Accurate determination method for phase content in stainless steel

Abstract

The invention discloses a method for accurately measuring the phase content in stainless steel. The method is suitable for stainless steel containing at least one phase of ferrite, austenite, cementite and martensite. The method comprises the following steps: S1, cutting, grinding and polishing a stainless steel sample to obtain a bright surface; s2, pre-judging the type and the number of contained phases through metallographic structure observation in combination with component analysis and a heat treatment process; and S3, directional detection is conducted according to the pre-judgment result, ferrite is measured through metallographic software after being subjected to electrolytic corrosion through a KOH electrolyte, austenite is measured through an X-ray diffraction technology, cementite is measured through a laser confocal microscope after being subjected to acid etching, and martensite is obtained by subtracting the sum of the measured phase content by 100%. The method integrates various technical advantages, is simple and convenient to operate, wide in detection range and high in precision, and can provide accurate data support for stainless steel process optimization and material development.

Description

Technical Field

[0001] This invention relates to the field of metallographic quantitative detection technology, specifically to a method for accurately determining the phase content in stainless steel, which is particularly suitable for determining the phase content of stainless steel containing at least one phase among ferrite, austenite, cementite, and martensite. Background Technology

[0002] Stainless steel, due to its excellent corrosion resistance and high plasticity, is widely used in key equipment in industries such as shipbuilding, marine engineering, and petrochemicals. With the upgrading of industrial demands, stainless steel has evolved from a single-phase material to a multiphase material: introducing austenitic phase into martensitic stainless steel can improve low-temperature toughness; adding ferrite phase to austenitic stainless steel can improve corrosion resistance and weldability; and introducing cementite phase can enhance hardness and wear resistance. The content ratio of each phase directly determines the performance of stainless steel; therefore, accurately determining the content of each phase in multiphase stainless steel is of great significance for material development, process optimization, and performance control.

[0003] However, existing methods for determining the phase content of stainless steel have significant drawbacks: (1) Traditional metallographic etching method: It relies on a single staining agent to distinguish phases, but the staining range is limited and the ability to distinguish phase types is weak. Especially for phases with irregular morphology (such as austenite in martensite), it is easy to cause large measurement errors and it is difficult to meet the requirements of accurate detection. (2) Scanning electron microscopy EBSD method: Although it can achieve high-precision measurement, the sample preparation process is complicated (requiring argon ion polishing, vibration polishing, etc.), and the scanning range is small and the time is long (the measurement of a single sample may take 2 to 3 days), which limits its applicability.

[0004] Existing technologies also include some analytical methods for the content of various phases in steel. For example, patent CN113899605B discloses a quantitative analytical method for the volumetric content of various phases in QP steel, which is applicable to the determination of the content of various phases in QP steel. However, it can only determine the three phases of retained austenite, martensite, and ferrite. If a fourth phase (such as cementite) is present, it cannot be evaluated. At the same time, because fine retained austenite is wrapped in martensite, improper control of the staining time can easily lead to the masking of fine phases, resulting in a large error when calculating the ferrite content. Patent CN117665026A discloses a quantitative analytical method for the percentage content of different microstructures in advanced high-strength steel. It uses EBSD to determine the phase content for advanced high-strength steel, which has strict requirements on the residual stress of the sample surface, requires complex polishing treatment, and the large-scale scanning is extremely time-consuming, making it impractical.

[0005] In summary, existing technologies cannot meet the requirements for accurate, efficient, and convenient determination of the phase content of multiphase stainless steel (including ferrite, austenite, cementite, martensite, etc.), and a new determination method is urgently needed. Summary of the Invention

[0006] To address the problems of low accuracy, complex procedures, narrow applicability, and long time consumption in existing methods for determining the phase content of stainless steel, this invention provides a precise method for determining the phase content of stainless steel, enabling accurate determination of at least one phase among ferrite, austenite, cementite, and martensite.

[0007] To achieve the above objectives, the specific solution adopted by the present invention is as follows: A method for accurately determining the phase content in stainless steel, wherein the phase in the stainless steel is at least one selected from ferrite, austenite, cementite, and martensite, mainly comprising the following steps: S1. Sample preparation: The stainless steel sample is cut, ground and polished to obtain a bright surface to be tested; S2. Phase composition prediction: Determine the types and quantities of phases by observing the metallographic structure, combining composition analysis and heat treatment process; S3. Detection of directional phase content: If it is a single phase, the content of that phase is considered to be 100%; if there are at least two phases, based on the prediction result of step S2, perform the following steps: S31, Measurable Phase Detection (a) When ferrite phase is present: the polished sample is placed in an electrolyte containing KOH for electrolytic corrosion, so that the ferrite phase appears reddish-brown. Metallographic images of the sample after electrolytic corrosion are taken and the images are imported into the metallographic content determination software. The ferrite phase content is determined based on the proportion of the reddish-brown area. (b) When austenitic phase is present: its content is determined by X-ray diffraction. (c) When the cementite phase is present: the sample is acid etched, and the three-dimensional morphology is obtained by laser confocal microscopy. The cementite phase content is determined based on the height difference between the phases. S32. If step S2 determines that the phase does not contain martensite, then the process ends. If step S2 predicts the presence of martensitic phase, then based on the assumption that the total amount of each phase in stainless steel is 100%, and combined with the measured phase content, the martensite content is calculated using the following formula: Martensite content = 100% - ∑ measured phase content; The phase being measured is at least one of the ferrite phase, austenite phase, and cementite phase detected in step S31.

[0008] Furthermore, in step S2, the metallographic observation is based on the phase morphology characteristics to predict the phase type: ferrite is polygonal, austenite contains twins, cementite is granular, and martensite is lath or lenticular.

[0009] Further, in step S1, the specific method of the grinding process is as follows: grinding is performed sequentially using 180#, 280#, 320#, and 500# sandpaper, with the sample rotated 90° after each grinding pass.

[0010] Furthermore, in step S1, during mechanical polishing, the polishing cloth is made of woolen cloth, the coarse polishing agent is a 3.5μm diamond suspension, and the polishing speed is 850~950r / min; the fine polishing agent is a 1.5μm diamond suspension, and the polishing speed is 550~600r / min.

[0011] Furthermore, in step S2, the electrolyte is prepared from KOH and water; the specific parameters during electrolysis are: voltage 6.5-7.5V, current ≤5A, and time 120-180s.

[0012] Furthermore, in step S3, the specific parameters for detecting the austenite phase content using an X-ray stress analyzer are as follows: the target material is a Cr target, the equipment voltage is boosted to 30kV, the current is 6.7mA, the diffraction angles of the goniometer are adjusted to 156.4° for detector A and 130° for detector B, the exposure time is 20s, the diffraction crystal planes are 211 and 220 respectively, the rotation angle is 0°, and the tilt angle is 0°.

[0013] Further, in step S3, the acid etching treatment uses a solution containing 5% ferric chloride and 50% hydrochloric acid, and the etching time is ≤10s; the detection parameters of the laser confocal microscope include: 260V detector voltage, 0.5AU pinhole, and 2048×2048 pixel resolution.

[0014] This invention achieves accurate determination of the content of each phase in stainless steel by optimizing the measurement process and parameters. Its beneficial effects, combined with the specific steps, raw materials, and parameters, are analyzed as follows: (1) High measurement accuracy and comprehensive coverage of covering phases For the ferrite phase: electrolytic corrosion with KOH electrolyte (voltage 6.5-7.5V, current ≤5A, time 120-180s) can make ferrite specifically reddish-brown, which is significantly distinguishable from other phases (such as austenite and martensite). Combined with software analysis of the proportion of reddish-brown area, the problem of phase identification ambiguity in traditional staining methods is avoided, and the ferrite measurement error is ≤0.5%. For the austenite phase: X-ray diffraction using a Cr target (30kV, 6.7mA) and specific diffraction planes (211, 220) resulted in strong diffraction peak signals with minimal interference. Through fitting processing, the austenite content of the face-centered cubic structure can be accurately calculated with a relative error ≤1%. For the cementite phase: an acid etching solution containing 5% ferric chloride and 50% hydrochloric acid (etching ≤5s) can quickly distinguish cementite from the matrix (cementite has good corrosion resistance and low corrosion degree). Combined with the high resolution (2048×2048 pixels) of a laser confocal microscope, it can clearly capture nanoscale height differences, and the accuracy of cementite measurement can reach 0.1%. For the martensitic phase: the calculation using "100% - ∑ measured phase content" avoids the problem of large direct measurement errors caused by martensite easily encapsulating fine phases (such as retained austenite). Combined with the high-precision measurement results of other phases, the calculation error of martensite content is ≤1.5%. In summary, this invention can simultaneously and accurately measure four phases: ferrite, austenite, cementite, and martensite, solving the problems of incomplete phase coverage (such as the inability to measure cementite in patent CN113899605B) or insufficient accuracy in existing technologies.

[0015] (2) The process is simple, the time consumption is short, and the predictive step improves efficiency. Sample preparation involves gradient grinding with 180#-500# sandpaper (90° rotation per pass) and graded polishing with diamond suspension (rough polishing 850-950 r / min, fine polishing 550-600 r / min). A qualified surface can be obtained in just 2-3 hours, eliminating the need for complex treatments such as argon ion polishing (as CN117665026A requires vibration polishing). The key is "pre-judgment before measurement": for example, if it is predicted that there is no cementite phase, acid etching and laser confocal microscopy are unnecessary and can be skipped directly; if it is predicted that there is only ferrite and austenite, only the corresponding detection steps need to be performed, avoiding invalid operations. The entire measurement time for a single sample is ≤4 hours, far less than the 2-3 days of the EBSD method, significantly improving efficiency.

[0016] (3) It has significant guiding significance for process optimization. This invention can accurately obtain the content ratio of "ferrite-austenite-cementite-martensite". Combined with the heat treatment process (such as quenching temperature and tempering time) and performance (such as hardness and corrosion resistance) data of stainless steel, it can establish a quantitative relationship of "process parameters-phase content-material properties". For example, if the test finds that the austenite content is too low, resulting in insufficient low-temperature toughness, the process can be optimized by adjusting the Ni element content (to improve austenite stability) or reducing the cooling rate (to increase austenite retention). This forms a closed-loop control of "process-structure-performance", shortens the process optimization cycle, and reduces R&D costs. Attached Figure Description

[0017] Figure 1 This is a microstructure image of the sample after electrolytic corrosion in Example 1.

[0018] Figure 2 For metallographic content determination software Figure 1The processing result image shows that the reddish-brown area is the ferrite phase, and the distribution morphology of ferrite and its boundary with other phases can be clearly observed. The area ratio of the ferrite phase, i.e. the ferrite phase content, can be directly obtained through software analysis.

[0019] Figure 3 This is a distribution diagram of diffraction peaks of a body-centered cubic phase obtained by X-ray diffraction. The position and intensity of the diffraction peaks can help analyze the presence and content characteristics of the body-centered cubic phase.

[0020] Figure 4 The image shows the diffraction peak distribution of the face-centered cubic phase obtained by X-ray diffraction. The austenite phase content can be accurately calculated by combining the diffraction peak fitting process.

[0021] Figure 5 The confocal metallographic image of the stainless steel sample taken by laser confocal microscopy shows the granular morphology of the cementite phase and its distribution in the matrix.

[0022] Figure 6 The image shows a three-dimensional elevation topography obtained by laser confocal microscopy. The different elevation differences clearly reflect the microscopic morphological differences between the cementite phase (which has good corrosion resistance, low corrosion degree, and relatively high elevation) and other phases in the matrix, providing an intuitive basis for the determination of cementite phase content.

[0023] Figure 7 The image shows a three-dimensional morphology of the stainless steel sample surface obtained by laser confocal microscopy, which can more intuitively show the spatial distribution and height differences between the cementite phase and other phases.

[0024] Figure 8 This is a schematic diagram of the analysis results of the cementite phase content by laser confocal analysis software. By processing the three-dimensional scanning data, the volume fraction of the cementite phase, i.e., the cementite phase content, can be directly obtained. Detailed Implementation

[0025] The technical solution of the present invention will be clearly and completely described below with reference to specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0026] This invention discloses a method for accurately determining the phase content in stainless steel, wherein the phase in the stainless steel is at least one of ferrite, austenite, cementite, and martensite. The steps are described in detail below.

[0027] S1. Sample Preparation The stainless steel sample is cut, ground, and polished to obtain a bright surface to be tested.

[0028] Among them, the grinding process involves using 180#, 280#, 320#, and 500# sandpaper in sequence for grinding. After each grinding pass, the sample is rotated 90° to ensure that the surface is flat and free of obvious scratches.

[0029] Polishing process: Mechanical polishing is adopted, and woolen cloth is selected for polishing. For rough polishing, 3.5μm diamond suspension is used, and the polishing speed is 850~950r / min. For fine polishing, 1.5μm diamond suspension is used, and the polishing speed is 550~600r / min. Finally, a scratch-free bright surface is obtained, which is then cleaned and dried for later use.

[0030] S2, Phase Composition Prediction By observing the metallographic structure (preliminary observation under an optical microscope), combined with the compositional analysis of stainless steel (such as the content of elements like C, Cr, and Ni) and the heat treatment process (such as quenching and tempering temperatures), the type and quantity of phases contained in the stainless steel to be tested (such as whether it contains one or more of ferrite, austenite, cementite, and martensite) can be determined. This phase composition prediction step can directly determine which phases need to be tested in the next step, thus accelerating the testing cycle.

[0031] S3, Detection of directional phase content If only a single phase is present, the content of that phase is directly considered to be 100%; if at least two phases are present, the content of each phase is determined based on the prediction result of S2 as follows: S31, Measurable Phase Detection (a) Ferrite phase detection: The polished sample was placed in an electrolyte containing KOH for electrolytic corrosion (the electrolyte was prepared by KOH and water, and the electrolysis parameters were: voltage 6.5-7.5V, current ≤5A, time 120-180s) to make the ferrite phase appear reddish-brown; the sample after electrolytic corrosion was photographed under an optical microscope, and the photograph was imported into the metallographic content determination software to determine the ferrite phase content based on the proportion of the reddish-brown area.

[0032] (b) Austenite phase detection: X-ray diffraction was used for determination. Specific parameters: target material was Cr target, equipment voltage was 30kV, current was 6.7mA, diffraction angle of goniometer (A detector 156.4°, B detector 130°), exposure time was 20s, diffraction crystal planes were 211 and 220 crystal planes, rotation angle was 0°, and tilt angle was 0°. The austenite phase content was calculated by fitting the diffraction peaks.

[0033] (c) Detection of cementite phase: The sample was acid-etched (the acid solution was a solution containing 5% ferric chloride and 50% hydrochloric acid, and the etching time was ≤10s), and then detected by laser confocal microscopy (parameters: detector voltage 260V, pinhole 0.5AU, resolution 2048×2048 pixels); the content of cementite phase was determined based on the microscopic height difference between the cementite phase (high hardness and good corrosion resistance) and other phases in the matrix.

[0034] S32. If step S2 determines that the phase does not contain martensite, then the process ends. If step S2 predicts that martensite phase is included, then based on the total amount of each phase being 100%, and combined with the content of the phases (ferrite, austenite, cementite) measured in S31, the martensite content is calculated using the formula "martensite content = 100% - ∑ content of measured phases".

[0035] The technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0036] Example 1 This embodiment provides a method for accurately determining the content of each phase in a four-phase stainless steel, which has been pre-determined to contain ferrite, austenite, cementite, and martensite. The method mainly includes the following steps: Step 1: Sample Preparation Stainless steel samples were cut using a cutting machine, with sample dimensions of 20mm*20mm*20mm. The cut samples were then ground sequentially with 180#, 280#, 320#, and 500# sandpaper. After grinding with each grade of sandpaper, the sample was rotated 90° before being ground with the next grade. After grinding, mechanical polishing was performed using woolen cloth. The coarse polishing agent was a 3.5µm diamond suspension, and the polishing speed was 900 rpm. The fine polishing agent was a 1.5µm diamond suspension, and the polishing speed was 550 rpm. The samples were polished to a scratch-free, bright surface, rinsed with tap water, and then air-dried. Two samples were prepared using this method.

[0037] Step 2: Electrolytic Corrosion Prepare the electrolytic etching solution by adding 16g KOH to 100ml of water and mixing thoroughly. Set the electrolysis parameters: voltage to 7.0V, current to 3A. Select a sample, with the polished surface of the sample facing the electrode, and electrolyze for 126s.

[0038] Step 3: Determination of Ferrite Phase Content The electrolytically etched sample was photographed under an optical microscope at 100x magnification. The focus was adjusted to ensure image clarity. The reddish-brown color in the image represents the ferrite phase. Figure 1 The image was imported into software to determine the percentage of reddish-brown ferrite phase. (See attached image.) Figure 2The ferrite phase content is 9.05%, as shown in Table 1.

[0039] Table 1. Results of Ferrite Phase Content Detection Step 4: Detect the austenite phase content using an X-ray stress analyzer. Adjust the instrument level and detection distance, and precisely adjust the equipment support column. Observe with a level to ensure the equipment is in a horizontal position. Adjust the goniometer to 8.7 mm from the sample surface. Set the equipment parameters: select a Cr target as the target material, boost the voltage to 30 kV, set the current to 6.7 mA, adjust the goniometer diffraction angles to 156.4° for detector A and 130° for detector B, set the exposure time to 20 s, and the diffraction crystal planes to 211 and 220, with a rotation angle of 0° and a tilt angle of 0°. After setting the equipment parameters, perform detection on the detection area and perform fitting processing on the diffraction peaks. Figure 3 and Figure 4 The results of the austenite phase content test were recorded. The test results are shown in Tables 2 and 3. The austenite phase content was 2.2%.

[0040] Table 2 Phase detection results Table 3. Percentage of austenite phase Step 5: Determination of cementite content Prepare an etching solution containing 5% ferric chloride and 50% hydrochloric acid. Immerse another sample prepared in step one in the etching solution for 5 seconds, then remove, rinse, and dry quickly. Place the treated sample under a laser confocal microscope, adjust the objective lens magnification to 10X, and the total magnification to 100X to take metallographic images. Set the laser detector voltage to 260V, the pinhole size to 0.5AU, and the image scanning resolution to 2048x2048 pixels. After taking a scanned 3D image, import it into the analysis software, select an appropriate height, analyze the cementite content, and record the results. See [link to relevant documentation]. Figures 5-8 The cementite content was found to be 10.0% upon testing.

[0041] Step Six: Calculation of Martensite Phase Content Substituting into the calculation formula: 100% - ferrite content - austenite content - ferrite content, the martensite content is calculated to be 100% - 9.05% - 2.2% - 10.0% = 78.75%.

[0042] The phase contents of multiphase stainless steel are as follows: ferrite 9.05%, austenite 2.2%, cementite 10.0%, and martensite 78.75%.

[0043] This invention integrates technologies such as electrolytic corrosion, X-ray diffraction, and laser confocalization, combined with precise parameter control, to achieve efficient and accurate determination of the multiphase content in stainless steel, providing a reliable basis for the material development and process optimization of stainless steel.

[0044] The above description is merely a preferred embodiment of the present invention and is not intended to limit the scope of the invention in any way. All equivalent transformations or modifications made in accordance with the essence of the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for accurately determining the phase content in stainless steel, wherein the phase in the stainless steel is at least one selected from ferrite, austenite, cementite, and martensite, characterized in that... The main steps include the following: S1. Sample preparation: The stainless steel sample is cut, ground and polished to obtain a bright surface to be tested; S2. Phase composition prediction: Determine the types and quantities of phases by observing the metallographic structure, combining composition analysis and heat treatment process; S3. Detection of directional phase content: If it is a single phase, the content of that phase is considered to be 100%; if there are at least two phases, based on the prediction result of step S2, perform the following steps: S31, Measurable Phase Detection (a) When ferrite phase is present: the polished sample is placed in an electrolyte containing KOH for electrolytic corrosion, so that the ferrite phase appears reddish-brown. Metallographic images of the sample after electrolytic corrosion are taken and the images are imported into the metallographic content determination software. The ferrite phase content is determined based on the proportion of the reddish-brown area. (b) When austenitic phase is present: its content is determined by X-ray diffraction. (c) When the cementite phase is present: the sample is acid etched, and the three-dimensional morphology is obtained by laser confocal microscopy. The cementite phase content is determined based on the height difference between the phases. S32. If step S2 determines that the phase does not contain martensite, then the process ends. If step S2 predicts the presence of martensitic phase, then based on the assumption that the total amount of each phase in stainless steel is 100%, and combined with the measured phase content, the martensite content is calculated using the following formula: Martensite content = 100% - ∑ measured phase content; The phase being measured is at least one of the ferrite phase, austenite phase, and cementite phase detected in step S31.

2. The method according to claim 1, characterized in that, In step S2, the metallographic observation is based on the phase morphology characteristics to predict the phase type: ferrite is polygonal, austenite contains twins, cementite is granular, and martensite is lath or lenticular.

3. The method according to claim 1, characterized in that, In step S1, the specific method of the grinding process is as follows: the sample is ground sequentially using 180#, 280#, 320# and 500# sandpaper, and rotated 90° after each grinding pass.

4. The method according to claim 1, characterized in that, In step S1, during mechanical polishing, the polishing cloth is made of woolen cloth, the coarse polishing agent is a 3.5μm diamond suspension, and the polishing speed is 850~950r / min; the fine polishing agent is a 1.5μm diamond suspension, and the polishing speed is 550~600r / min.

5. The method according to claim 1, characterized in that, In step S2, the electrolyte is prepared from KOH and water; the specific parameters during electrolysis are: voltage 6.5-7.5V, current ≤5A, and time 120-180s.

6. The method according to claim 1, characterized in that, In step S3, the specific parameters for detecting the austenite phase content using an X-ray stress analyzer are as follows: the target material is a Cr target, the equipment voltage is boosted to 30kV, the current is 6.7mA, the diffraction angles of the goniometer are adjusted to 156.4° for detector A and 130° for detector B, the exposure time is 20s, the diffraction crystal planes are 211 and 220 respectively, the rotation angle is 0°, and the tilt angle is 0°.

7. The method according to claim 1, characterized in that, In step S3, the acid etching treatment uses an acid etching solution containing 5% ferric chloride and 50% hydrochloric acid, and the etching time is ≤10s; the detection parameters of the laser confocal microscope include: 260V detector voltage, 0.5AU pinhole, and 2048×2048 pixel resolution.

Images