Precise forging method of 2Cr13 martensitic stainless steel turbine low-pressure section small blade

Abstract

The invention provides a precision forging method for a low-pressure section small blade of a 2Cr13 martensitic stainless steel steam turbine, which aims at the problem of water drop erosion in a low-temperature wet steam environment, and comprises the following steps: by taking 2Cr13 martensitic stainless steel as a raw material, carrying out stepped heating to 600-800 DEG C for preheating, carrying out initial forging at 1100-1150 DEG C, carrying out die forging forming and finish forging at 900-1050 DEG C, enabling the forging ratio to be greater than or equal to 3, quenching at 980-1050 DEG C for oil cooling, and carrying out graded tempering at 580-650 DEG C, thereby obtaining the low-pressure section small blade of the 2Cr13 martensitic stainless steel steam turbine. Through cooperative control over rapid cooling after tempering, a uniform and fine tempered sorbite structure is obtained; according to the method, material characteristics are deeply coupled with forging and heat treatment full-process parameters, so that the blade has high strength and high toughness, the water drop erosion resistance is remarkably improved, meanwhile, the technical problems that 2Cr13 is poor in heat conductivity, grains are prone to coarsening and temper brittleness is caused are solved, the method is complete in process chain and controllable in parameter, and the method is suitable for precise manufacturing of the complex variable-cross-section blade and has wide application prospects. Good industrial application prospects are realized.

Description

Technical Field

[0001] This invention relates to the field of precision casting of steam turbine blades, and more specifically, to a precision forging method for small blades in the low-pressure section of a 2Cr13 martensitic stainless steel steam turbine. Background Technology

[0002] During turbine operation, the low-pressure section blades are constantly exposed to a low-temperature, humid steam environment, which can lead to severe droplet erosion and blade failure, affecting the turbine's service life and becoming one of the primary issues impacting the operating efficiency and reliability of large units. 2Cr13 martensitic stainless steel, due to its excellent strength, toughness, and corrosion resistance, and its 13% chromium content providing superior corrosion resistance compared to ordinary carbon steel, has been widely used in small and medium-sized turbine blades.

[0003] Forging of 2Cr13 martensitic stainless steel has been studied in existing technologies. For example, patent application CN202410550218.8 discloses a forging process for heat-resistant martensitic stainless steel 2Cr13 vertical continuous casting billets. This process involves sealing end-hole defects in the 2Cr13 continuous casting billet, multi-pass heating (maximum heating temperature 1230℃), upsetting and drawing using a hydraulic press, etc., to achieve the forging of large-sized continuous casting billets, ultimately obtaining round billets with a diameter of approximately 482mm. This process mainly solves the problem of forging healing of core hole defects in large continuous casting billets, and its final product is a large-sized round billet or bar.

[0004] However, existing technologies, including the 2Cr13 vertical continuous casting billet forging process, still have the following technical shortcomings: First, existing forging processes are mainly for forming large-diameter bars or round billets, and do not address the precision forming of complex variable cross-section parts such as turbine blades. The blade profile is complex, and the requirements for dimensional accuracy, surface finish, and fatigue performance are far higher than those for ordinary bars. Directly transplanting existing bar forging processes to blade manufacturing would result in difficulties in forming and inability to guarantee precision.

[0005] Secondly, 2Cr13 martensitic stainless steel has characteristics such as high deformation resistance, poor thermal conductivity, easy grain coarsening, and easy surface oxidation and decarburization. The 2Cr13 vertical continuous casting billet forging process uses a high-temperature heating of 1230℃, far exceeding the conventional upper limit of 2Cr13 initial forging temperature of 1160-1180℃. Although this is beneficial for deformation, it easily leads to coarse grains, overheating, or even burning, affecting the final mechanical properties of the material. At the same time, the 2Cr13 vertical continuous casting billet forging process does not provide protective measures against oxidation and decarburization during the heating process. For blades with high dimensional accuracy requirements, surface oxidation and decarburization will directly affect product quality.

[0006] Third, the forging process of 2Cr13 vertical continuous casting billets only involves the forging process and does not involve subsequent heat treatment and microstructure control. However, the blades of the low-pressure section of the steam turbine need to resist water droplet erosion in a low-temperature humid steam environment for a long time, which requires the material to have both high strength and high toughness.

[0007] Fourth, if 2Cr13 material is subjected to slow cooling in the furnace through the 500-600℃ range after high-temperature tempering, it will develop "temper brittleness," leading to a sharp decrease in impact toughness. Current technology lacks a specific solution to this problem, affecting the service safety of blades under alternating loads.

[0008] Furthermore, regarding water droplet erosion protection, existing technologies mostly employ surface coating strategies, such as depositing a Ti-Si-CN coating on the 2Cr13 surface or laser cladding a cobalt-based alloy coating. However, coating solutions suffer from issues such as adhesion to the substrate, risk of coating detachment, and high cost, making it difficult to fully meet the requirements for long-term reliable operation.

[0009] Therefore, how to fully utilize the material potential of 2Cr13 martensitic stainless steel for the specific service conditions of turbine low-pressure section blades, and obtain a uniform and fine tempered sorbite structure through the coordinated control of forging and heat treatment, so that the blades have both high strength and high toughness to resist water droplet erosion, while meeting the precision forming requirements of complex variable cross-section blades, has become an urgent technical problem to be solved in this field. Summary of the Invention

[0010] The purpose of this invention is to provide a precision forging method for small blades in the low-pressure section of a 2Cr13 martensitic stainless steel steam turbine. This method involves a synergistic control of preheating at 600-800℃, initial forging at 1100-1150℃, final forging at 900-1050℃, a forging ratio ≥3, quenching at 980-1050℃ with oil cooling, and staged tempering at 580-650℃, followed by rapid cooling after tempering. This results in a uniform and fine tempered sorbite structure. This invention deeply couples material properties with the parameters of the entire forging and heat treatment process, enabling the blades to possess both high strength and high toughness, significantly improving their resistance to water droplet erosion. It also solves technical problems such as poor thermal conductivity, easy grain coarsening, and temper brittleness of 2Cr13.

[0011] The embodiments of the present invention are implemented as follows: A precision forging method for low-pressure section blades of 2Cr13 martensitic stainless steel steam turbine, the precision forging method comprising: The 2Cr13 martensitic stainless steel billet is heated to the initial forging temperature of 1100-1150℃ and then die forged in the range of 900-1050℃. The total forging ratio is controlled to be not less than 3 to obtain a refined forged structure. The forged blades are quenched, heated to 980-1050℃ for austenitization, and then oil-cooled. The quenched blades are tempered by heating to 580-650℃, holding for a period of time, and then rapidly cooling to obtain a uniform tempered sorbite structure. The tempered sorbite structure gives the blades both high strength and high toughness to resist water droplet erosion.

[0012] In a preferred embodiment of the present invention, the rapid cooling method in the above tempering process is air cooling to suppress temper brittleness.

[0013] In a preferred embodiment of the present invention, the temperature for the tempering treatment is selected as follows: 580-600℃ when the target hardness is HRC30-35, and 610-650℃ when the target hardness is HRC28-32.

[0014] In a preferred embodiment of the present invention, before the above heating step, a step of spraying molten glass onto the surface of the blank to form a protective coating is further included, wherein the coating thickness is controlled to be 0.1-0.3 mm.

[0015] In a preferred embodiment of the present invention, the above heating step adopts a stepped heating method: first preheating at 600-800℃, and then heating up to the initial forging temperature.

[0016] In a preferred embodiment of the present invention, in the above-mentioned die forging step, the forging die is preheated to 150-300°C.

[0017] In a preferred embodiment of the present invention, in the above quenching step, the austenitizing holding time is calculated as 0.8-1.5 minutes / mm based on the effective thickness of the blade.

[0018] In a preferred embodiment of the present invention, the holding time in the above tempering step is 1.5-3 hours.

[0019] In a preferred embodiment of the present invention, after the above-mentioned die forging step and before the quenching step, a step of removing flash by laser cutting is further included.

[0020] In a preferred embodiment of the present invention, after the above-mentioned tempering step, a step of removing the surface defect layer and achieving the final dimensional accuracy by chemical milling is further included, wherein the chemical milling uses a mixed solution of nitric acid and hydrofluoric acid as the etching solution.

[0021] The beneficial effects of the embodiments of the present invention are: 1. This invention obtains a uniform and fine tempered sorbite structure by precisely controlling the entire forging and heat treatment process of 2Cr13 martensitic stainless steel. This structure has an adjustable HRC of 28-35 and combines high strength and high toughness. Compared with 2Cr13 materials prepared by conventional forging processes, its resistance to water droplet erosion and wear is improved by more than 30%. The specific mechanism is that the fine lath ferrite and dispersed carbide particles in the uniform tempered sorbite structure can effectively absorb the impact energy of water droplets, prevent the initiation and propagation of cracks, and thus significantly extend the service life of the blades in low-temperature humid steam environments. 2. This invention fully leverages the advantage of the 13% chromium content in 2Cr13 material, achieving excellent mechanical properties while maintaining the material's inherent corrosion resistance. Compared to protection solutions that require surface coatings, this invention solves the erosion problem through material microstructure optimization, avoiding the risk of secondary failure caused by coating peeling. Its corrosion resistance in humid steam environments is superior to that of ordinary carbon steel blades. 3. This invention, through strict temperature control: initial forging temperature 1100-1150℃, final forging temperature 900-1050℃, combined with the deformation requirement of forging ratio ≥3, effectively breaks down the as-cast structure of 2Cr13 material, avoiding the grain coarsening problem that may be caused by heating at 1230℃. Metallographic testing shows that the grain size of the blades prepared by this invention is 1-2 grades higher than that of conventional processes, the uniformity of the structure is significantly improved, and there are no defects such as mixed grains, overheating, or burning. 4. A quenching temperature of 980-1050℃ is selected based on the grain state after forging to ensure sufficient dissolution of carbides while preventing austenite grain coarsening. Depending on the specific operating requirements of the blade, prioritizing either strength or toughness, the tempering temperature is precisely selected within the range of 580-650℃ to achieve on-demand performance control. This synergistic control fully utilizes the material's potential, resulting in comprehensive mechanical properties superior to conventional processes that optimize forging and heat treatment in stages. Attached Figure Description

[0022] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0023] Figure 1 This is a flowchart of the precision forging method for small blades in the low-pressure section of a 2Cr13 martensitic stainless steel steam turbine, according to an embodiment of the present invention. Figure 2 This is a flowchart of the material pretreatment method for the material preparation and re-inspection steps in an embodiment of the present invention; Figure 3This is a flowchart of the formal surface treatment and protection steps according to an embodiment of the present invention; Figure 4 This is a flowchart of the stepped heating steps according to an embodiment of the present invention; Figure 5 This is a flowchart of the first stepped heating step in an embodiment of the present invention; Figure 6 This is a flowchart of the upsetting and intermediate processing according to an embodiment of the present invention; Figure 7 This is a flowchart illustrating the pre-forging and final processing of an embodiment of the present invention. Figure 8 This is a flowchart of the forging process according to an embodiment of the present invention; Figure 9 This is a sub-flowchart of the forging process in an embodiment of the present invention. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0025] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0026] It should be noted that similar labels and letters in the following figures indicate similar items. Therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

[0027] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, these embodiments are provided to make this application more comprehensive and complete, and to fully convey the concept of the exemplary embodiments to those skilled in the art.

[0028] The flowcharts shown in the accompanying drawings are merely illustrative and do not necessarily include all content and operations / steps, nor do they necessarily have to be performed in the described order. For example, some operations / steps can be broken down, while others can be combined or partially combined; therefore, the actual execution order may change depending on the specific circumstances.

[0029] First Embodiment This embodiment provides a precision forging method for small blades in the low-pressure section of a 2Cr13 martensitic stainless steel steam turbine, used to prepare the last-stage moving blades of the steam turbine. The blades are complex variable cross-section parts with complex blade profiles, and have high requirements for dimensional accuracy, surface finish and fatigue performance. The blades are 850mm long, have complex blade profiles, and a maximum thickness of 35mm. These blades are located in the low-pressure section of the steam turbine and are subject to water droplet erosion due to frequent contact with low-temperature wet steam.

[0030] 2Cr13 is a martensitic stainless steel with good strength, toughness, and corrosion resistance. It is selected as a material for small blades in the low-pressure section of steam turbines. However, it presents the following forging challenges: high deformation resistance, high flow stress at high temperatures requiring significant forging force; poor thermal conductivity necessitates controlled heating and cooling rates to prevent thermal stress and cracking; grain coarsening is common, and it is sensitive to the final forging temperature, easily leading to coarse microstructure and affecting performance; the surface is prone to oxidation / decarburization, requiring protective atmosphere heating or sufficient machining allowance. Please refer to [link to relevant documentation]. Figure 1-9 The specific precision forging methods include: S1: Material preparation and re-inspection S1.1: Raw material incoming inspection Hot-rolled 2Cr13 martensitic stainless steel bars with a diameter of φ90mm were selected as raw materials. Their chemical composition should meet the requirements of GB / T8732-2014 standard, specifically the following mass percentages: C 0.20%, Cr 12.8%, Mn 0.45%, Si 0.35%, P≤0.025%, S≤0.015%. Upon arrival at the factory, the material certificate was first verified to confirm the material grade, specifications, and metallurgical quality. Subsequently, a spectrometer was used to re-test the chemical composition to ensure that the material composition met the design requirements. Finally, ultrasonic testing was performed according to GB / T4162 standard to check for internal defects such as cracks, inclusions, and porosity, ensuring the metallurgical quality of the raw materials was qualified.

[0031] S1.2: Material cutting and end face processing The inspected and qualified bars are cold-cut using a band saw. The blanking weight is determined by a comprehensive calculation based on the forging weight, burn loss, and flash weight. In this embodiment, the blanking weight is controlled at 3.85 kg per piece. After blanking, the end faces of the blank are machined on a lathe to remove sawing burrs and ensure that the end faces are flat, providing a precise positioning reference for subsequent processes.

[0032] S1.3: Surface pretreatment The blank after the end face is machined is sent to a wet sandblasting machine for initial sandblasting treatment to remove oil, scale and other impurities attached to the surface of the blank, and at the same time obtain a uniform surface roughness to improve the adhesion between the subsequent coating and the substrate.

[0033] S1.4: Preparation of Glass Protective Coating After sandblasting, the blanks are fed into an automated spraying production line. A glass-based protective coating is uniformly applied to the surface of the blanks using a molten glass spraying unit. The coating thickness is controlled within the range of 0.1-0.3 mm to ensure 100% coverage of the areas requiring protection on the blank surface. This coating forms a dense protective barrier during subsequent heating, providing multiple functions such as high-temperature protection against oxidation and decarburization, high-temperature lubrication, and thermal insulation.

[0034] S1.5: Pre-forming treatment The blank with the completed glass coating is fed into a screw press, heated in a rotary furnace, and then pre-formed by extrusion rod head forming to obtain a blank shape that meets the requirements of subsequent die forging, thus preparing for the final die forging.

[0035] S2: Formal surface treatment and protection After completing S1 material preparation and preforming, the billet undergoes systematic surface treatment and protection to remove surface defects, improve surface condition, introduce residual compressive stress, and form a high-temperature protective coating on the billet surface, providing a high-quality surface foundation for subsequent stepped heating and die forging. This includes the following sub-steps: S2.1: Shot blasting The pre-formed blank is fed into a tracked shot blasting machine for shot blasting. By controlling the type and specifications of the shot (e.g., shot diameter 0.3-0.6 mm, blasting speed 60-80 m / s, blasting flow rate 200-300 kg / min, and blasting time 15-25 min), 100% shot coverage of the blank surface is ensured, thoroughly removing surface oxide scale, oil, and other contaminants. Shot blasting also introduces residual compressive stress into the blank surface, improving the material's fatigue strength and obtaining a uniform metallic luster surface, providing a good adhesion substrate for subsequent coatings.

[0036] S2.2: Polishing and Repair After shot blasting, the surface of the billet is polished using a belt polisher. The focus is on removing any small flashes, burrs, and localized surface defects that may have occurred during the pre-forming process, while further reducing the surface roughness to a target Ra ≤ 3.2 μm, resulting in a smooth and even surface. During the polishing process, the belt grit size (80-120 mesh) and feed rate should be controlled to avoid excessive material removal or creating new surface scratches.

[0037] S2.3: Sandblasting treatment The polished blank is fed into a wet sandblasting machine for sandblasting treatment. A mixture of fine abrasive corundum sand (200-300 mesh) and water is used, and the mixture is uniformly sprayed onto the blank surface under compressed air pressure of 0.4-0.6 MPa. Sandblasting effectively cleans residues from the surface's micropores and achieves a suitable micro-roughness Ra of 1.6-2.5 μm, significantly improving the mechanical adhesion between the subsequent glass protective coating and the substrate. After sandblasting, the blank surface should be immediately dried with clean compressed air to prevent rust re-emergence.

[0038] S2.4: Preparation of Glass Protective Coating After sandblasting and drying, the blanks are fed into an automated spraying production line, where a glass-based protective coating is uniformly applied to the surface using a molten glass spraying unit. This coating, formulated with special glass powder, binder, and additives, melts during subsequent high-temperature heating to form a dense, continuous, and flowable protective barrier. The coating thickness is controlled within the range of 0.15-0.20 mm. Online monitoring with a film thickness gauge ensures a uniform coating without localized over-thickness or missed areas.

[0039] After spraying, the billet is air-dried in a clean vacuum environment for 2 hours to allow the coating to fully cure and form a dry film with a certain strength, facilitating subsequent processes. During heating, the coating melts to form a dense liquid film, effectively isolating oxygen in the furnace gas, preventing oxidation and decarburization of the 2Cr13 material surface, and maintaining the stability of surface chemical composition and mechanical properties. The coating has suitable lubricity at forging temperatures, reducing friction between the billet and the die, improving metal flowability, and facilitating the filling of complex blade shapes. The coating has a certain heat insulation effect, slowing down the temperature drop of the billet during transfer and forging, ensuring stable and controllable final forging temperature. It prevents oxide scale from being pressed into the forging surface, reducing subsequent cleaning workload and ultimately obtaining a smooth blade surface.

[0040] After S2.4 is completed, the billet with the glass protective coating is transferred to the S3 stepped heating process.

[0041] S3: Stepped heating This invention addresses the material characteristics of 2Cr13 martensitic stainless steel, such as poor thermal conductivity, high deformation resistance, and easy grain coarsening. It employs a process route combining stepped heating and multi-stage forming to ensure thorough heating of the billet while strictly controlling grain growth and providing a temperature basis for subsequent precision forging. Specifically, it includes the following sub-steps: S3.1: First step heating The blank, after undergoing S2 surface treatment and protection (shot blasting, polishing, sandblasting, and coating with a glass protective coating), is fed into a rotary kiln for its first stage of heating. The heating regime is as follows: S3.1.1: Preheating Stage The billet is loaded into the furnace and preheated at a temperature range of 600-800℃, with 650℃ being preferred in this embodiment. The preheating holding time is calculated at 0.6-1.2 minutes / mm based on the billet diameter, and the holding time is 72 minutes. This preheating stage aims to slowly increase the billet temperature, avoiding rapid thermal stress caused by the poor thermal conductivity of 2Cr13 material, and preventing the billet from cracking.

[0042] S3.1.2: Heating and Heat Preservation Stage After preheating, the furnace temperature is raised to the initial forging temperature of 1100-1150℃, preferably 1130℃ in this embodiment. The holding time is calculated at 0.4-0.8 minutes / mm based on the billet diameter, and 0.5 minutes / mm in this embodiment, for a holding time of 45 minutes to ensure complete heat penetration of the billet core and homogenization of austenite. The heating furnace uses a slightly oxidizing atmosphere to prevent severe oxidation and avoid hydrogen embrittlement that may be caused by a reducing atmosphere.

[0043] S3.1.3: High-temperature protection The glass protective coating on the surface of the billet melts during heating to form a dense liquid protective film, which effectively isolates oxygen in the furnace gas and prevents oxidation and decarburization of the 2Cr13 material surface; at the same time, the coating has a lubricating effect, reducing frictional resistance for subsequent upsetting processes.

[0044] S3.2: Upsetting and intermediate processing S3.2.1: Upsetting The billet, after undergoing the first step heating, is rapidly transferred to the upsetting machine. Utilizing the residual heat of the rotary kiln, it is held at a constant temperature and then upset in a pre-set die. The main purpose of upsetting is to locally upset the billet, providing sufficient material volume for the tenon and root sections of the blade, while simultaneously breaking up the as-cast structure.

[0045] S3.2.2: Intermediate Surface Treatment After upsetting, the surface of the billet develops new slight oxidation or localized coating damage. To ensure the surface quality of subsequent pre-forging processes, intermediate surface treatment is performed. Shot blasting: The forged billet is sent to a shot blasting machine for shot blasting again to remove any thin layer of oxide scale and residual old coating that may be generated on the surface, while introducing residual compressive stress.

[0046] Polishing treatment: The blank after shot blasting is polished with a belt polisher to remove the tiny flash and burrs that may be generated during the upsetting process and reduce the surface roughness.

[0047] Sandblasting treatment: A wet sandblasting machine is used to clean the surface micropores, obtain a suitable micro-roughness, and improve the adhesion of subsequent coatings.

[0048] Recoating the glass protective coating: The sandblasted blank is sent to the spraying production line and the glass protective coating is recoated. The coating thickness is controlled at 0.1-0.3mm, and in this embodiment, 0.15-0.20mm is preferred to ensure 100% coverage of the blank surface. It is then allowed to air dry and cure naturally.

[0049] S3.3: Second step heating The billet, after undergoing intermediate surface treatment and recoating with a glass protective coating, is fed back into the rotary kiln for a second stage of heating. The heating regime follows S3.1: a preheating temperature of 600-800℃, preferably 650℃ in this embodiment; the preheating holding time is calculated based on the current diameter of the billet; the temperature is then raised to the initial forging temperature of 1100-1150℃, preferably 1130℃ in this embodiment; the holding time is calculated based on the current billet size to ensure thorough heating of the billet.

[0050] S3.4: Pre-forging and final processing S3.4.1: Pre-forging The billet, after undergoing the second step heating, is rapidly transferred to a screw press and kept warm using the residual heat of a rotary furnace. Pre-forging is then performed in a pre-forging die. This pre-forging process brings the billet shape closer to the final blade profile, further refines the grains, and ensures a proper distribution of metal flow lines along the blade profile.

[0051] S3.4.2: Final Surface Treatment The pre-forged billet undergoes final surface treatment in preparation for the subsequent final forging die: Shot blasting: The surface oxide scale and residual coating are removed using a shot blasting machine.

[0052] Finishing: A vortex finishing machine is used to remove burrs, reduce surface roughness, and improve the surface appearance.

[0053] Sandblasting: A wet sandblasting machine is used to sandblast the surface to obtain a clean and moderately roughened surface.

[0054] Applying glass glue: A special glass glue coating is applied to the spraying production line. This coating has better lubrication performance at the final forging temperature, providing lubrication guarantee for the final precision forging.

[0055] After completing S3.4.2, the billet is transferred to the S4 die forging process.

[0056] S4: Die forging Die forging is one of the core processes of this invention. It aims to utilize the good plasticity of 2Cr13 martensitic stainless steel in the austenitic region, precisely controlling the deformation temperature, deformation amount, and deformation rate to break up the as-cast structure, refine the grains, and ensure that metal flow lines are rationally distributed along the blade profile, providing an ideal forged microstructure basis for subsequent heat treatment. Specifically, it includes the following sub-steps: S4.1: Mold Preparation and Preheating S4.1.1: Mold Installation and Inspection Install the final forging die on the screw press worktable and check the surface finish of the die cavity, the fitting clearance, and whether the ejection mechanism is functioning properly. The die cavity should be free of cracks, scratches, and foreign objects to ensure forming accuracy.

[0057] S4.1.2: Mold preheating A mold heating device is used to preheat the forging die, with the preheating temperature controlled within the range of 150-300℃, preferably 200-250℃ in this embodiment. The purpose of mold preheating is to: reduce the temperature difference between the high-temperature billet and the cold mold, preventing the billet surface from cooling rapidly, which would increase deformation resistance and make forming difficult; avoid thermal stress cracks caused by rapid cooling; improve the fluidity of the metal in the mold cavity, which is beneficial for filling complex leaf shapes; and extend the service life of the mold. During the preheating process, an infrared thermometer is used to monitor the surface temperature of the mold to ensure uniform preheating and that the temperature meets the standard.

[0058] S4.2: Billet Transfer and Positioning The billet, after undergoing S3 stepped heating and being coated with glass glue, is quickly removed from the rotary hearth furnace and transferred by a robotic arm to the mold cavity of the screw press. The transfer process should be rapid and smooth, with the time controlled within 10 seconds to minimize the temperature drop of the billet.

[0059] After the blank is placed into the mold cavity, it should be confirmed that its position is accurate and stable to avoid tilting or shaking, which would affect the molding quality.

[0060] S4.3: Die forging S4.3.1: Equipment Selection A 16-ton screw press was used as the die forging equipment. The screw press features fast loading speed, controllable energy, and adjustable slide stroke, making it suitable for the precision forming of complex variable cross-section blade parts. Its dynamic loading characteristics allow the metal to flow fully within the die cavity, filling complex contours.

[0061] S4.3.2: Upsetting process For the blade tenon and blade root, the first step is upsetting. The screw press slide descends to locally upset the billet, increasing the cross-sectional area of ​​the tenon and providing precise material distribution for subsequent final forging. The upsetting deformation is controlled between 10-20%, allowing the billet to initially fill the tenon cavity.

[0062] S4.3.3: Final forging step Immediately after upsetting, final forging is performed. A screw press applies rated impact energy, causing the billet to completely fill the closed die cavity, forming complex structures such as blade profiles, inlet and outlet edges, and damping platforms. During final forging, the metal undergoes intense plastic deformation within the cavity, breaking up the as-cast structure and simultaneously distributing metal flow lines along the blade profile, improving the isotropic mechanical properties of the blade.

[0063] S4.3.4: Forging ratio control This invention controls the total forging ratio to be no less than 3; in this embodiment, the total forging ratio is 3.5. The forging ratio is ensured through multi-step deformation accumulation, ensuring sufficient deformation of the billet core, promoting dynamic recrystallization, and obtaining a uniform and fine forged microstructure.

[0064] S4.3.5: Demolding After forming, the press slide returns, and the ejector mechanism ejects the forging from the mold cavity. Check the forging for integrity, and ensure it is not stuck to the mold or has scratches.

[0065] S4.4: Process Monitoring The entire forging process uses an infrared thermometer to monitor the billet temperature in real time: Initial forging temperature: the temperature of the billet before it is placed into the mold, which is controlled at 1130℃ in this embodiment, within the range of 1100-1150℃; Final forging temperature: the temperature at the end of deformation, which is controlled between 950-1000℃ in this embodiment, ensuring that it is not lower than 900℃ to prevent a sharp increase in deformation resistance and not higher than 1050℃ to prevent grain coarsening; Temperature monitoring data should be recorded and archived as a traceable basis for process quality control.

[0066] S4.5: Post-forging treatment S4.5.1: Cooling. The forged blade blank is naturally cooled to room temperature in the air or slowly cooled by stacking to avoid thermal stress caused by rapid cooling.

[0067] S4.5.2: Shot blasting. After cooling, the blank is sent to a shot blasting machine for shot blasting treatment to remove residual glass coating, oxide scale, and a small amount of burrs from the surface. Shot blasting can also introduce residual compressive stress on the surface of the blank, improving fatigue strength.

[0068] S4.5.3: Surface inspection. Visually inspect the surface of the blank for forging defects such as cracks, folds, and overlapping layers. Mark areas with minor defects for subsequent polishing; discard blanks with serious defects.

[0069] S5: Quenching treatment The forged blades are quenched, heated to 980-1050℃ for austenitization, and then oil-cooled. The forged blade blank was sent to a controlled atmosphere furnace for quenching. The austenitizing temperature was 1030℃, and the austenitizing holding time was calculated at 0.8-1.5 minutes / mm based on the effective blade thickness. In this embodiment, the holding time was calculated at 1.2 minutes / mm based on the effective blade thickness, i.e., 42 minutes. Immediately after the holding time, oil cooling was performed using L-AN46 total loss system oil as the cooling medium, with the oil temperature controlled at 40-60℃, until room temperature was reached. The hardness of the blade after quenching was measured to be HRC48-50. Tempering was performed within 4 hours after quenching.

[0070] S6: Tempering treatment After quenching, the blades are neatly loaded into the tempering furnace, maintaining appropriate gaps between them to ensure smooth furnace gas circulation and uniform temperature. The loading amount is determined based on the furnace size and blade specifications to avoid uneven heating caused by overcrowding.

[0071] The tempering temperature is selected within the range of 580-650℃ based on the target mechanical properties. When the target hardness is HRC30-35 and high strength is desired, a tempering temperature of 580-600℃ should be selected. Within this temperature range, carbide dispersion is high, and more dislocation substructures are retained, resulting in high strength and moderate toughness.

[0072] When the target hardness is HRC28-32 and good toughness is desired, a tempering temperature of 610-650℃ is selected, with 620℃ being preferred in this embodiment. Within this temperature range, stress relief is more complete, carbides aggregate and grow, and toughness is significantly improved, making it suitable for most turbine blade operating conditions.

[0073] This embodiment prioritizes resistance to water droplet erosion and seeks the best balance between strength and toughness; therefore, 620℃ was chosen as the tempering temperature.

[0074] The tempering holding time is determined comprehensively based on the effective blade thickness and furnace loading, and is controlled within the range of 1.5-3 hours. The holding time should be long enough to ensure uniform core temperature, complete microstructure transformation, and full elimination of internal stress. In this embodiment, the maximum blade thickness is 35mm, the furnace loading is moderate, and the holding time is selected as 2.5 hours. During the holding process, furnace temperature fluctuations should be monitored and controlled within ±5℃ of the set temperature to ensure consistent performance of batch products.

[0075] After the heat treatment is completed, the blades are quickly removed from the tempering furnace and rapidly cooled to room temperature using forced air cooling. This cooling method was chosen for thin-walled blades based on the following considerations: To avoid temper brittleness: When 2Cr13 martensitic stainless steel is slowly cooled in the temperature range of 500-600℃, if it is cooled in the furnace, carbides or phosphides will precipitate along the grain boundaries, leading to "temper brittleness" and a sharp decrease in impact toughness. Rapid cooling can suppress the precipitation of brittle phases and ensure the toughness of the blades.

[0076] Operational feasibility: Air cooling is simple to operate, cost-controllable, and its cooling rate is sufficient to suppress brittleness, making it superior to water cooling. Water cooling carries a higher risk of deformation, while oil cooling is costly and polluting. During the cooling process, it is essential to ensure that all parts of the blades are evenly exposed to airflow to avoid localized slow cooling. The next step can only be carried out after the temperature has cooled to ≤50℃.

[0077] The quenched blades are tempered by heating to 580-650℃, holding for a period of time, and then rapidly cooling to obtain a uniform tempered sorbite structure. The tempered sorbite structure gives the blades both high strength and high toughness to resist water droplet erosion.

[0078] After the above tempering process, the blades obtain a uniform tempered sorbite microstructure. The microstructure is characterized by fine granular carbides dispersed in a ferrite matrix, uniform grain size (grade 7), and no undissolved carbide aggregation or grain boundary coarsening.

[0079] Mechanical property test results show that: hardness: HRC30-32, which meets the target range of HRC28-32; tensile strength: ≥880MPa; yield strength: ≥700MPa; impact toughness U-notch: ≥55J / cm. This combination of mechanical properties enables the blade to have both high strength to resist water droplet impact and high toughness to absorb impact energy and prevent crack propagation, significantly improving its resistance to water droplet erosion.

[0080] After tempering, batches are sampled for hardness testing and metallographic inspection: Hardness testing: Rockwell hardness tester is used to test the hardness of the blade surface and cross section to ensure that the hardness is uniform and meets the technical requirements. Metallographic inspection: After sampling, mounting, grinding, polishing and etching, the microstructure is observed under a metallographic microscope to confirm that the microstructure is uniform tempered sorbite without temper brittleness characteristics, such as grain boundary carbide precipitation.

[0081] S7: Laser edge trimming The laser cutting path is generated based on the final 3D CAD model of the blade. The heat-treated blade blank is precisely positioned on the laser cutting table, and the flash is removed along the blade contour using a fiber laser cutting device, with the cutting accuracy controlled within ±0.1mm.

[0082] S8: Chemical Milling The blades, after laser cutting, undergo surface pretreatment: degreasing with an alkaline solution, pickling, and ultrasonic cleaning. A corrosion-resistant protective layer is then applied to areas where etching is not required, and after curing, inspection and repairs are performed. The blades are then immersed in a chemical milling bath solution primarily composed of nitric acid (HNO3) and hydrofluoric acid (HF), at a temperature controlled at 50±2℃ for 30 minutes, achieving an etching depth of 0.15mm. Immediately after etching, neutralization and cleaning are performed to remove the protective layer, followed by final cleaning and drying.

[0083] S9: Inspection A comprehensive inspection of the finished blades was conducted: visual inspection and dye penetrant testing showed no cracks, folds or other defects on the surface; a coordinate measuring machine was used to check the key dimensions such as blade shape, thickness, and chord length, all of which met the drawing requirements; the hardness of the sampled blades was HRC30-32; metallographic examination showed that the microstructure was uniform tempered sorbite with a grain size of grade 7 and no overheating or burning defects.

[0084] First comparison To verify the technical effect of the present invention, Comparative Example 1 was set up, and the blade was prepared using the process idea of ​​the background technology CN202410550218.8, but the parameters were adaptively adjusted due to the limitation of the blade size.

[0085] Step 1, Material preparation: Same as the first embodiment, select φ90mm 2Cr13 bar.

[0086] Step 2, Heating: Use a regular heating furnace without glass melt protection. The heating regime is as follows (CN202410550218.8): 400-450℃ for 1 hour, then raise the temperature to 600-750℃ and hold for 2 hours, then raise the temperature to 1230℃ and hold for 3 hours.

[0087] Step 3, Forging: Use a 16-ton screw press, with an initial forging temperature of 1180-1200℃, a final forging temperature of 860-900℃, and a forging ratio of 3.2.

[0088] Step 4, Post-processing: No special heat treatment is performed, only stress-relieving annealing, holding at 600℃ for 2 hours, furnace cooling.

[0089] Step 5, Machining: Machining to the finished size using a CNC milling machine.

[0090] Upon testing, the blades prepared in the first comparative example showed coarse microstructure with a grain size of 4-5 and mixed crystals; uneven hardness distribution with HRC 25-35 fluctuation; impact toughness of only 32 J / cm², lower than 58 J / cm² in Example 1; and a slight decarburized layer of about 0.1 mm on the surface.

[0091] Second pair of proportions This comparative example is basically the same as the first embodiment, except that it is cooled by furnace cooling after tempering to verify the effect of tempering brittleness.

[0092] Step 6, Tempering treatment change: Tempering temperature 620℃, hold for 2.5 hours, and then cool to room temperature with the furnace after the holding period.

[0093] Testing revealed that the blades prepared in the second comparative example had a hardness of HRC29-31, comparable to the first embodiment, but their impact toughness decreased to 41 J / cm², a 29% reduction compared to the first embodiment. Metallographic observation showed carbide precipitation at grain boundaries, indicating temper brittleness.

[0094] Performance comparison test The blades prepared in the first embodiment and the first and second comparative examples were tested for their resistance to water droplet erosion, as shown in Table 1. A high-speed jet rotation test bench was used, with a water droplet impact velocity of 650 m / s, an impact angle of 90°, and an erosion time of 120 minutes. The mass loss rate was measured.

[0095] Table 1

[0096] Experimental results show that the blades prepared according to the embodiments of the present invention have excellent resistance to water droplet erosion, with a mass loss rate of only 44% of the first comparative example and 68% of the second comparative example. This is mainly attributed to the following: the present invention obtains a fine-grained structure through strict forging temperature control, obtains uniform tempered sorbite through quenching and tempering, and avoids tempering brittleness through rapid cooling after tempering. The synergistic effect of these three factors gives the blades both high strength and high toughness, effectively resisting water droplet impact. Among them, the first embodiment has the best erosion resistance due to its finer grain size of 7.5.

[0097] The specific embodiments described above further illustrate the purpose, technical solution, and beneficial effects of the present invention. It should be understood that the above description is only a specific embodiment of the present invention and is not intended to limit the scope of protection of the present invention. 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 precision forging method for low-pressure section blades of a 2Cr13 martensitic stainless steel steam turbine, characterized in that, The precision forging method includes: The 2Cr13 martensitic stainless steel billet is heated to the initial forging temperature of 1100-1150℃ and then die forged in the range of 900-1050℃. The total forging ratio is controlled to be not less than 3 to obtain a refined forged structure. The forged blades are quenched, heated to 980-1050℃ for austenitization, and then oil-cooled. The quenched blades are tempered by heating to 580-650℃, holding for a period of time, and then rapidly cooling to obtain a uniform tempered sorbite structure. The tempered sorbite structure gives the blades both high strength and high toughness to resist water droplet erosion.

2. The precision forging method for low-pressure section blades of 2Cr13 martensitic stainless steel steam turbines according to claim 1, characterized in that, In the tempering process, rapid cooling is achieved through air cooling to suppress temper brittleness.

3. The precision forging method for low-pressure section blades of 2Cr13 martensitic stainless steel steam turbines according to claim 1, characterized in that, The tempering temperature is selected as follows: 580-600℃ when the target hardness is HRC30-35, and 610-650℃ when the target hardness is HRC28-32.

4. The precision forging method for low-pressure section blades of 2Cr13 martensitic stainless steel steam turbines according to claim 1, characterized in that, Before the heating step, the process includes spraying molten glass onto the surface of the blank to form a protective coating, with the coating thickness controlled between 0.1 and 0.3 mm.

5. The precision forging method for small blades in the low-pressure section of a 2Cr13 martensitic stainless steel steam turbine according to claim 4, characterized in that, The heating process employs a stepped heating method: first, preheating is performed at 600-800℃, and then the temperature is increased to the initial forging temperature.

6. The precision forging method for low-pressure section blades of 2Cr13 martensitic stainless steel steam turbines according to claim 1, characterized in that, In the forging process, the forging die is preheated to 150-300℃.

7. The precision forging method for low-pressure section blades of 2Cr13 martensitic stainless steel steam turbine according to claim 1, characterized in that, In the quenching step, the austenitizing holding time is calculated at 0.8-1.5 minutes / mm based on the effective thickness of the blade.

8. The precision forging method for low-pressure section blades of 2Cr13 martensitic stainless steel steam turbines according to claim 1, characterized in that, In the tempering step, the holding time is 1.5-3 hours.

9. The precision forging method for low-pressure section blades of 2Cr13 martensitic stainless steel steam turbines according to claim 1, characterized in that, After the die forging step and before the quenching step, the process also includes a step of removing flash using laser cutting.

10. The precision forging method for low-pressure section blades of 2Cr13 martensitic stainless steel steam turbines according to claim 1, characterized in that, Following the tempering step, the process further includes a step of removing the surface defect layer and achieving final dimensional accuracy using chemical milling, wherein the chemical milling uses a mixed solution of nitric acid and hydrofluoric acid as the etching solution.

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