or it can be the internal communication of two components. For ordinary technicians in this field, the specific meanings of the above terms in the present invention can be understood according to specific circumstances.

[0026] The directional words "left" and "right" in the text correspond to the directions of the drawings in the specification.

[0027] A method for characterizing the phase change temperature during heat treatment of carbon steel using in-situ thermoelectric potential measurement technology comprises the following steps:

[0028] S1. Figure 1, copper wires are welded at both ends of the carbon steel sample 1, and the copper wires at both ends of the carbon steel sample 1 are connected to the digital DC potentiometer 2 to form a loop; calibrated thermocouples 3 are provided at both ends of the carbon steel sample 1, and the thermocouples 3 are connected to the furnace temperature tester 4 respectively; the digital DC potentiometer 2 and the furnace temperature tester 4 are both connected to the computer 5 for communication; the computer 5 can obtain the data of the digital DC potentiometer 2 and the furnace temperature tester 4; the present application directly measures the temperature difference ΔT and the temperature difference thermoelectric potential ΔV at both ends of the carbon steel sample 1, and the measurement position is directly the sample itself, so the present application is an in-situ tracking technology with high accuracy; the digital DC potentiometer is a UJ33D-1 digital DC potentiometer, and the resolution of the potentiometer is 1×10 -6 V; the furnace temperature tester is a CENTER-309USB furnace temperature tester, the measurement temperature range of the furnace temperature tester is 200 to 1370°C, and the measurement accuracy is 0.1°C; the thermocouple 3 is a nickel-chromium-nickel-silicon thermocouple;

[0029] S2. The carbon steel sample 1 is placed in a tubular furnace 6 with a temperature sensor for heat treatment. The furnace temperature tester 4 collects temperature data at both ends of the carbon steel sample 1 in real time and sends the temperature data to a computer 5. The computer calculates the temperature difference ΔT at both ends of the carbon steel sample 1 in real time; the digital DC potentiometer 2 collects temperature difference thermoelectric potential ΔV data at both ends of the carbon steel sample 1 in real time, and sends the temperature difference thermoelectric potential ΔV data to the computer 5; the temperature sensor collects temperature T data of the tubular furnace 6 in real time and sends the temperature T data to the computer 5;

[0030] S3. Calculate the thermoelectric power S of the carbon steel sample 1 relative to the reference electrode copper wire using the following formula: S = ΔV / ΔT; take temperature T as the horizontal coordinate and thermoelectric power S as the vertical coordinate to establish a curve of the thermoelectric power S of the carbon steel sample 1 changing with temperature T; the thermoelectric power S can be calculated manually or by computer.

[0031] The method also includes step S4. Analyze the curve of thermoelectric power S changing with temperature T. The temperature T corresponding to the inflection point of the curve is the phase transition point temperature.

[0032] After determining the phase transition point temperature, in order to further determine which phase transition the curve inflection corresponds to, such as ferrite transition, pearlite transition, etc., the calculated thermoelectric power of the sample at different temperatures can be compared with the data of known phase transitions to determine the specific phase transition type at the phase transition point temperature.

[0033] Preferably, in step S2, heat treatment refers to precision controlled heat treatment, heat treatment with precisely controlled cooling rate, ensuring that there is a stable temperature difference at both ends of the carbon steel sample 1, and the heat treatment process is as follows: the carbon steel sample 1 is heated to 850-950°C (specifically, it can be heated to 850°C or 870°C or 900°C, etc.) and austenite is homogenized for 20-60 minutes (specifically, it can be austenite homogenization for 20 or 40 or 60 minutes, etc.), after which the sample is cooled to 600°C at a rate of 0.008-0.08°C / s (specifically, it can be 0.008 or 0.02 or 0.04 or 0.08°C / s, etc.), and then furnace cooled to room temperature.

[0034] The copper wires at both ends of the carbon steel sample 1 are two copper wires of equal length and uniform material. The two copper wires form a loop, which can measure in situ the thermoelectric potential generated by the temperature difference during the precise heat treatment of the carbon steel sample 1.

[0035] In step S1, the carbon steel sample 1 is connected to the thermocouple in such a way that the end of the nickel-chromium-nickel-silicon thermocouple is fixed at the welding point between the copper wire and the carbon steel sample 1 (specifically, the end of the thermocouple can be welded at the welding point between the copper wire and the carbon steel sample 1), etc., as long as it can have good thermal contact with both ends of the carbon steel sample 1.

[0036] The computer 5 has the capabilities of data storage, data acquisition and data analysis, which are inherent in the computer itself and will not be described in detail here.

[0037] The present application is further described below with more specific embodiments.

[0038] Example 1

[0039] A method for characterizing the phase change temperature during heat treatment of carbon steel using in-situ thermoelectric potential measurement technology comprises the following steps:

[0040] S1. The carbon steel sample 1 is 0.45% carbon steel. Copper wires of equal length and uniform material are welded at both ends of the carbon steel sample 1. The copper wires at both ends of the carbon steel sample 1 are connected to a digital DC potentiometer 2 to form a loop. Calibrated thermocouples 3 are connected at both ends of the carbon steel sample 1. The ends of the thermocouples are fixed at the welding points of the copper wires and the carbon steel sample 1. The thermocouples 3 are connected to a furnace temperature tester 4. The digital DC potentiometer 2 and the furnace temperature tester 4 are both connected to a computer 5 for communication. The digital DC potentiometer is a UJ33D-1 digital DC potentiometer with a resolution of 1×10 -6 V; the furnace temperature tester is a CENTER-309USB furnace temperature tester, the measurement temperature range of the furnace temperature tester is 200 to 1370°C, and the measurement accuracy is 0.1°C; the thermocouple 3 is a nickel-chromium-nickel-silicon thermocouple;

[0041] S2. Place the carbon steel sample 1 in a tubular furnace 6 with a temperature sensor for heat treatment. The heat treatment is a precision-controlled heat treatment. There is a stable temperature difference at both ends of the carbon steel sample 1. The precision-controlled heat treatment process is: the sample is heated to 950°C and austenite is homogenized for 20 minutes. After that, the sample is cooled to 600°C at a speed of 0.02°C / s, and then furnace-cooled to room temperature. The tubular furnace 6 is a vacuum tubular furnace with a precisely controllable cooling rate. The furnace temperature tester 4 collects temperature data at both ends of the carbon steel sample 1 in real time and sends the temperature data to the computer 5. The computer calculates the temperature difference ΔT at both ends of the carbon steel sample 1 in real time. The digital DC potentiometer 2 collects the temperature difference thermoelectric potential ΔV data at both ends of the carbon steel sample 1 in real time, and sends the temperature difference thermoelectric potential ΔV data to the computer 5. The temperature sensor collects the temperature T data of the tubular furnace 6 in real time and sends the temperature T data to the computer 5.

[0042] S3. Calculate the thermoelectric power S of carbon steel sample 1 relative to the reference electrode copper wire in real time using the following formula: S = ΔV / ΔT; take temperature T as the horizontal coordinate and thermoelectric power S as the vertical coordinate to establish the thermoelectric power S curve of carbon steel sample 1 versus temperature T, as shown in Figure 2 shown.

[0043] Depend on Figure 2 Analysis shows that when the cooling rate is controlled at 0.02℃ / s, the thermoelectric power versus temperature curve of the carbon steel sample to be tested deflects upward at 775℃. It can be judged that the carbon steel sample undergoes phase transformation at 775℃. Figure 3 It can be seen that at this time, surface (local) ferrite transformation occurred on the surface of carbon steel sample 1 to obtain a single-phase ferrite layer.

[0044] Example 2

[0045] The difference between Example 2 and Example 1 is that the precision controlled heat treatment process of Example 2 is that the sample is heated to 950°C and austenite is homogenized for 20 minutes, after which the sample is cooled to 600°C at a rate of 0.05°C / s and then furnace cooled to room temperature. The rest of Example 2 is the same as Example 1. The thermoelectric power S versus temperature T curve of carbon steel sample 1 established in Example 2 is shown in FIG. Figure 4 shown.

[0046] Depend on Figure 4 It can be seen that when cooling at a controlled cooling rate of 0.05℃ / s, the thermoelectric power versus temperature curve of the carbon steel sample to be tested deflects upward at 777℃, from which it can be judged that the carbon steel sample has undergone a phase change at 777℃.

[0047] The above embodiments illustrate that the temperature at which phase change occurs during the precise heat treatment of the carbon steel to be tested can be accurately measured using the online in-situ thermoelectric potential measurement technology.

[0048] The above is only a preferred feasible implementation example of the present invention, which cannot limit the scope of rights of the present invention. For those skilled in the art, all other corresponding changes made using the technical solutions and technical concepts of the present invention should fall within the scope of protection of the claims of the present invention.

Claims

1. A method for characterizing the phase transition temperature of carbon steel heat treatment using in-situ thermoelectric potential measurement technology, characterized in that: The steps include: S1. Copper wires are welded at both ends of the carbon steel sample (1), and the copper wires at both ends of the carbon steel sample (1) are connected to a digital DC potentiometer (2) to form a loop; calibrated thermocouples (3) are provided at both ends of the carbon steel sample (1), and the thermocouples (3) are connected to a furnace temperature tester (4) respectively; the digital DC potentiometer (2) and the furnace temperature tester (4) are connected to a computer (5); S2. The carbon steel sample (1) is placed in a tubular furnace (6) with a temperature sensor for heat treatment. The furnace temperature tester (4) collects temperature data at both ends of the carbon steel sample (1) in real time and sends the temperature data to a computer (5). The computer calculates in real time to obtain the temperature difference ΔT at both ends of the carbon steel sample (1); the digital DC potentiometer (2) collects temperature difference thermoelectric potential ΔV data at both ends of the carbon steel sample (1) in real time and sends the temperature difference thermoelectric potential ΔV data to the computer (5); the temperature sensor collects temperature T data of the tubular furnace (6) in real time and sends the temperature T data to the computer (5); in step S2, the heat treatment refers to precision controlled heat treatment; in step S2, the heat treatment process is that the carbon steel sample (1) is heated to 850-950°C and austenite is homogenized for 20-60 min, after which the sample is cooled to 600°C at a speed of 0.008-0.08°C / s and then furnace cooled to room temperature; S3. Calculate the thermoelectric power S of the carbon steel sample (1) relative to the reference electrode copper wire using the following formula: S=ΔV / ΔT; take temperature T as the abscissa and thermoelectric power S as the ordinate to establish a curve of the thermoelectric power S of the carbon steel sample (1) versus temperature T; The method also includes step S4. Analyze the curve of thermoelectric power S changing with temperature T. The temperature T corresponding to the inflection point of the curve is the phase transition point temperature.

2. The method according to claim 1, characterized in that The digital DC potentiometer is a UJ33D-1 digital DC potentiometer, and the resolution of the potentiometer is 1×10 -6 V.

3. The method according to claim 1, characterized in that The furnace temperature tester is a CENTER-309 USB furnace temperature tester, the measurement temperature range of the furnace temperature tester is 200~1370℃, and the measurement accuracy is 0.1℃.

4. The method according to claim 1, characterized in that In step S1, the thermocouple (3) is a nickel-chromium-nickel-silicon thermocouple.

5. The method according to claim 1, characterized in that In step S1, the copper wires are two copper wires of equal length and uniform material.

6. The method according to claim 1, characterized in that In step S1, the carbon steel sample is a carbon steel sample with phase transformation.

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