A method for displaying prior austenite grain boundaries of a steel for a high-pressure end of a steam turbine rotor

Abstract

The application discloses a prior austenite grain boundary display method for a steel for a high-pressure end of a steam turbine rotor and belongs to the technical field of metallographic detection. A surface treatment is performed on a steel sample to be detected to obtain a metallographic sample without visible scratches on the surface; the metallographic sample is subjected to electrolytic corrosion, washing and metallographic observation; wherein the electrolyte is a mixture of phosphoric acid and sulfuric acid, and the electrolysis parameters are 14-16 V direct current voltage and electrolytic corrosion for 50-70 s. The application utilizes a customized acid electrolyte, realizes controllable and selective corrosion under optimized electrochemical conditions by utilizing the microscopic difference in chemical activity between the prior austenite grain boundary and the surrounding bainite matrix, and thus safely and efficiently reveals the grain boundary network which is crucial to the material performance.

Description

Technical Field

[0001] This invention belongs to the field of metallographic testing technology, specifically relating to a method for displaying the original austenite grain boundaries of steel used in the high-pressure end of a steam turbine rotor. Background Technology

[0002] As a core power component of a generator set, the turbine rotor operates under high temperature, high pressure, and complex centrifugal force coupling for extended periods. Its material properties directly impact the safety, reliability, and service life of the unit. The high-pressure end of the rotor typically undergoes heat treatment using methods such as forced draft or oil cooling, resulting in a predominantly bainitic microstructure. The original austenitic grain boundaries constitute the initial microstructure framework formed at high temperatures, and their morphology, size, and continuity have a decisive influence on the material's creep resistance, fracture toughness, and stress corrosion susceptibility. Clear and accurate characterization of the original austenitic grain boundaries can effectively diagnose whether the material has undergone degradation behaviors such as grain coarsening and thermal embrittlement. This is a crucial foundation for assessing the condition of bainitic steel materials used in turbine rotors, diagnosing microstructure degradation, and predicting remaining service life, and is essential for ensuring the long-term safe operation of the generator set.

[0003] Currently, the mainstream methods for clearly displaying the original austenitic grain boundaries can be divided into chemical etching and electrolytic etching. Chemical etching using picric acid or electrolytic etching requiring complex organic agents generally suffer from poor safety, cumbersome operation, or easy damage to the sample surface. More importantly, most of these methods were not developed specifically for bainitic steels. Because the bainitic structure itself is dense and fine, it severely interferes with grain boundary contrast. Furthermore, conventional etchants do not show a significant difference in corrosion rate between the bainitic structure and the original austenitic grain boundaries, making it difficult to preferentially and clearly etch out the grain boundary lines. If the electrolytic etching parameters are not properly controlled, it can easily lead to over-etching or blurred visualization of the structure, failing to obtain a complete and continuous grain boundary network and thus failing to meet the requirements for accurate characterization of this type of material. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the purpose of this invention is to provide a method for revealing the original austenitic grain boundaries of steel used in the high-pressure end of turbine rotors. The method provided by this invention is safe, non-toxic, and efficient, and is applicable to the original austenitic grain boundaries of bainitic steel in the high-pressure end of turbine rotors. It is of great significance for meeting the needs of production testing and quality control.

[0005] To achieve the above objectives, the technical solution of the present invention is as follows:

[0006] A first aspect of the present invention provides a method for revealing the original austenite grain boundaries in steel used at the high-pressure end of a steam turbine rotor, comprising:

[0007] The steel sample to be tested is surface treated to obtain a metallographic sample with no visible scratches on the surface;

[0008] Metallographic samples were subjected to electrolytic etching, rinsed, and observed. The electrolyte was a mixture of phosphoric acid and sulfuric acid, and the electrolysis parameters were 14-16 V DC voltage and 50-70 s electrolytic etching.

[0009] In some embodiments of the present invention, the surface treatment includes cutting, grinding and mechanical polishing the steel sample to be tested.

[0010] In some embodiments of the present invention, the polishing is performed in stages from 240 grit to 2000 grit.

[0011] In some embodiments of the present invention, the metallographic specimen has dimensions of (9-11) mm × (9-11) mm × (4-6) mm.

[0012] In some embodiments of the present invention, during electrolytic corrosion, the metallographic sample is fixed at the anode, the cathode is a copper sheet, and the effective immersion area of ​​the cathode in the electrolyte is greater than 400 mm. 2 .

[0013] In some embodiments of the present invention, the distance between the cathode and the anode is 5-10 cm.

[0014] In some embodiments of the present invention, the volume ratio of phosphoric acid to sulfuric acid is (2.8~3.2):(1.8~2.2).

[0015] In some embodiments of the present invention, the mass concentration of phosphoric acid is ≥85%, and the mass concentration of sulfuric acid is 95~98%.

[0016] In some embodiments of the present invention, the electrolytic corrosion is completed and then rinsed with water and ethanol in sequence; the time interval between the completion of electrolytic corrosion and water rinsing does not exceed 5 seconds.

[0017] In some embodiments of the present invention, the steel sample to be tested includes 25Cr2NiMo1V steel and 30Cr2Ni4MoV steel that have been cooled and have a bainitic microstructure.

[0018] The beneficial effects of this invention are as follows:

[0019] This invention provides a method for revealing the original austenitic grain boundaries of steel used in the high-pressure end of steam turbine rotors. The mixed electrolyte of phosphoric acid and sulfuric acid used completely avoids the explosion and toxicity risks associated with the traditional method using picric acid, significantly improving operational safety. The core mechanism of this method lies in the ingenious combination of conductivity and passivation behavior: in a concentrated acid and low-water system, conductivity does not rely on free ion migration, but rather on proton transition mechanisms, where protons transfer charge along the hydrogen bond network to maintain the electrolysis process. Simultaneously, phosphoric acid forms a high-viscosity phosphate protective film (passivation film) on the anode surface, while sulfuric acid precisely regulates the density and stability of this film. Due to the presence of compositional segregation or defects at the original austenitic grain boundaries, the protective film on its surface is relatively fragile and will preferentially be broken down and dissolved under the action of an electric field, forming grooves. The grain body, due to the integrity of the protective film, remains flat, thus clearly outlining the grain boundary contours. This method achieves clear visualization through a single, short-duration electrolysis process. The procedure is simple and efficient, and the electrolyte composition is straightforward, effectively avoiding defects such as pitting and etching on the sample surface that can be caused by complex additives, thus ensuring the integrity of the observed surface. This method is optimized for the microstructure of bainitic steel, selectively and clearly etching out a continuous network of proto-austenite grain boundaries, solving the problem of poor visualization results for this type of material found in existing methods. Attached Figure Description

[0020] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0021] Figure 1 This is a structural diagram of the electrolytic corrosion apparatus used in the embodiments of the present invention.

[0022] Figure 2 This is the original austenite grain boundary diagram obtained by the method of the present invention for 25Cr2NiMo1V bainitic steel in Example 1 of the present invention.

[0023] Figure 3 This is the original austenite grain boundary diagram obtained by the method of the present invention for 30Cr2Ni4MoV bainitic steel in Example 2 of the present invention.

[0024] Figure 4 This is the original austenite grain boundary diagram obtained by the method of the present invention for 25Cr2NiMo1V bainitic steel in Example 3 of the present invention.

[0025] Figure 5 This is the original austenite grain boundary diagram obtained by the method of the present invention for 25Cr2NiMo1V bainitic steel in Example 4 of the present invention.

[0026] Figure 6 The image shown is a metallographic image of 25Cr2NiMo1V bainitic steel obtained by chemical etching in Comparative Example 1 of this invention.

[0027] Figure 7 The image shown is a metallographic image of the 30Cr2Ni4MoV bainitic steel in Comparative Example 2 of this invention obtained by chemical etching.

[0028] Figure 8 This is a metallographic photograph of the 25Cr2NiMo1V bainitic steel obtained by using the method of the present invention under excess phosphoric acid in Comparative Example 3 of the present invention.

[0029] Figure 9 This is a photograph of the metallographic structure of 25Cr2NiMo1V bainitic steel obtained by using the method of the present invention under excess sulfuric acid in Comparative Example 4 of the present invention.

[0030] Figure 10 The image shows the metallographic structure of the 25Cr2NiMo1V bainitic steel in Comparative Example 5 of this invention obtained using the method of this invention when the distance between the anode and cathode is too large.

[0031] Figure 11 The image shows the metallographic structure of the 25Cr2NiMo1V bainitic steel in Comparative Example 6 of this invention obtained using the method of this invention when the distance between the anode and cathode is too small.

[0032] Figure 12 The image shows the metallographic structure of 25Cr2NiMo1V bainitic steel obtained by using the method of the present invention in pure phosphoric acid in Comparative Example 7 of the present invention.

[0033] Figure 13 The image shows the metallographic structure of 25Cr2NiMo1V bainitic steel obtained by using the method of this invention under pure sulfuric acid conditions in Comparative Example 8 of this invention.

[0034] Among them, 1: negative electrode wire, 2: copper sheet, 3: positive electrode wire, and 4: stainless steel sample holder. Detailed Implementation

[0035] To further understand the present invention, preferred embodiments of the present invention are described below in conjunction with examples. However, it should be understood that these descriptions are only for further illustrating the features and advantages of the present invention, and not for limiting the scope of the claims of the present invention.

[0036] Given that existing methods for displaying the original austenitic grain boundaries have problems such as the use of hazardous chemicals, complex operation, and easy damage to the sample surface, and generally have not been optimized for the dense microstructure and poor grain boundary contrast of bainitic steel, resulting in poor grain boundary display effect for this material, this invention proposes a method for displaying the original austenitic grain boundaries of steel used in the high-pressure end of steam turbine rotors.

[0037] This invention provides a method for revealing the original austenite grain boundaries in steel used at the high-pressure end of a steam turbine rotor, comprising:

[0038] The steel sample to be tested is surface treated to obtain a metallographic sample with no visible scratches on the surface;

[0039] Metallographic samples were subjected to electrolytic etching, rinsed, and observed. The electrolyte was a mixture of phosphoric acid and sulfuric acid, and the electrolysis parameters were 14-16 V DC voltage and 50-70 s electrolytic etching.

[0040] This invention achieves selective and preferential corrosion of the original austenitic grain boundaries in bainitic steel by combining a phosphoric acid-sulfuric acid mixed electrolyte with optimized electrolysis parameters, thereby enabling clear visualization under a microscope. Specifically, the mixture of phosphoric acid and sulfuric acid provides moderate oxidizing and complexing capabilities. Under energized conditions, this electrolyte preferentially attacks regions with irregular atomic arrangements and higher energy at grain boundaries. The bainitic structure itself is dense, and conventional etchants struggle to highlight its original grain boundaries. The electrolyte formulation of this invention effectively overcomes this interference, significantly creating grooves at grain boundaries while corroding the matrix, resulting in sufficient contrast. A voltage range of 14-16 V is sufficient to drive an effective anodic dissolution reaction, while the controlled time window ensures sufficient etching and visualization of grain boundaries while avoiding excessive "over-corrosion" of the overall structure. Fine-tuning of the current by simply adjusting the anode-cathode distance allows the operator to precisely control the process based on specific sample conditions (such as minor compositional differences), ensuring stable and repeatable results. Immediate rinsing with water and ethanol after electrolysis can quickly terminate the reaction and prevent residual electrolyte from continuing to corrode and destroy the already formed clear grain boundary morphology. In summary, this invention utilizes a customized acidic electrolyte under optimized electrochemical conditions, taking advantage of the microscopic differences in chemical activity between the original austenite grain boundaries and the surrounding bainite matrix, to achieve controllable and selective corrosion, thereby safely and efficiently revealing the grain boundary network that is crucial to material properties.

[0041] In this invention, the surface treatment includes cutting, grinding and mechanically polishing the steel sample to be tested.

[0042] Surface treatment eliminates interference, ensures uniform electrolytic etching, and exposes the true microstructure. Through cutting, grinding, and mechanical polishing, a near-perfect observation plane is prepared for subsequent electrolytic etching and microscopic observation, providing an indispensable prerequisite for obtaining clear and reliable proto-austenite grain boundaries.

[0043] In this invention, the polishing is performed in stages from 240 grit to 2000 grit, specifically by using sandpaper of grades 240#, 400#, 800#, 1000#, 1500#, and 2000# in sequence.

[0044] Fine scratches remaining on the sample surface can easily be confused with the actual original austenite grain boundaries, leading to misjudgment. Macroscopic defects are removed by cutting and rough grinding, while fine, stepwise grinding (from 240# to 2000#) can greatly eliminate scratches, deformation layers, and contaminants on the sample surface.

[0045] In this invention, the mechanical polishing is performed by sequentially using diamond polishing paste with particle sizes of 1.5 μm and 0.5 μm.

[0046] A smooth, stress-free surface ensures uniform contact and reaction between the electrolyte and the metal. If the surface is rough and uneven, the electrolytic corrosion rate will vary, leading to inconsistent grain boundary visibility or even localized areas that are not visible, severely impacting the accuracy and repeatability of the results. Polishing to achieve a mirror finish allows the true microstructure of the metal (bainitic matrix and target grain boundaries) to be exposed without interference. Only in this way can subsequent selective electrolytic corrosion occur precisely at the grain boundaries, thus highlighting a clear and continuous grain boundary network against a uniform background.

[0047] In this invention, the size of the metallographic sample is (9-11) mm × (9-11) mm × (4-6) mm, preferably 10 mm × 10 mm × 5 mm.

[0048] Controlling the metallographic sample size within the range of (9-11) mm × (9-11) mm × (4-6) mm balances ease of operation, stability of the electrolysis process, and representativeness of the observation results. This size facilitates fine surface grinding and polishing by hand while ensuring the sample is stably clamped as the anode in the electrolytic cell, which helps maintain a relatively fixed distance between the anode and cathode, thus achieving a uniform and repeatable electrolytic corrosion effect. Simultaneously, this size provides a statistically significant field of view without being too small to handle or too large, resulting in material waste and potential edge effects.

[0049] In this invention, during electrolytic corrosion, the metallographic sample is fixed at the anode, the cathode is a copper sheet, and the effective immersion area of ​​the cathode in the electrolyte is greater than 400 mm². 2 .

[0050] The cathode is a copper sheet, and its effective immersion area is greater than 400 mm. 2 This ensures a stable, uniform, and controllable electrolysis process. Copper, as an excellent conductor and with stable chemical properties, provides a continuous and stable current output. A sufficiently large cathode area ensures a more uniform current distribution on the anode (sample) surface, preventing excessively high local current density caused by an insufficiently small cathode area. This avoids problems such as pitting, over-corrosion, or uneven display on the sample surface, and facilitates obtaining clear, complete, and repeatable images of the original austenite grain boundaries.

[0051] In some embodiments of the present invention, the volume ratio of phosphoric acid to sulfuric acid is (2.8~3.2):(1.8~2.2), preferably 3:2.

[0052] Phosphoric acid has a high viscosity and possesses certain corrosion inhibition and complexation properties, forming a high-viscosity phosphate protective film on the anode surface during electrolysis. The addition of sulfuric acid can adjust the density and dissolution rate of the passivation film. Too low a sulfuric acid content results in slow film formation and insufficient corrosion; too high a sulfuric acid content will damage the protective properties of the passivation film, leading to over-corrosion. A 3:2 mixture of phosphoric acid and sulfuric acid produces a synergistic effect, allowing the electrolyte to preferentially corrode the high-energy austenite grain boundaries under the set voltage and time, thereby etching a continuous and clear grain boundary network on the dense bainitic matrix while avoiding over-corrosion of the overall microstructure.

[0053] In this invention, the mass concentration of phosphoric acid is ≥85%, and the mass concentration of sulfuric acid is 95~98%.

[0054] Controlling the concentrations of phosphoric acid and sulfuric acid ensures that the electrolyte possesses stable, potent, and repeatable corrosive properties. High acid concentrations imply extremely low water content, providing a sufficient and stable supply of H₂. + The ion concentration is controlled to ensure sufficient and consistent corrosion driving force during the electrolysis process. This avoids problems such as unstable corrosion rate and complex reaction byproducts caused by fluctuations in acid concentration or excessive moisture, ensuring that each experiment can achieve precise and controllable selective corrosion of the original austenite grain boundaries of bainitic steel under the set voltage and time, ultimately obtaining reliable and repeatable clear grain boundary morphology.

[0055] In this invention, the distance between the cathode and the anode is 5-10 cm.

[0056] The electrolytic current can be easily fine-tuned by adjusting the distance between the anode and cathode, making the process parameters flexible to control and the results highly repeatable, which greatly meets the comprehensive requirements of safety, efficiency and accuracy in production testing.

[0057] Specifically, controlling the distance between the anode and cathode optimizes the electric field distribution and current density during electrolysis, thereby achieving precise control over the corrosion rate and uniformity. This distance range balances operational safety and electrolysis efficiency. Too small a distance leads to excessively strong local electric fields and current densities, easily causing pitting or over-corrosion on the sample surface; too large a distance weakens the electric field and reduces current efficiency, potentially resulting in insufficient corrosion or uneven corrosion. A distance of 5-10 cm ensures a stable and moderate current at a DC voltage of 14-16 V, allowing the electrolyte to perform uniform and controllable selective corrosion of the original austenite grain boundaries, obtaining a complete and clear grain boundary network and ensuring method repeatability.

[0058] In this invention, after the electrolytic corrosion is completed, the parts are rinsed with water and then with ethanol in sequence; the time interval between the end of the electrolytic corrosion and the water rinsing does not exceed 5 seconds.

[0059] 5 seconds is the time to remove the electrolysis device and rinse the sample after electrolysis is completed under normal experimental conditions. During this process, the sample surface is still coated with electrolyte, and chemical corrosion is in progress. In order to avoid over-corrosion phenomena such as pitting caused by chemical corrosion, the time from the end of electrolysis to rinsing should not exceed 5 seconds.

[0060] In this invention, the steel samples to be tested include cooled 25Cr2NiMo1V steel and 30Cr2Ni4MoV steel with a bainitic microstructure. These two steels are typical key materials for the high-pressure end of large steam turbine rotors, with dense bainitic microstructure and difficulty in displaying the original austenitic grain boundaries.

[0061] This invention addresses the problems of poor safety, cumbersome operation, and unsatisfactory results in displaying the original austenite grain boundaries of bainitic steel used in turbine rotors, as seen in existing methods. It proposes a display scheme using a phosphoric acid-sulfuric acid mixture with a specific volume ratio as the electrolyte, combined with optimized DC voltage and a single, short-duration electrolysis. This scheme, through precisely controlled electrolysis, achieves safe, efficient, and selective corrosion of the original austenite grain boundaries in typical bainitic steels such as 25Cr2NiMo1V and 30Cr2Ni4MoV, resulting in a clear, complete, and continuous grain boundary display effect. It also boasts advantages such as simple operation, easy parameter control, and good repeatability, effectively meeting the urgent need for accurate assessment of the material condition of key components in practical engineering.

[0062] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0063] The raw materials used in the following examples are all commercially available products that can be purchased.

[0064] Example 1

[0065] This embodiment provides a method for revealing the austenitic grain boundaries of steel used in the high-pressure end of a steam turbine rotor. A sample of a 25Cr2NiMo1V steam turbine rotor high-pressure end manufactured by a company is dissected and cut. This part of the rotor is heat-treated by forced-air cooling, and the room-temperature microstructure is bainite. The specific steps are as follows:

[0066] S1. Sample preparation: Cut the above-mentioned rotor into 10×10×5 mm block samples, clean them with alcohol and acetone to remove surface oil and impurities, and polish them with 240#, 400#, 800#, 1000#, 1500# and 2000# sandpaper in sequence. Then, mechanically polish them with 1.5 μm and 0.5 μm diamond polishing paste until the sample surface is as bright as a mirror and there are no visible scratches.

[0067] S2. Assemble the electrolysis unit: The structure of the electrolysis unit is as follows... Figure 1 As shown, the cathode is a large-area copper sheet 2 with a fixed position, and the immersion area of ​​the copper sheet 2 in the electrolyte is greater than 400 mm. 2 The anode is a movable stainless steel sample holder 4. The distance between the cathode and anode is changed by moving the position of the stainless steel sample holder 4. The distance between the cathode and anode is set to 5 cm. The electrolytic cell is a 15 cm × 10 cm × 6 cm rectangular corrosion-resistant glass vessel.

[0068] S3. Prepare the electrolyte: The electrolyte is based on phosphoric acid and mixed with sulfuric acid. The volume ratio of phosphoric acid (H3PO4, ≥85wt%) to sulfuric acid (H2SO4, 95~98wt%) is 3:2. The electrolyte is poured into the electrolytic cell to a depth greater than 3 cm.

[0069] S4. Electrolytic corrosion: The prepared sample is clamped in a stainless steel sample holder 4 and electrolyzed in the prepared electrolyte. The electrolysis voltage is set to 15 V and the electrolysis time is 65 s. After electrolysis, the sample is taken out and quickly rinsed with water. The time interval between the end of electrolysis and rinsing with water should not exceed 5 s. Then, it is rinsed with anhydrous ethanol and dried before metallographic observation.

[0070] Example 2

[0071] This embodiment provides a method for revealing the austenitic grain boundaries of steel used in the high-pressure end of a steam turbine rotor. The method involves dissecting and cutting a portion of the 30Cr2Ni4MoV steam turbine rotor steel produced by a manufacturer. This part of the rotor undergoes heat treatment with forced-air cooling, resulting in a bainitic microstructure at room temperature. The specific steps are as follows:

[0072] S1. Sample preparation: Cut the above-mentioned rotor into 10×10×5 mm block samples, clean them with alcohol and acetone to remove surface oil and impurities, and polish them with 240#, 400#, 800#, 1000#, 1500# and 2000# sandpaper in sequence. Then, mechanically polish them with 1.5 μm and 0.5 μm diamond polishing paste until the sample surface is as bright as a mirror and there are no visible scratches.

[0073] S2. Assemble the electrolysis unit: The structure of the electrolysis unit is as follows... Figure 1 As shown, the cathode is a large-area copper sheet 2 with a fixed position, and the immersion area of ​​the copper sheet 2 in the electrolyte is greater than 400 mm. 2 The anode is a movable stainless steel sample holder 4. The distance between the cathode and anode is changed by moving the position of the stainless steel sample holder 4. The distance between the cathode and anode is set to 5 cm. The electrolytic cell is a 15 cm × 10 cm × 6 cm rectangular corrosion-resistant glass vessel.

[0074] S3. Prepare the electrolyte: The electrolyte is based on phosphoric acid and mixed with sulfuric acid. The volume ratio of phosphoric acid (H3PO4, ≥85wt%) to sulfuric acid (H2SO4, 95~98wt%) is 3:2. The electrolyte is poured into the electrolytic cell to a depth greater than 3 cm.

[0075] S4. Electrolytic corrosion: The prepared sample is clamped in a stainless steel sample holder 4 and electrolyzed in the prepared electrolyte. The electrolysis voltage is set to 15 V and the electrolysis time range is 50 s. After electrolysis, the sample is taken out and quickly rinsed with water. The time interval between the end of electrolysis and water rinsing should not exceed 5 s. Then, it is rinsed with anhydrous ethanol and dried before metallographic observation.

[0076] Example 3

[0077] This embodiment provides a method for revealing the original austenitic grain boundaries of steel used in the high-pressure end of a steam turbine rotor. The difference from Embodiment 1 is that in step S4, the electrolysis voltage is set to 14 V and the electrolysis time is 70 s. The remaining steps are exactly the same as in Embodiment 1.

[0078] Example 4

[0079] This embodiment provides a method for revealing the original austenitic grain boundaries of steel used in the high-pressure end of a steam turbine rotor. The difference from Embodiment 1 is that in step S3, the volume ratio of phosphoric acid (H3PO4, ≥85 wt%) to sulfuric acid (H2SO4, 95~98 wt%) is 2.8:1.8, and the remaining steps are exactly the same as in Embodiment 1.

[0080] Comparative Example 1

[0081] This comparative example provides a method for revealing the austenitic grain boundaries of steel used in the high-pressure end of a steam turbine rotor. A sample of a 25Cr2NiMo1V steam turbine rotor high-pressure end manufactured by a company was dissected and cut. This part of the rotor underwent forced-air cooling during heat treatment, and the room-temperature microstructure is bainite. The traditional nitric acid-alcohol etching method was followed, with the specific steps as follows:

[0082] S1. Sample preparation: Cut the above-mentioned rotor into 10×10×5 mm block samples, clean them with alcohol and acetone to remove surface oil and impurities, and polish them with 240#, 400#, 800#, 1000#, 1500# and 2000# sandpaper in sequence. Then, mechanically polish them with 1.5 μm and 0.5 μm diamond polishing paste until the sample surface is as bright as a mirror and there are no visible scratches.

[0083] S2. Preparation of etching agent: Prepare a 4% nitric acid alcohol solution according to the formula of concentrated nitric acid to ethanol volume ratio of 4:96.

[0084] S3. Metallographic Etching: Take an appropriate amount of the prepared nitric acid alcohol solution with a dropper and drop it evenly onto the polished sample surface. The etching time is 20-30 seconds. After etching, rinse quickly with water, then rinse with anhydrous ethanol and blow dry. Observe the metallographic structure.

[0085] Comparative Example 2

[0086] This comparative example provides a method for revealing the austenitic grain boundaries of steel used in the high-pressure end of a steam turbine rotor. The 30Cr2Ni4MoV steam turbine rotor steel produced by the company is dissected and cut. The heat treatment method for this part of the rotor is the same as in Example 1, and the room temperature microstructure is bainite. The specific steps are as follows:

[0087] S1. Sample preparation: Cut the above-mentioned rotor into 10×10×5 mm block samples, clean them with alcohol and acetone to remove surface oil and impurities, and polish them with 240#, 400#, 800#, 1000#, 1500# and 2000# sandpaper in sequence. Then, mechanically polish them with 1.5 μm and 0.5 μm diamond polishing paste until the sample surface is as bright as a mirror and there are no visible scratches.

[0088] S2. Preparation of etching agent: Prepare a 4% nitric acid alcohol solution according to the formula of concentrated nitric acid to ethanol volume ratio of 4:96.

[0089] S3. Metallographic Etching: Take an appropriate amount of the prepared nitric acid alcohol solution with a dropper and drop it evenly onto the polished sample surface. The etching time is 20-30 seconds. After etching, rinse quickly with water, then rinse with anhydrous ethanol and blow dry. Observe the metallographic structure.

[0090] Comparative Example 3

[0091] This comparative example provides a method for revealing the original austenitic grain boundaries of steel used in the high-pressure end of a steam turbine rotor. The difference from Example 1 is that in step S3, the volume ratio of phosphoric acid (H3PO4, ≥85 wt%) to sulfuric acid (H2SO4, 95~98 wt%) is 5:2, and the remaining steps are exactly the same as in Example 1.

[0092] Comparative Example 4

[0093] This comparative example provides a method for revealing the original austenitic grain boundaries of steel used in the high-pressure end of a steam turbine rotor. The difference from Example 1 is that in step S3, the volume ratio of phosphoric acid (H3PO4, ≥85 wt%) to sulfuric acid (H2SO4, 95~98 wt%) is 2:3, and the remaining steps are exactly the same as in Example 1.

[0094] Comparative Example 5

[0095] This comparative example provides a method for revealing the original austenitic grain boundaries of steel used in the high-pressure end of a steam turbine rotor. The difference from Example 1 is that in step S2, the distance between the cathode and anode is set to 15 cm, while the remaining steps are exactly the same as in Example 1.

[0096] Comparative Example 6

[0097] This comparative example provides a method for revealing the original austenitic grain boundaries of steel used in the high-pressure end of a steam turbine rotor. The difference from Example 1 is that in step S2, the distance between the cathode and anode is set to 2 cm, while the remaining steps are exactly the same as in Example 1.

[0098] Comparative Example 7

[0099] This comparative example provides a method for revealing the original austenitic grain boundaries of steel used in the high-pressure end of a steam turbine rotor. The difference from Example 1 is that in step S3, the electrolyte is phosphoric acid (H3PO4, ≥85 wt%), which is poured into the electrolytic cell to meet the condition of a depth greater than 3 cm. The remaining steps are exactly the same as in Example 1.

[0100] Comparative Example 8

[0101] This comparative example provides a method for revealing the original austenitic grain boundaries of steel used in the high-pressure end of a steam turbine rotor. The difference from Example 1 is that in step S3, the electrolyte is sulfuric acid (H2SO4, 95~98 wt%), which is poured into the electrolytic cell to meet the condition of a depth greater than 3 cm. The remaining steps are exactly the same as in Example 1.

[0102] Figure 2The image shows the proto-austenite grain boundary diagram obtained using the display method of this invention for the 25Cr2NiMo1V bainitic steel in Example 1. As shown in the figure, the proto-austenite grain boundaries are clearly and continuously presented, with distinct grain boundary outlines and no obvious blurring or discontinuity. Simultaneously, the morphological characteristics of lath-like bainite structure can be clearly observed within the proto-austenite grains. The microstructure surface displayed by this method is uniform, without surface defects such as pitting or corrosion caused by improper corrosion, thus ensuring the authenticity and integrity of the microstructure display. This result provides a reliable and high-quality image basis for subsequent accurate grain size assessment and related microstructure analysis.

[0103] Figure 3 The image shows a metallographic photograph of the proto-austenite grain boundaries obtained using the method of this invention for the 30Cr2Ni4MoV bainitic steel in Example 2. As shown in the figure, the proto-austenite grain boundaries exhibit a clear and complete network distribution, with fine and continuous grain boundary lines, and no obvious pitting or corrosion caused by excessive corrosion. This result demonstrates that the method of this invention can still stably and effectively expose the proto-austenite grain boundaries in 25Cr2NiMo1V bainitic steel, which is different from that in Example 1. This further verifies the applicability and reliability of this method in bainitic steel for the high-pressure end of turbine rotors, providing an effective technical means for grain size analysis and microstructure evaluation of related materials.

[0104] Figure 4 The image shows the original austenite grain boundary diagram obtained by the display method of the present invention for 25Cr2NiMo1V bainitic steel in Example 3. The difference from Example 1 is that the electrolysis voltage in this example is 14V. As can be seen from the figure, the original austenite grain boundary outline can still be clearly and continuously presented under this voltage, without obvious blurring or discontinuity. In addition, no obvious over-corrosion phenomenon is observed. It can be seen that the original austenite grain boundary display of turbine rotor bainitic steel can be achieved within the range of 14-16V.

[0105] Figure 5 This is the original austenite grain boundary diagram obtained by the display method of the present invention for 25Cr2NiMo1V bainitic steel in Example 4. The difference from Example 1 is that the volume ratio of phosphoric acid to sulfuric acid in the electrolyte is 2.8:1.8. As can be seen from the diagram, under this volume ratio, electrolytic corrosion still clearly and continuously reveals the original austenite grain boundary outline, without obvious blurring or discontinuity. Furthermore, no obvious over-corrosion phenomenon is observed. This proves that the original austenite grain boundaries can be well displayed within the volume ratio range of (2.8~3.2):(1.8~2.2).

[0106] Figure 6The image shows a metallographic image of the 25Cr2NiMo1V bainitic steel in Comparative Example 1 obtained by chemical etching. As can be seen from the image, the bainitic microstructure has been exposed by etching; however, the original austenite grain boundaries are blurred and difficult to identify, exhibiting a discontinuous or discontinuous state, making it impossible to clearly define the boundaries of each original austenite grain. Due to the unclear grain boundary display, this result is difficult to use for subsequent quantitative determination of the original austenite grain size and related microstructure analysis, thus limiting its practical application in material evaluation and quality control.

[0107] Figure 7 The image shows the metallographic structure of the 30Cr2Ni4MoV bainitic steel in Comparative Example 2 obtained using the conventional chemical etching method. As can be seen from the image, this method can obtain the metallographic morphology of acicular bainite, but the proto-austenite grain boundary outlines are not clearly and continuously displayed; the grain boundaries are blurred and difficult to distinguish, and a complete grain boundary network cannot be formed. This result further indicates that although the conventional etching method can corrode the matrix structure, it is difficult to effectively reveal the proto-austenite grain boundaries, and therefore cannot meet the technical requirements for quantitative analysis of proto-austenite grain size and related microstructure evaluation of this type of material.

[0108] Figure 8 The image shows the metallographic structure of 25Cr2NiMo1V bainitic steel obtained by using the method of this invention under excess phosphoric acid conditions in Comparative Example 3. The image shows that although the original austenite grain boundaries were corroded, the corrosion effect was discontinuous, with multiple breaks in the grain boundaries. It can be inferred that with a phosphoric acid-sulfuric acid volume ratio of 5:2, the phosphoric acid content was too high, resulting in excessive protection from the passivation film and thus poor corrosion performance.

[0109] Figure 9 The image shows the metallographic structure of 25Cr2NiMo1V bainitic steel in Comparative Example 4 obtained using the method of this invention under conditions of excess sulfuric acid. As can be seen from the image, with a phosphoric acid-sulfuric acid volume ratio of 2:3, grooves have been etched into the original austenite grain boundaries. However, due to the excessive sulfuric acid content, the density and stability of the passivation film are compromised, leading to excessive corrosion of the bainite structure within the grains. This results in the bainite structure being difficult to distinguish from the original austenite grain boundaries, creating a cluttered image that is unsuitable for quantitative grain size analysis and related microstructure evaluation.

[0110] Figure 10 The image shows the metallographic structure of 25Cr2NiMo1V bainitic steel in Comparative Example 5 obtained using the method of this invention when the distance between the anode and cathode is too large. As can be seen from the image, when the distance between the anode and cathode is 15 cm, the corrosion is shallow and the original austenite grain boundaries cannot be clearly and continuously characterized. This indicates that an excessively large distance between the anode and cathode will lead to an increase in the internal resistance of the solution and a decrease in the current, thus affecting the corrosion of the original austenite grain boundaries.

[0111] Figure 11The image shows the metallographic structure of the 25Cr2NiMo1V bainitic steel in Comparative Example 6 obtained using the method of this invention when the distance between the anode and cathode is too small. As can be seen from the image, the sample surface is excessively corroded. Although the original austenite grain boundaries are also corroded to form grooves, the bainite structure inside the grains is also corroded, making it difficult to distinguish from the original austenite grain boundaries. The image is too cluttered, which is not conducive to the technical requirements of quantitative grain size analysis and related microstructure evaluation.

[0112] Figure 12 The image shows the metallographic structure of 25Cr2NiMo1V bainitic steel obtained using the method of this invention in pure phosphoric acid. Due to the corrosion inhibition and complexation effects of phosphoric acid, a high-viscosity phosphate protective film forms on the sample surface when electrolyzing with pure phosphoric acid (mass concentration ≥85%), hindering further corrosion. Therefore, the corrosion degree is relatively light and the effect is extremely poor as seen in the image.

[0113] Figure 13 The image shows the metallographic structure of 25Cr2NiMo1V bainitic steel in Comparative Example 8 obtained using the method of this invention under pure sulfuric acid conditions. Due to the high sulfuric acid concentration (95-98 wt%) and low water content, there is not much freely moving H₂ in the electrolyte. + This results in extremely poor conductivity of the electrolyte, leading to almost no electrolytic corrosion. Furthermore, the strong oxidizing properties of concentrated sulfuric acid cause an oxide film to form on the sample surface, preventing further corrosion. Therefore, the figure shows that even with pure sulfuric acid as the electrolyte, the degree of corrosion in the sample remains low.

[0114] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for revealing the original austenite grain boundaries in steel used at the high-pressure end of a steam turbine rotor, characterized in that, include: The steel sample to be tested is surface treated to obtain a metallographic sample with no visible scratches on the surface; The metallographic sample was subjected to electrolytic etching, rinsed, and then observed. The electrolyte was a mixture of phosphoric acid and sulfuric acid, and the electrolysis parameters were 14-16 V DC voltage and 50-70 s electrolytic etching. The dimensions of the metallographic specimen are (9-11) mm × (9-11) mm × (4-6) mm; During electrolytic corrosion, the metallographic sample is fixed at the anode, the cathode is a copper sheet, and the distance between the cathode and the anode is 5-10 cm; The volume ratio of phosphoric acid to sulfuric acid is (2.8~3.2):(1.8~2.2); The phosphoric acid has a mass concentration of ≥85%, and the sulfuric acid has a mass concentration of 95~98%.

2. The method for displaying the original austenite grain boundaries as described in claim 1, characterized in that, The surface treatment includes cutting, grinding, and mechanically polishing the steel sample to be tested.

3. The method for displaying the original austenite grain boundaries as described in claim 2, characterized in that, The polishing process involves progressively increasing the grit from 240 to 2000 grit.

4. The method for displaying the original austenite grain boundaries as described in claim 1, characterized in that, The effective immersion area of ​​the cathode in the electrolyte is greater than 400 mm. 2 .

5. The method for displaying the original austenite grain boundaries as described in claim 1, characterized in that, After electrolytic corrosion is completed, rinse with water and then with ethanol in sequence; the time interval between the end of electrolytic corrosion and water rinsing should not exceed 5 seconds.

6. The method for displaying the original austenite grain boundaries as described in claim 1, characterized in that, The steel samples to be tested include cooled 25Cr2NiMo1V steel and 30Cr2Ni4MoV steel with bainitic microstructure.

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