A quantitative statistical method for sigma phase in austenitic heat-resistant steel
By employing a combination of two mechanical polishing processes and one electrolytic polishing process, along with perchloric acid and glacial acetic acid electrolytes and an EBSD probe, the quantitative statistical problem of the Sigma phase in austenitic heat-resistant steel was solved, achieving efficient and accurate Sigma phase quantification while reducing costs and time.
Patent Information
- Authority / Receiving Office
- CN · China
- Patent Type
- Applications(China)
- Current Assignee / Owner
- DATANG BOILER & PRESSURE VESSEL INSPECTION CENTER CO LTD
- Filing Date
- 2026-01-26
- Publication Date
- 2026-05-29
AI Technical Summary
Existing technologies are difficult to efficiently and accurately quantify the Sigma phase in austenitic heat-resistant steel. Traditional methods suffer from problems such as easy dissolution of the Sigma phase, reliance on experience in operation, and low resolution.
The method employs two mechanical polishing steps and one electrolytic polishing step. An electrolyte solution prepared with perchloric acid and glacial acetic acid in a 1:3 ratio is used for electrolytic polishing in an ice-water mixture constant temperature bath. Image data is acquired using an EBSD probe with an appropriate acquisition step size set.
It achieves efficient and accurate quantitative statistics of sigma phase with a resolution of over 95%, reducing polishing time and cost and improving quantitative accuracy.
Smart Images
Figure CN122109153A_ABST
Abstract
Description
Technical Field
[0001] This invention relates to the field of steel structure quality inspection technology, and in particular to a quantitative statistical method for the sigma phase in austenitic heat-resistant steel. Background Technology
[0002] Austenitic heat-resistant steels are widely used in thermal power, nuclear power, and other industries due to their excellent high-temperature strength and resistance to high-temperature oxidation corrosion. Observation and research of their microstructure is an important way to analyze their service performance. Super304H, TP347HFG, and HR3C austenitic steels are currently the most widely used heat-resistant materials in superheater and reheater components of supercritical thermal power units. In recent years, with the extension of the service life of these units, reports have emerged of problems such as rapid aging of the microstructure and high-temperature performance degradation of austenitic steels after 6-10 years of service, leading to tube rupture. Especially with the rapid development of new energy sources, thermal power units need to reduce their output power to participate in peak shaving to maintain the stability of the power system. During peak shaving and low-load operation, the steam pressure decreases and the flow rate slows down, causing local overheating of the heating surface pipes, and the problem of accelerated microstructure aging caused by overheating becomes increasingly serious. Studies have found that the performance degradation of austenitic heat-resistant steel is closely related to the abnormal precipitation of sigma phases in the microstructure.
[0003] The Sigma phase in austenitic steel is mainly composed of Fe and Cr elements. EDS energy dispersive spectroscopy analysis results show that the Cr phase is commonly found in austenitic steel. 23 C6 carbides are similar and difficult to distinguish; in BSE backscattered electron images, the sigma phase and Cr phase are clearly distinguishable. 23 C6 does not exhibit significant compositional contrast differences and cannot be directly distinguished; the field of view in TEM transmission images is small, and the sigma phase is relatively large (approximately 1 μm), making it unsuitable for quantitative statistics. Currently, the main method for quantifying the sigma phase involves treating it with a chromogenic chemical etchant (such as oxalic acid solution, potassium ferricyanide solution, etc.) followed by observation under an optical microscope, statistically analyzing the area fraction of the sigma phase in multiple optical images. Traditional electrolytic polishing methods for heat-resistant steel are performed at room temperature, using an electrolyte typically consisting of 10% perchloric acid + 90% anhydrous ethanol. This method has strong corrosive power, and the electrolysis time is difficult to control, often resulting in the sigma phase being etched away within seconds. Furthermore, the selection of chemical reagents and the processing time require highly experienced operators, and the resolution of optical microscopes is limited; therefore, the efficiency and accuracy of this quantitative method need improvement. Summary of the Invention
[0004] The technical problem to be solved by the present invention is to provide an efficient and accurate quantitative statistical method for the sigma phase in austenitic heat-resistant steel.
[0005] The present invention solves the above-mentioned technical problems through the following technical solution: a quantitative statistical method for the sigma phase in austenitic heat-resistant steel, the quantitative statistical method comprising the following steps: first, wire cutting the sample according to the sample size requirements of the electron microscope; then grinding and polishing the sample; after polishing, electrolytic polishing the sample; after electrolytic polishing, cleaning and drying the sample; and finally, image data acquisition.
[0006] Using the method of this invention, quantitative statistical sample preparation of sigma phase in austenitic heat-resistant steel can be completed quickly and efficiently, completely preserving the microstructure containing sigma phase, with a calibration rate of over 95%, and the sigma phase content can be accurately obtained.
[0007] Preferably, the specific process of grinding and polishing the sample includes: using silicon carbide sandpaper to grind the sample step by step from the initial grit to the target grit, until only fine grinding marks in a single direction exist on the grinding surface.
[0008] Preferably, the specific process of grinding and polishing the sample further includes: performing a first mechanical polishing on the sample; the first mechanical polishing includes using a polishing cloth made of velvet or wool, and using polishing paste of W5 or W3.5.
[0009] Preferably, when performing the first mechanical polishing of the sample, the polishing machine speed is maintained at 300~400r / min, and the force is kept uniform during the polishing process so that the observation surface of the sample is in complete contact with the polishing cloth. The polishing time is not less than 10min, and polishing is stopped when the observation surface of the sample is free of scratches and is bright and smooth after cleaning.
[0010] Preferably, the specific process of grinding and polishing the sample further includes: performing a second mechanical polishing on the sample; using a fine polishing cloth made of fine polishing velvet material, and using polishing paste W1 or W0.5.
[0011] Preferably, when performing the second mechanical polishing on the sample, the polishing machine speed is maintained at 300~400 r / min, and the force is kept uniform during the polishing process so that the observation surface of the sample is in complete contact with the polishing cloth. The polishing time is not less than 10 minutes, and polishing is stopped when the observation surface of the sample is a bright mirror surface.
[0012] Preferably, the step of electrolytic polishing the sample after polishing includes the following specific process: preparing an electrolyte by mixing perchloric acid and glacial acetic acid with a purity of 70-72% in a 1:3 ratio; placing the electrolyte in a constant temperature water bath containing an ice-water mixture at around 0°C; connecting the cathode of a DC power supply to a stainless steel or iron sheet and the anode to the sample; setting the power supply voltage to 10-15V and the current not exceeding 0.5A; and immersing the sample in the electrolyte.
[0013] Preferably, in the electrolytic polishing step, the electrolysis time is approximately 60 to 120 seconds depending on the composition of the heat-resistant steel. The specific time needs to be determined based on the small amplitude oscillation of the sample during the electrolysis process, with the observation of black film-like fragments falling off the sample surface and into the electrolyte as the benchmark.
[0014] Preferably, in the electropolishing step, after the sample is electrolyzed, it is quickly removed, rinsed with water, then ultrasonically cleaned in anhydrous ethanol, and finally dried with a hair dryer.
[0015] Preferably, the image data acquisition step specifically includes: using a scanning electron microscope equipped with an EBSD probe to acquire data from the sample, wherein the acquisition step size is set to the range of 1 / 3 to 1 / 2 of the average diameter of the sigma phase.
[0016] Setting the step size too large will result in low acquisition accuracy and inaccurate sigma phase quantification; setting the step size too small, although theoretically the acquisition accuracy will be higher, will result in excessively long acquisition time and high acquisition costs.
[0017] The advantages of this invention are:
[0018] (1) This application provides an electropolishing method for EBSD characterization of austenitic heat-resistant steel with sigma phase precipitation. In this method, an innovative electrolyte is prepared by mixing perchloric acid and glacial acetic acid with a purity of 70~72% in a 1:3 ratio. The electrolyte is placed in a constant temperature bath of ice water mixture for electropolishing. Experiments have shown that the sigma phase of the sample under these conditions will not dissolve and fall off during electropolishing, thus solving the drawback of traditional stainless steel electropolishing which easily leads to the dissolution and fall off of the sigma phase. (2) The method of this application adopts two mechanical polishing and one electrolytic polishing, which greatly reduces the polishing time and cost compared with the traditional argon ion beam polishing method; (3) This application also provides an EBSD acquisition step size suitable for sigma phase calibration. By acquiring sigma phase within the acquisition step size range set in this application, the problems of low resolution and large data error of the quantitative method of optical microscope are solved, and the cost is reduced as much as possible while ensuring accuracy. Attached Figure Description
[0019] Figure 1 This is a schematic diagram of the electropolishing method of the present invention; Figure 2 This is a topographic image of the area collected after the TP347HFG steel sample was prepared in Example 1 of this invention. Figure 3 The above refers to the EBSD image data acquisition results of TP347HFG steel in Embodiment 1 of the present invention. Figure 4This is a topographic image of the area collected after the Super304H steel sample was prepared in Embodiment 2 of the present invention. Figure 5 The above are the EBSD image data acquisition results of Super304H steel in Embodiment 2 of the present invention; Figure 6 This is a topographic image of the area collected after the TP347HFG steel sample was prepared in Comparative Example 1 of this invention. Figure 7 This is a high-magnification SEM image of the TP347HFG steel sample after preparation in Comparative Example 1 of this invention. Detailed Implementation
[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below in conjunction with specific embodiments and with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.
[0021] Example 1: like Figure 1 As shown in the figure, this embodiment provides a quantitative statistical method for the Sigma phase in austenitic heat-resistant steel. The statistical method includes the following steps: (1) Cut the sample by wire according to the sample size requirements of the electron microscope; (2) Grinding: Heat-resistant steel materials are generally ground from the initial grit silicon carbide sandpaper to the target grit, until only fine grinding marks in one direction exist on the grinding surface; (3) First mechanical polishing: Use a polishing cloth made of velvet or wool, and use W5 or W3.5 polishing paste. Adjust the speed of the polishing machine to 300~400r / min. Apply even force during the polishing process so that the observation surface of the sample is in complete contact with the polishing cloth. Polish for no less than 10 minutes until there are no scratches on the observation surface of the sample. After cleaning, the surface is bright and smooth. The purpose of using polishing compound W5 or W3.5 is to achieve fine polishing and remove minor scratches, which is equivalent to 3000 grit sandpaper. The polishing machine speed is controlled at 300~400 r / min because the in-situ EBSD sample is relatively large and it is not easy to clamp and fix it on an automatic polishing machine. Therefore, this speed is safer and more efficient when using manual polishing. (4) Second mechanical polishing: Use fine polishing cloth with fine polishing velvet material, use polishing paste W1 or W0.5, adjust the speed of the polishing machine to 300~400 r / min, apply even force during the polishing process, so that the observation surface of the sample is in complete contact with the polishing cloth, polish for no less than 10 minutes until the observation surface of the sample is a bright mirror surface. The purpose of using polishing compound W1 or W0.5 is to achieve fine polishing and a highly glossy surface, equivalent to sandpaper of 5000 grit or higher. (5) Electrolytic polishing: Prepare an electrolyte solution by mixing perchloric acid and glacial acetic acid with a purity of 70-72% in a 1:3 ratio. Place the electrolyte solution in a constant temperature water bath containing an ice-water mixture at around 0°C. Connect the cathode of the DC power supply to a stainless steel or iron sheet and the anode to the sample. Set the power supply voltage to 10-15V and the current to no more than 0.5A. Place the sample into the electrolyte solution. The electrolysis time is about 60-120s depending on the composition of the heat-resistant steel. After the electrolysis is completed, quickly remove the sample, rinse it with water, then place it in anhydrous ethanol for ultrasonic cleaning, and finally dry it with a hair dryer. (6) Image data acquisition: The sample was acquired using a scanning electron microscope equipped with an EBSD probe. To improve the accuracy of the data, the acquisition step size was set to the range of 1 / 3 to 1 / 2 of the average diameter of the sigma phase.
[0022] Specifically, this embodiment takes TP347HFG steel as the specific object and uses the aforementioned quantitative statistical method for the Sigma phase in austenitic heat-resistant steel to perform quantitative statistical analysis of the Sigma phase in TP347HFG steel to verify the feasibility of the method of this application. The specific steps are as follows: (1) Cut the sample by wire according to the sample size requirements of the electron microscope; (2) Grind on silicon carbide sandpaper, from 180 grit to 2000 grit, until there are only fine scratches in one direction on the grinding surface, and rinse the sample surface with water. (3) Use velvet polishing cloth and W5 polishing paste. Adjust the speed of the polishing machine to 400 r / min. Apply even force during the polishing process to ensure that the observation surface of the sample is in complete contact with the polishing cloth. Polish for 15 minutes until there are no scratches on the observation surface of the sample. Clean the sample with water and anhydrous ethanol in sequence. (4) Use fine polishing cloth and W1 polishing paste. Adjust the speed of the polishing machine to 400r / min. Apply even force during the polishing process to ensure that the observation surface of the sample is in complete contact with the polishing cloth. Polish for 10 minutes until the observation surface of the sample is a bright mirror surface. Clean the sample with water and anhydrous ethanol in sequence. (5) Electrolytic polishing: Prepare an electrolyte by mixing perchloric acid and glacial acetic acid with a purity of 70~72% in a 1:3 ratio. Place the electrolyte in a constant temperature water bath containing an ice-water mixture at about 0°C. Connect the cathode of the DC power supply to a stainless steel sheet and the anode to the sample. Set the power supply voltage to 10V and the current to no more than 0.5A. Place the sample into the electrolyte and electrolyze for about 60 seconds. After electrolysis, quickly remove the sample, rinse it with water, then ultrasonically clean it in anhydrous ethanol, and finally dry it with a hair dryer. (6) Image data acquisition: The sample was acquired using a scanning electron microscope equipped with an EBSD probe, with an acquisition step size of 400 nm; (7) Figure 2 This is a topographic image of the area collected after the TP347HFG steel sample was prepared in Example 1 of the present invention. No obvious sigma phase detachment was observed. Figure 3 The data shows the EBSD image acquisition results of TP347HFG steel in Example 1 of this invention. According to the data, the resolution of the sample in this example is 98.85%, and the sigma phase content is about 1.2%.
[0023] Example 2: To further verify the feasibility of the method in this application, this embodiment uses Super304H steel as the object and performs quantitative statistics on the sigma phase in Super304H steel. The steps are as follows: (1) Cut the sample by wire according to the sample size requirements of the electron microscope; (2) Grind on silicon carbide sandpaper, from 180 grit to 2000 grit, until there are only fine scratches in one direction on the grinding surface, and rinse the sample surface with water. (3) Use velvet polishing cloth and W5 polishing paste. Adjust the speed of the polishing machine to 400 r / min. Apply even force during the polishing process to ensure that the observation surface of the sample is in complete contact with the polishing cloth. Polish for 15 min until there are no scratches on the observation surface of the sample. Clean the sample with water and anhydrous ethanol in sequence. (4) Use fine polishing cloth and W1 polishing paste. Adjust the speed of the polishing machine to 400 r / min. Apply even force during the polishing process to ensure that the observation surface of the sample is in complete contact with the polishing cloth. Polish for 10 minutes until the observation surface of the sample is a bright mirror surface. Clean the sample with water and anhydrous ethanol in sequence. (5) Electrolytic polishing: Prepare an electrolyte by mixing perchloric acid and glacial acetic acid with a purity of 70~72% in a 1:3 ratio. Place the electrolyte in a constant temperature water bath containing an ice-water mixture at about 0°C. Connect the cathode of the DC power supply to a stainless steel sheet and the anode to the sample. Set the power supply voltage to 10V and the current to no more than 0.5A. Place the sample into the electrolyte and electrolyze for about 90 seconds. After electrolysis, quickly remove the sample, rinse it with water, then ultrasonically clean it in anhydrous ethanol, and finally dry it with a hair dryer. (6) Image data acquisition: The sample was acquired using a scanning electron microscope equipped with an EBSD probe, with an acquisition step size of 400 nm; (7) Figure 4 This is a topographic image of the area collected after the Super304H steel sample was prepared in Example 2 of this invention. No obvious sigma phase detachment was observed. Figure 5 This is the EBSD image data acquisition result of Super304H steel in Example 2 of the present invention. According to the data, the resolution of the sample in this example is 96.56%, and the sigma phase content is approximately 1.6%.
[0024] Comparative Example 1: To further verify the feasibility of the method in this application, this comparative example uses TP347HFG steel, which is exactly the same as that in the examples, to perform quantitative statistics on the sigma phase in TP347HFG steel. The steps are as follows: (1) Cut the sample by wire according to the sample size requirements of the electron microscope; (2) Grind on silicon carbide sandpaper, from 180 grit to 2000 grit, until there are only fine scratches in one direction on the grinding surface, and rinse the sample surface with water. (3) Use velvet polishing cloth and W5 polishing paste. Adjust the speed of the polishing machine to 400 r / min. Apply even force during the polishing process to ensure that the observation surface of the sample is in complete contact with the polishing cloth. Polish for 15 minutes until there are no scratches on the observation surface of the sample. Clean the sample with water and anhydrous ethanol in sequence. (4) Use fine polishing cloth and W1 polishing paste. Adjust the speed of the polishing machine to 400r / min. Apply even force during the polishing process to ensure that the observation surface of the sample is in complete contact with the polishing cloth. Polish for 10 minutes until the observation surface of the sample is a bright mirror surface. Clean the sample with water and anhydrous ethanol in sequence. (5) Electrolytic polishing: Prepare an electrolyte solution by mixing perchloric acid with a purity of 70~72% in a 1:9 ratio with anhydrous ethanol. Connect the cathode of the DC power supply to a stainless steel sheet and the anode to the sample. Set the power supply voltage to 15V and the current to no more than 0.5A. Place the sample in the room temperature electrolyte solution and electrolyze for about 15s. After electrolysis, quickly remove the sample, rinse it with water, then place it in anhydrous ethanol for ultrasonic cleaning, and finally dry it with a hair dryer. (6) Figure 6 The image shows the morphology of the area acquired by EBSD after the TP347HFG steel sample was prepared in Comparative Example 1. It reveals that most of the sigma phase has detached. Figure 7 The image shown is a high-magnification SEM image of the TP347HFG steel sample after preparation in Comparative Example 1, confirming the detachment of the sigma phase.
[0025] The biggest difference between Comparative Example 1 and Example 1 lies in the different electrolyte ratios, electrolysis environments, and resulting electrolysis times during electropolishing. The final results show that Example 1 of this application achieved a resolution of 98.85%, clearly calculating the sigma phase content to be approximately 1.2%; while in Comparative Example 1, most of the sigma phase detached, rendering the sigma phase statistics meaningless.
[0026] This application provides an electropolishing method for EBSD characterization of austenitic heat-resistant steel with sigma phase precipitation. This method innovatively uses a 1:3 ratio of perchloric acid and glacial acetic acid (70-72% purity) to prepare the electrolyte, and then places the electrolyte in a constant-temperature bath containing an ice-water mixture for electropolishing. Experiments show that under these conditions, the sigma phase does not dissolve or fall off during electropolishing, overcoming the drawback of traditional stainless steel electropolishing which easily leads to the dissolution and shedding of the sigma phase. This method employs two mechanical polishing processes and one electropolishing process, significantly reducing polishing time and cost compared to traditional argon ion beam polishing methods. Furthermore, it provides a suitable EBSD acquisition step size for sigma phase calibration. By acquiring the sigma phase within the acquisition step size range set in this application, the problems of low resolution and large data errors in optical microscopy quantitative methods are solved, minimizing costs while meeting statistical accuracy requirements.
[0027] It should be noted that, in this document, relational terms such as "first" and "second" are used merely to distinguish one entity or operation from another, and do not necessarily require or imply any such actual relationship or order between these entities or operations. Furthermore, the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Terms such as "upper," "lower," "left," "right," "front," and "rear" used in the invention are merely for clarity of description and are not intended to limit the scope of the invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the invention.
[0028] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.
Claims
1. A quantitative statistical method for the Sigma phase in austenitic heat-resistant steel, characterized in that, The quantitative statistical method includes the following steps: First, the sample is wire-cut according to the sample size requirements of the electron microscope; then, the sample is ground and polished; after polishing, the sample is electrolytically polished; after electrolytic polishing, the sample is cleaned and dried, and finally, image data is acquired.
2. The quantitative statistical method for the Sigma phase in austenitic heat-resistant steel according to claim 1, characterized in that: The specific process of grinding and polishing the sample includes: using silicon carbide sandpaper to grind the sample step by step from the initial grit to the target grit, until only fine grinding marks in a single direction exist on the grinding surface.
3. The quantitative statistical method for the Sigma phase in austenitic heat-resistant steel according to claim 2, characterized in that: The specific process of grinding and polishing the sample further includes: performing a first mechanical polishing on the sample; the first mechanical polishing includes using a polishing cloth made of velvet or wool, and using polishing paste of W5 or W3.
5.
4. The quantitative statistical method for the Sigma phase in austenitic heat-resistant steel according to claim 3, characterized in that, When performing the first mechanical polishing of the sample, the polishing machine speed is maintained at 300~400r / min, and the force is kept uniform during the polishing process so that the observation surface of the sample is in complete contact with the polishing cloth. The polishing time is not less than 10min, and polishing is stopped when the observation surface of the sample is free of scratches and is bright and smooth after cleaning.
5. The quantitative statistical method for the Sigma phase in austenitic heat-resistant steel according to claim 2, characterized in that, The specific process of grinding and polishing the sample includes: performing a second mechanical polishing on the sample; using a fine polishing cloth made of fine polishing velvet material, and using polishing paste W1 or W0.
5.
6. The quantitative statistical method for the Sigma phase in austenitic heat-resistant steel according to claim 5, characterized in that, When performing the second mechanical polishing on the sample, the polishing machine speed is maintained at 300~400r / min, and the force is kept uniform during the polishing process so that the observation surface of the sample is in complete contact with the polishing cloth. The polishing time is not less than 10min, and polishing is stopped when the observation surface of the sample is a bright mirror surface.
7. The quantitative statistical method for the Sigma phase in austenitic heat-resistant steel according to claim 1, characterized in that, The specific process of electropolishing the sample after polishing includes: preparing an electrolyte solution by mixing perchloric acid and glacial acetic acid with a purity of 70-72% in a 1:3 ratio; placing the electrolyte solution in a constant temperature water bath containing an ice-water mixture at around 0°C; connecting the cathode of a DC power supply to a stainless steel or iron sheet and the anode to the sample; setting the power supply voltage to 10-15V and the current not exceeding 0.5A; and immersing the sample in the electrolyte solution.
8. The quantitative statistical method for the Sigma phase in austenitic heat-resistant steel according to claim 7, characterized in that, In the electrolytic polishing step, the electrolysis time is approximately 60 to 120 seconds depending on the composition of the heat-resistant steel. The specific time depends on the small amplitude swinging of the sample during the electrolysis process, with the observation of black film-like fragments falling off the sample surface and into the electrolyte as the benchmark.
9. A quantitative statistical method for the Sigma phase in austenitic heat-resistant steel according to claim 8, characterized in that, In the electropolishing step, after the sample is electrolyzed, it is quickly removed, rinsed with water, then ultrasonically cleaned in anhydrous ethanol, and finally dried with a hair dryer.
10. The quantitative statistical method for the Sigma phase in austenitic heat-resistant steel according to claim 1, characterized in that, The image data acquisition step specifically includes: using a scanning electron microscope equipped with an EBSD probe to acquire data from the sample, wherein the acquisition step size is set to the range of 1 / 3 to 1 / 2 of the average diameter of the sigma phase.