2cr13 stainless steel and heat treatment method for steam turbine blade against water droplet erosion

Abstract

The application provides 2Cr13 stainless steel for steam turbine blades resistant to water drop erosion and a heat treatment method thereof. Through a combined process of post-forging air cooling quenching, high-temperature tempering and air cooling after tempering, the production process is greatly simplified, the technical problem of tempering brittleness of 2Cr13 stainless steel is accurately solved, the accurate regulation and control of mechanical properties are realized, and finally high-quality steam turbine blades with excellent comprehensive performance and especially suitable for water drop erosion resistant working conditions are prepared.

Description

Technical Field

[0001] This invention relates to the field of turbine blade processing, and more specifically, to a 2Cr13 stainless steel for turbine blades resistant to water droplet erosion and its heat treatment method. Background Technology

[0002] 2Cr13, a martensitic stainless steel, is widely used in the manufacture of turbine blades operating in humid steam environments due to its excellent corrosion resistance and comprehensive mechanical properties. These blades, especially the last-stage blades of turbines, not only need to withstand complex mechanical stresses but also must possess excellent resistance to water droplet erosion. To meet different performance requirements, various heat treatment processes for 2Cr13 stainless steel have been developed, such as quenching followed by high-temperature tempering to obtain a tempered sorbite structure, thus balancing strength and toughness.

[0003] In existing combined forging and heat treatment processes, patent application number CN202310856933.X discloses a forging process for billets. This process is for steel billets with a specific chemical composition, the weight percentages of which are: C: 0.40-0.44%, Si: 1.12-1.14%, Mn: 0.47-0.48%, Cr: 4.9-5.1%, Mo: 1.401-1.410%, V: 0.98-1.01%, S: 0.0012-0.0015%, Ce: 0.024-0.026%. After forging, the billets undergo complex normalizing and spheroidizing annealing treatments, and the final heat treatment employs an oil quenching followed by two tempering processes. However, the process route in Prior Art Document 1 is designed for high-alloy steel with specific composition, and its processing is complex. It does not cover how to provide excellent water droplet erosion resistance to 2Cr13 stainless steel turbine blades through a simplified heat treatment process, especially by using the residual heat of forging for quenching.

[0004] Furthermore, 2Cr13 stainless steel is prone to second-type temper brittleness when slowly cooled through the 500-600℃ temperature range after high-temperature tempering, resulting in a significant decrease in impact toughness. Although conventional heat treatment specifications mention "rapid cooling" after tempering, they lack refined control schemes for specific cooling methods, key parameters such as wind speed, cooling endpoint temperature, and their correlation with blade size and performance targets. Meanwhile, existing technologies still need improvement in how to utilize the good hardenability of 2Cr13 to achieve quenching directly through controlled air cooling after forging, thus eliminating the complex process of traditional reheating and quenching, and precisely controlling the final microstructure and properties to meet the specific requirements of water-drop erosion resistant blades for strength, tensile strength Rm≥700MPa, yield strength Rp0.2≥500MPa, and room temperature impact energy Akv≥30J. Summary of the Invention

[0005] The purpose of this invention is to provide a 2Cr13 stainless steel for turbine blades resistant to water droplet erosion and its heat treatment method. Through an innovative combination process of "forging followed by air cooling quenching + high-temperature tempering + rapid cooling after tempering", the production process is not only greatly simplified, but also the technical problem of temper brittleness of 2Cr13 stainless steel is solved more precisely, and the mechanical properties are precisely controlled. Finally, high-quality turbine blades with excellent comprehensive performance are produced, which are particularly suitable for water droplet erosion conditions.

[0006] The embodiments of the present invention are implemented as follows: A heat treatment method for 2Cr13 stainless steel used in steam turbine blades to resist water droplet erosion, the heat treatment method comprising: After forging and cooling, the forged 2Cr13 stainless steel blades are subjected to a first air cooling to achieve quenching hardening and obtain a martensitic structure. Tempering involves subjecting the blades, after the first air cooling, to high-temperature tempering at 580°C to 650°C for 1.5 to 3 hours. After the second air cooling and tempering process, the blades are immediately removed from the furnace for a second air cooling to rapidly cool to room temperature in order to suppress temper brittleness.

[0007] In a preferred embodiment of the present invention, in the above-mentioned post-forging cooling step, the cooling rate of the first air cooling is achieved by controlling the wind speed, which is adjusted according to the effective thickness of the blade.

[0008] In a preferred embodiment of the present invention, the wind speed of the first air cooling is 2 to 5 m / s.

[0009] In a preferred embodiment of the present invention, in the above-mentioned cooling step after forging, the ambient temperature of the first air cooling is room temperature, specifically 15°C to 35°C, and the cooling ambient temperature is uniform, without cross drafts or local overcooling.

[0010] In a preferred embodiment of the present invention, in the first air-cooling step described above, air is continuously blown until the surface temperature of the blade is below 130°C.

[0011] In a preferred embodiment of the present invention, in the above-mentioned cooling step after forging, the airflow direction during air cooling is parallel to the blade length direction, or from the blade root to the blade crown direction, and a reasonable spacing is maintained between the blades to ensure that the airflow passes smoothly through all blade surfaces.

[0012] In a preferred embodiment of the present invention, the tempering temperature in the above tempering step is 600℃±10℃.

[0013] In a preferred embodiment of the present invention, the holding time in the above tempering step is 2 hours.

[0014] In a preferred embodiment of the present invention, the interval between the forging cooling step and the tempering step shall not exceed 4 to 8 hours.

[0015] A turbine blade resistant to water droplet erosion, the blade is made of 2Cr13 stainless steel and is treated by any of the aforementioned heat treatment methods. The 2Cr13 stainless steel blade after heat treatment has the following mechanical properties: tensile strength Rm ≥ 700 MPa, yield strength Rp0.2 ≥ 500 MPa, hardness HB 260 to 320, and room temperature impact energy Akv ≥ 30 J.

[0016] The beneficial effects of the embodiments of the present invention are: 1. Simplified Process Flow, Significantly Improved Production Efficiency and Reduced Energy Consumption: The process route disclosed in CN202310856933.X is complex, requiring multiple steps such as "forging-normalizing-spheroidizing annealing-reheating and quenching-two temperings," and includes special operations such as coating with mixed powders. This technical solution, however, innovatively adopts a "direct air-cooling quenching after forging" process route, specifically targeting the hardenability characteristics of 2Cr13 stainless steel. This eliminates the cumbersome steps of post-forging normalizing, spheroidizing annealing, and subsequent reheating and quenching in traditional processes. This not only significantly shortens the production cycle and improves production efficiency but also significantly reduces energy consumption and process costs, embodying the concept of green manufacturing.

[0017] 2. Precise Suppression of Temper Brittleness and Significant Improvement of Blade Toughness: While CN202310856933.X employs air cooling after final tempering, its steel composition differs from that of 2Cr13, and it does not specifically address the brittleness-sensitive range after high-temperature tempering. This technical solution deeply recognizes the problem of second-type temper brittleness easily occurring in 2Cr13 stainless steel when slowly cooled through the 500-600℃ range after high-temperature tempering. It mandates a second air cooling after high-temperature tempering. Through meticulous control of key parameters such as the air cooling method, wind speed (2-5 m / s), final cooling temperature below 150℃, and operational details like wind direction parallel to the blades and maintaining spacing, the precipitation of brittle phases at grain boundaries is effectively suppressed. This ensures the blades achieve excellent impact toughness, with room temperature impact energy Akv ≥ 30J, which is crucial for turbine blades subjected to water droplet erosion.

[0018] 3. Precise control of process parameters to achieve excellent matching of mechanical properties: CN202310856933.X adjusts properties through two tempering processes, with a wide process window that is not directly related to the blade's performance. This technical solution precisely controls the tempering temperature (580-650℃, preferably 600±10℃) and the holding time (1.5-3 hours, preferably 2 hours), combined with the aforementioned rapid cooling, to stably obtain the target microstructure of tempered sorbite. The final treated blade achieves an excellent balance between strength, plasticity, and toughness, simultaneously meeting the comprehensive performance requirements of water-drop erosion resistant blades: high tensile strength Rm≥700MPa, yield strength Rp0.5≥500MPa, moderate hardness HB 260-320, and high toughness Akv≥30J. This allows it to resist deformation and absorb impact energy in a high-speed rotating wet steam environment, preventing brittle fracture.

[0019] 4. Utilizing forging residual heat to optimize martensitic phase transformation: In this technical solution, the first air cooling after forging directly utilizes the residual heat for quenching. Compared to the process in Comparative Document 1, which involves air cooling the forged billet to room temperature before reheating and quenching, this solution avoids energy waste and stress accumulation caused by repeated phase transformations. Controllable air cooling directly from the austenitizing temperature is more conducive to obtaining a uniform and fine martensitic structure, laying a good microstructural foundation for subsequent tempering to achieve excellent final properties.

[0020] 5. Reduce the risk of deformation and cracking, and improve product yield: In CN202310856933.X, the final heat treatment uses oil quenching, which poses a significant risk of internal stress and deformation cracking. This technical solution uses air cooling throughout the process, especially for thin-walled or complex-shaped blades, where the cooling intensity is more moderate compared to oil cooling. By precisely controlling the low-to-medium wind speed of 2-5 m / s and ensuring uniform cooling, avoiding drafts, and ensuring the wind direction is parallel to the blade length, sufficient martensite mass is obtained while minimizing thermal and structural stresses. This effectively reduces the tendency of blades to quench deformation and cracking, significantly improving product manufacturing yield and dimensional stability. Attached Figure Description

[0021] 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.

[0022] Figure 1 This is a flowchart of the heat treatment process according to an embodiment of the present invention; Figure 2This is a parameter setting diagram for the key subsystem of the air-cooled system in an embodiment of the present invention. Detailed Implementation

[0023] 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.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] In this embodiment, a closed-loop circulating air cooling system is used for the first air cooling after forging and quenching, and the second air cooling after tempering. The specific equipment configuration is as follows: The sealed cooling chamber is used to isolate the workshop environment, avoid cross drafts and local overcooling, and ensure uniform cooling. Its length × width × height dimensions are 6000mm × 3000mm × 2500mm. The shell is welded with 6mm thick Q235 steel plate and lined with a 50mm thick rock wool insulation layer. The chamber's airtightness is ≤5% under 500Pa pressure.

[0028] The air intake device is installed below the bottom of the cooling chamber and includes a variable frequency fan and a flared air intake shroud. The fan model is a 4-72 centrifugal fan with a power of 15kW, an air volume of 15000-30000 m³ / h, and a total pressure of 800-1200 Pa. The frequency converter has an adjustment range of 0-50Hz, corresponding to a continuously adjustable airflow of 0-8 m / s. Precise airflow control is achieved by providing adjustable cooling airflow.

[0029] The air outlet is installed on the top of the cooling chamber and extends to the outside of the workshop through air ducts; the air outlet size is 800mm×800mm and equipped with louvers to exhaust hot air to the outside and reduce heat pollution in the workshop.

[0030] The trolley system enables batch loading, unloading, and positioning of blades, carrying blades in and out of the cooling chamber. The trolley dimensions are 5000mm×2000mm. The trolley is equipped with special blade fixtures to ensure that the blade spacing is ≥100mm and that the blade length direction is parallel to the airflow direction.

[0031] The temperature control system includes an online infrared temperature probe (Raytek MM series, range 0-1200℃, accuracy ±1.5℃) and a PLC control system. The temperature probes are installed on the side walls and top of the cooling chamber, with three measuring points in each chamber to monitor the blade surface temperature in real time. Automatic temperature monitoring enables closed-loop control of the cooling process.

[0032] The environmental control system ensures that the air entering the cooling chamber meets the process requirements. This includes temperature and humidity sensors at the air inlet of the cooling chamber, with a range of -20 to 80°C and 0 to 100%RH humidity, and a pretreatment unit, which includes filters and dehumidifiers.

[0033] The ambient temperature control range is 15-35℃, with an optimal range of 20-25℃. An air conditioning pre-treatment unit is installed at the air inlet. When the temperature is too high in summer, the cooling is activated; when the temperature is too low in winter, the external air intake is stopped, and the indoor circulating air is switched to heating.

[0034] The relative humidity is controlled at 40-60%, and a dehumidification device is installed at the air inlet. Dehumidification is activated when the ambient humidity exceeds the standard. During the rainy season or humid weather, indoor circulating air is preferred.

[0035] The cleanliness is ensured by installing a pre-filter at the air inlet, with a filtration efficiency of G4 level, which has a filtration efficiency of ≥90% for particles ≥5μm, ensuring that the number of dust particles with a diameter ≥5μm in the air is ≤1 million particles / m³.

[0036] The airflow uniformity of the cooling chamber ensures that the wind speed deviation between any two points is ≤ ±0.5 m / s, achieved through the design of guide plates and air distribution perforated plates; the air inlet perforated plate has an opening rate of 35%, a hole diameter of φ8 mm, and is arranged in a quincunx pattern.

[0037] The angle between the wind direction and the blade length direction is ≤15°. By positioning the blade with tooling and adjusting the guide vanes, we ensure that the wind flows from the blade root to the blade crown, thus guaranteeing wind direction consistency.

[0038] The cooling chamber pressure is 10-30Pa slightly positive pressure, which is maintained by adjusting the opening of the air inlet fan and the air outlet valve to prevent untreated external air from seeping in.

[0039] The PLC control system used in this invention automatically adjusts based on real-time monitored environmental parameters: Temperature regulation: When the ambient temperature is >30℃, start the intake air cooling to reduce the intake air temperature to 25±2℃; when the ambient temperature is <15℃, switch to internal circulation mode to use the residual heat of the blades to heat the air, or start the auxiliary heater to raise the intake air temperature to 20±2℃.

[0040] Humidity control: When the relative humidity is >70%, start the dehumidifier to control the inlet dew point temperature below 10℃; when the relative humidity is >85%, stop the external air intake and switch to full internal circulation mode to prevent water vapor from condensing on the high temperature blade surface and causing local abnormal cooling.

[0041] Wind speed adjustment: The fan frequency is automatically adjusted according to the effective thickness of the blades. When the thickness is ≤20mm, the wind speed is set to 2.0-3.0m / s; when the thickness is 20-50mm, the wind speed is set to 3.0-4.5m / s; when the thickness is ≥50mm, the wind speed is set to 4.5-6.0m / s.

[0042] Example 1 This embodiment provides a heat treatment method for 2Cr13 stainless steel turbine blades resistant to water droplet erosion. The blades are the last stage blades of the turbine, and their effective thickness is 40mm.

[0043] Please see Figure 1 The specific steps are as follows: Post-forging cooling and quenching steps: The 2Cr13 stainless steel blades, with a forging final temperature of not less than 980℃, were immediately transported via a trolley system into a sealed cooling chamber and transferred to the air-cooling zone for their first air cooling. The blades were positioned using specialized tooling to ensure a blade spacing of 120mm, with the blade length parallel to the length of the cooling chamber, and the airflow direction from the blade root to the blade crown.

[0044] The ambient temperature in the air-cooled area is controlled at 25℃, with an actual monitored temperature of 24.5℃. Relative humidity: set at 50%, adjusted by a dehumidifier to an actual monitored 52%RH. Cleanliness: Intake air undergoes a G4-level pre-filter, with dust particles ≥5μm in diameter ≤800,000 particles / m³. Cooling chamber pressure: maintained at a slightly positive pressure of 20Pa.

[0045] Axial flow fans are used for cooling, with the airflow direction strictly adjusted to be parallel to the blade length, i.e., from the blade root to the blade crown. Based on the effective blade thickness of 40mm, the airflow speed is adjusted to a low-to-medium speed of 3.5 m / s. The fan frequency is set to 35Hz, corresponding to an average airflow speed of 3.5 m / s within the cooling chamber. Monitoring at five airflow measurement points within the cooling chamber shows that the actual airflow speed range is 3.3-3.7 m / s, meeting the uniformity requirements.

[0046] Online infrared temperature probes (Raytek MM series, 0-1200℃) are fixedly installed at the outlet of the air-cooled area and in the middle of the cooling zone. The probes are positioned approximately 1.5 meters from the blade surface and are equipped with water-cooling jackets and lens cleaners to prevent oxide scale interference. The probes are aimed at the largest cross-section in the middle of the blade. The temperature measurement system is linked to the PLC control system to record the temperature change curve of each blade in real time.

[0047] Continuous airflow cooling is employed, with the control system monitoring the temperature in real time. Three online infrared temperature probes installed on the side walls and top of the cooling chamber continuously monitor the blade surface temperature, and the data is recorded in real time by the PLC system. When any probe detects a blade temperature below 120°C, the system automatically reduces the fan frequency to 10Hz to maintain circulation. When all probes detect temperatures below 120°C, actual measurements in this embodiment show that after approximately 35 minutes of continuous airflow, the temperatures at the three measuring points were 115°C, 118°C, and 122°C, with an average of 118.3°C. At this point, the system issues a prompt and automatically shuts down the fan, and the trolley is removed from the cooling chamber.

[0048] This step utilizes the residual heat from forging to directly quench the blades, resulting in a martensitic structure. Random sampling revealed that the surface hardness of the quenched blades was HRC 47-49.

[0049] Tempering steps: After the first air cooling, the blades are allowed to cool naturally to room temperature. Within 4 hours, the blades are loaded into a tempering furnace for high-temperature tempering. The tempering temperature is set at 600℃, and the holding time is calculated based on the load size. In this embodiment, the load size is moderate, and the holding time is 2 hours. The temperature uniformity inside the furnace is controlled within ±5℃.

[0050] Second air-cooling step: After tempering and heat preservation, the blades were immediately removed from the tempering furnace and quickly transported to a sealed cooling chamber for a second air cooling using a trolley system (transfer time ≤ 2 minutes). The air cooling environment was set the same as the first air cooling: ambient temperature 25℃, relative humidity 50%, and slight positive pressure 20Pa. The fan speed was controlled at a set frequency of 40Hz, corresponding to an average wind speed of 4.0m / s. The actual monitored wind speed range was 3.8-4.2m / s. Temperature monitoring: The same online infrared temperature measurement system was used for continuous monitoring. When the blade temperature rapidly passed through the 500-600℃ brittle sensitivity range, the system automatically recorded the cooling rate. In this embodiment, the measured average cooling rate in the 500-600℃ range was 45℃ / min. After continuous air cooling for about 20 minutes, the temperatures at the three measuring points dropped to 92℃, 95℃, and 98℃ respectively, with an average of 95℃. The system then automatically shut down the fan.

[0051] The mechanical properties of the treated blades were tested and found to be as follows: tensile strength Rm: 755 MPa; yield strength Rp0.2: 528 MPa; hardness: HB 285; room temperature impact energy Akv: 78 J, 42 J, 65 J, average 61.7 J.

[0052] This step aims to quickly pass through the tempering brittleness-sensitive temperature range of 500-600℃ to suppress the precipitation of brittle phases.

[0053] After the above steps, the 2Cr13 stainless steel blades were subjected to room temperature tensile tests according to GB / T 228.1, room temperature impact tests according to GB / T 229, Charpy V-notch tests, and Brinell hardness tests according to GB / T 231.1. The test results are as follows: tensile strength Rm: 755 MPa; yield strength Rp0.2: 528 MPa; hardness: HB 285; room temperature impact energy Akv: 78 J, 42 J, 65 J, with an average of 61.7 J for the three samples.

[0054] The results show that the mechanical properties of the blades treated in this embodiment meet the requirements of tensile strength Rm≥700MPa, yield strength Rp0.2≥500MPa, hardness HB260-320, room temperature impact energy Akv≥30J, and excellent impact toughness.

[0055] To verify the importance of environmental condition control, the following comparative experiments were conducted:

[0056] Data shows that controlling the ambient temperature between 15-35℃, relative humidity ≤85%, wind speed uniformity deviation ≤±0.5m / s, and ensuring the wind direction is consistent with the blade length direction are also very important for ensuring the overall performance of the blades.

[0057] Example 2 This embodiment is basically the same as Embodiment 1, except that the tempering temperature was adjusted to focus on improving strength. The effective thickness of the blade is 40 mm.

[0058] Cooling steps after forging: Same as in Example 1, with a wind speed of 3.5 m / s, cooling to a surface temperature of 118°C.

[0059] Tempering procedure: Set the tempering temperature to 585℃ and hold for 2 hours.

[0060] Second air-cooling step: Same as in Example 1, with an air speed of 4.0 m / s, cooling until the surface temperature is below 100°C, measured at 98°C.

[0061] The performance test results are as follows: tensile strength Rm: 802 MPa; yield strength Rp0.2: 565 MPa; hardness: HB312; room temperature impact energy Akv: 48 J, 36 J, 52 J, with an average of 45.3 J for the three samples.

[0062] The results show that when tempered at 585℃, the strength indices Rm, Rp0.2 and hardness are further improved, and although the impact toughness decreases, it is still much higher than the requirement of 30J, making it suitable for working conditions with higher strength requirements.

[0063] Example 3 This embodiment is basically the same as Embodiment 1, except that the tempering temperature is adjusted to focus on improving toughness, and the effective thickness of the blade is 40mm.

[0064] Cooling steps after forging: Same as in Example 1, with a wind speed of 3.5 m / s, cooling to a surface temperature of 118°C.

[0065] Tempering procedure: Set the tempering temperature to 630℃ and hold for 2 hours.

[0066] Second air-cooling step: Same as in Example 1, with an air speed of 4.0 m / s, cooling until the surface temperature is below 100°C, measured at 96°C.

[0067] The performance test results are as follows: tensile strength Rm: 718 MPa; yield strength Rp0.2: 512 MPa Hardness: HB 268; Room temperature impact energy Akv: 112 J, 96 J, 124 J, average of the three samples: 110.7 J.

[0068] The results show that when tempered at 630℃, the impact toughness is excellent while the strength index still meets the requirements, making it suitable for working conditions with extremely high impact resistance requirements.

[0069] Example 4 This embodiment demonstrates the adjustment of the first air-cooling wind speed under different blade thicknesses. Blade A has an effective thickness of 20mm and is a thin-walled small blade.

[0070] Post-forging cooling steps: Immediately after forging, the first air cooling is performed. The air temperature is 25℃, and the air direction is parallel to the blade length. Based on the characteristics of thin-walled blades being easily deformed and cooled, the air speed is adjusted to a low speed of 2.5 m / s. The blade spacing is 80 mm. Continuous air cooling is continued until the surface temperature is below 150℃; the actual measured temperature is 132℃.

[0071] Tempering procedure: Same as in Example 1, hold at 600°C for 2 hours.

[0072] Second air-cooling step: Air speed 3.5 m / s, cooling until the surface temperature is below 100℃, actual measured 88℃.

[0073] Blade B: Effective thickness is 60mm, thick blade.

[0074] Post-forging cooling procedure: Immediately after forging, perform the first air cooling. Air temperature: 25℃, air direction: parallel to the blade length. To ensure rapid cooling of the core, adjust the air speed to medium-high, 5.0 m / s. Blade spacing: 150 mm. Continuously blow air to cool until the surface temperature is below 150℃; actual measured temperature: 145℃.

[0075] Tempering procedure: Same as in Example 1, keep warm at 600℃ for 2.5 hours, and extend the holding time appropriately if the blades are thicker.

[0076] Second air-cooling step: Air speed 5.0 m / s, cooling until the surface temperature is below 100℃, actual measured 92℃.

[0077] Testing revealed that the mechanical properties of both blade A and blade B met the required specifications, with blade A showing no deformation and blade B exhibiting uniform core structure, thus proving the effectiveness of the wind speed control strategy of this invention.

[0078] Example 5 This embodiment shares the same basic process steps as Embodiment 1, the difference being the adjustment of the target final temperatures for the first and second air cooling cycles to verify whether the mechanical performance requirements can still be met at higher final temperatures. The blades are the same as in Embodiment 1, being the last-stage blades of a steam turbine, with an effective thickness of 40 mm. The same closed-loop circulating air cooling system as in Embodiment 1 is used for cooling.

[0079] The specific steps are as follows: Post-forging cooling and quenching steps: 2Cr13 stainless steel blades with a forging final temperature of not less than 980℃ are transported into a sealed cooling chamber via a trolley system. The blades are positioned using specialized tooling to ensure a blade spacing of 120mm, with the blade length direction parallel to the length direction of the cooling chamber, and the airflow direction from the blade root to the blade crown.

[0080] Air-cooled environment settings: Ambient temperature: set at 25℃, actual monitoring was 24.8℃, regulated by air conditioning; Relative humidity: set at 50%, actual monitoring was 51%RH; Cleanliness: intake air is filtered through G4 grade primary filter, with dust particles ≥5μm in diameter ≤800,000 particles / m³; Cooling chamber pressure: maintained at 20Pa slightly positive pressure.

[0081] Wind speed control: Based on the effective blade thickness of 40mm, the fan frequency is set to 35Hz, corresponding to an average wind speed of 3.5m / s in the cooling room. Monitoring at 5 wind speed measuring points in the cooling room shows that the actual wind speed range is 3.3-3.7m / s, meeting the uniformity requirements.

[0082] Temperature monitoring and endpoint control: Three online infrared temperature probes installed on the side walls and top of the cooling chamber continuously monitor the blade surface temperature, and the data is recorded in real time to the PLC system. In this embodiment, the endpoint temperature target for the first air cooling is set at 145±5℃ (i.e., the range of 140-150℃). When the system detects that the temperature of all probes is below 150℃, it begins to reduce the fan frequency to slow down the cooling process. When the temperature of all probes drops to within the range of 145±5℃, the actual measurement records in this embodiment show that after about 28 minutes of continuous air blowing, the temperatures at the three measuring points are 142℃, 146℃, and 148℃, with an average of 145.3℃. The system then issues a prompt and automatically shuts down the fan, and the trolley is moved out of the cooling chamber.

[0083] After testing, the surface hardness of the blade after quenching was HRC 47-49, which is comparable to that of Example 1, and the metallographic structure was martensite with a small amount of retained austenite.

[0084] (2) Tempering step: After the first air cooling is completed, the blades are loaded into a tempering furnace for high-temperature tempering within 4 hours. The tempering temperature is set at 600℃, and the holding time is 2 hours. The temperature uniformity inside the furnace is controlled within ±5℃.

[0085] (3) Second air-cooling step: After the tempering and heat preservation are completed, the blades are immediately removed from the tempering furnace and quickly transported into the sealed cooling chamber for a second air cooling using the trolley system, with a transfer time of ≤3 minutes.

[0086] Air-cooled environment settings: Same as the first air-cooling cycle, ambient temperature 25℃, relative humidity 50%, slight positive pressure 20Pa. Fan speed control: Fan frequency set to 40Hz, corresponding to an average wind speed of 4.0m / s. Actual monitored wind speed range was 3.8-4.2m / s.

[0087] Temperature Monitoring and Endpoint Control: In this embodiment, the target end temperature for the second air cooling is set at 125±5℃. The same online infrared temperature measurement system is used for continuous monitoring. When the blade temperature rapidly passes through the 500-600℃ brittle sensitivity range, the system automatically records the cooling rate. In this embodiment, the measured average cooling rate in the 500-600℃ range is 42℃ / min. After approximately 15 minutes of continuous air cooling, the system monitors three measuring points and finds that the temperatures have dropped to 122℃, 125℃, and 128℃ respectively, with an average of 125℃. At this point, the system automatically shuts down the fan.

[0088] Performance test results: After the above steps, the 2Cr13 stainless steel blades were subjected to room temperature tensile tests according to GB / T 228.1, room temperature impact tests (Charpy V-notch) according to GB / T 229, and Brinell hardness tests according to GB / T 231.1. The test results are as follows: tensile strength Rm (MPa) 748 ≥ 700, qualified; yield strength Rp (0.2 MPa) 519 ≥ 500, qualified; hardness HB 281 ∈ (260, 320), qualified; room temperature impact energy Akv (J) Specimen 1 72 ≥ 30, qualified; room temperature impact energy Akv (J) Specimen 2 58 ≥ 30, qualified; room temperature impact energy Akv (J) Specimen 3 63 ≥ 30, qualified; the average impact energy is 64.3 J.

[0089] Metallographic examination: Tempered sorbite with a small amount of ferrite, uniform structure, and no obvious temper brittleness characteristics.

[0090] Data from Example 5 shows that when the final temperature of the first air cooling is controlled within the range of 140-150℃ and the final temperature of the second air cooling is controlled within the range of 120-130℃, the treated 2Cr13 stainless steel blades can still stably meet the performance requirements of tensile strength Rm≥700MPa, yield strength Rp0.2≥500MPa, hardness HB260-320, and room temperature impact energy Akv≥30J.

[0091] Compared to Example 1, the higher final temperature reduces the cooling time by approximately 20%, which is beneficial for improving production efficiency and reducing energy consumption. Simultaneously, due to the higher initial air-cooling stop temperature, the internal temperature gradient of the blade is smaller, which helps to further reduce quenching stress and deformation risk.

[0092] Comparative Example 1 This comparative example references the process concept of CN202310856933.X, but replaces the steel grade with the 2Cr13 stainless steel of this invention for effect comparison. Since the specific composition of reference document 1 differs from this application, this comparative example only borrows its process framework of "forging-normalizing-spheroidizing annealing-reheating quenching-tempering," and adjusts the specific temperature parameters to the conventional processing range of 2Cr13.

[0093] The effective thickness of the blade is 40mm.

[0094] Forging steps: 2Cr13 steel ingots are heated and forged to a final forging temperature of approximately 950℃, and then air-cooled to room temperature after forging.

[0095] Preliminary heat treatment steps: Normalize the forging billet by heating it to 1000℃ and holding it, then air cool it, followed by spheroidizing annealing by heating it to 780℃ and holding it, then furnace cooling it.

[0096] Final heat treatment steps: Quenching: The spheroidized annealed blades were reheated to 1020℃ for austenitization, held at that temperature, and then oil-cooled to room temperature. The hardness after quenching was measured to be HRC 49.

[0097] Tempering: One tempering is used, while CN202310856933.X uses two temperings. Here, one tempering is used as usual. Heat to 600℃ and hold for 2 hours. After tempering, air cool to room temperature to simulate the "air cool to room temperature" in comparison document 1.

[0098] The performance test results are as follows: tensile strength Rm: 732 MPa; yield strength Rp0.2: 511 MPa; hardness: HB279; room temperature impact energy Akv: 22 J, 18 J, 25 J, with an average of 21.7 J for the three samples.

[0099] The results show that although the strength indicators barely meet the requirements, the average impact energy is only 21.7 J, lower than the requirement of 30 J, and the three samples exhibit significant dispersion. Analysis suggests this is because air cooling after tempering results in a slow cooling rate, causing the blades to remain in the 500-600℃ temperature range for an extended period, leading to type II temper brittleness and a significant decrease in impact toughness. Furthermore, the added normalizing, spheroidizing annealing, and reheating quenching processes prolong the process cycle and increase energy consumption.

[0100] Comparative Example 2 This comparative example is basically the same as Example 1, except that the second air-cooling step is changed to slow cooling, and the effective thickness of the blade is 40mm.

[0101] Cooling steps after forging: Same as in Example 1, with a wind speed of 3.5 m / s, cooling to a surface temperature of 118°C.

[0102] Tempering procedure: Set the tempering temperature to 600℃ and hold for 2 hours.

[0103] Cooling steps: After the heat preservation is completed, the blades are cooled to room temperature in the furnace under slow cooling conditions simulating temper brittleness.

[0104] The performance test results are as follows: tensile strength Rm: 748 MPa; yield strength Rp0.2: 519 MPa; hardness: HB281; room temperature impact energy Akv: 15 J, 11 J, 19 J, with an average of 15.0 J for the three samples.

[0105] The results show that the strength is comparable to that of Example 1, but the average impact energy is only 15.0 J, far below the requirement of 30 J. This clearly demonstrates that slow cooling or furnace cooling through the 500-600℃ range after tempering leads to severe temper brittleness and a sharp deterioration in impact toughness. The second air-cooling process used in this invention is the key technical means to solve this problem.

[0106] Comparative Example 3 This comparative example is basically the same as Example 1, except that the wind speed was too low during the first air cooling and the effective thickness of the blade was 40mm.

[0107] Post-forging cooling procedure: Immediately after forging, a first air cooling process is performed, but the air velocity is only 1.0 m / s, lower than the 2-5 m / s range of this invention. Other conditions are the same as in Example 1. Continuous air cooling is performed until the surface temperature reaches 118°C; due to the low air velocity, the required time is significantly extended.

[0108] Tempering procedure: Same as in Example 1, hold at 600°C for 2 hours.

[0109] Second air-cooling step: Same as in Example 1, with an air speed of 4.0 m / s, cooling until the surface temperature is below 100°C.

[0110] The performance test results are as follows: tensile strength Rm: 681 MPa, less than 700 MPa; yield strength Rp0.2: 472 MPa, less than 500 MPa; hardness: HB 251, less than 260; room temperature impact energy Akv: 35 J, 41 J, 29 J, average value 35.0 J.

[0111] The results show that due to the excessively low initial air cooling velocity and insufficient cooling rate, a sufficient and uniform martensitic structure was not obtained, resulting in the final tempered strength and hardness failing to meet requirements. Although the impact toughness was acceptable, the overall mechanical properties were substandard. This indicates that controlling the initial air cooling velocity within a reasonable range of 2-5 m / s is crucial for ensuring the quenching effect and final performance.

[0112] 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 heat treatment method for 2Cr13 stainless steel for steam turbine blades resistant to water droplet erosion, characterized by, The heat treatment method includes: After forging and cooling, the forged 2Cr13 stainless steel blades are subjected to a first air cooling to achieve quenching hardening and obtain a martensitic structure. Tempering involves subjecting the blades, whose temperature was below 150°C after the first air cooling, to high-temperature tempering at a temperature of 580°C to 650°C for 1.5 to 3 hours. After the tempering step is completed, the blades are immediately removed from the furnace for a second air cooling, which quickly cools them to below 130°C. Then the air cooling system is turned off and the blades are cooled to room temperature to suppress temper brittleness.

2. The heat treatment method for 2Cr13 stainless steel turbine blades resistant to water droplet erosion according to claim 1, characterized in that, In the post-forging cooling step, the cooling rate of the first air cooling is achieved by controlling the wind speed, which is adjusted according to the effective thickness of the blade.

3. The heat treatment method for 2Cr13 stainless steel turbine blades resistant to water droplet erosion according to claim 2, characterized in that, The wind speed for the first air cooling is 2 to 5 m / s.

4. The heat treatment method for 2Cr13 stainless steel turbine blades resistant to water droplet erosion according to claim 1, characterized in that, In the post-forging cooling step, the ambient temperature of the first air cooling is room temperature, specifically 15°C to 35°C, and the cooling ambient temperature is uniform, without drafts or local overcooling.

5. The heat treatment method for 2Cr13 stainless steel turbine blades resistant to water droplet erosion according to claim 1 or 4, characterized in that, In the first air-cooling step, air is continuously blown until the surface temperature of the blades is below 130°C.

6. The heat treatment method for 2Cr13 stainless steel turbine blades resistant to water droplet erosion according to claim 1, characterized in that, In the post-forging cooling step, the airflow direction during air cooling is parallel to the blade length direction, or from the blade root to the blade crown, and a reasonable spacing is maintained between the blades to ensure that the airflow passes smoothly through all blade surfaces.

7. The heat treatment method for 2Cr13 stainless steel turbine blades resistant to water droplet erosion according to claim 1, characterized in that, In the tempering step, the tempering temperature is 600℃±10℃.

8. The heat treatment method for 2Cr13 stainless steel turbine blades resistant to water droplet erosion according to claim 1 or 7, characterized in that, In the tempering step, the holding time is 2 hours.

9. The heat treatment method for 2Cr13 stainless steel turbine blades resistant to water droplet erosion according to claim 1, characterized in that, The interval between the post-forging cooling step and the tempering step shall not exceed 4 to 8 hours.

10. A turbine blade resistant to water droplet erosion, characterized in that, The blade is made of 2Cr13 stainless steel and is treated by the heat treatment method described in any one of claims 1 to 10. The mechanical properties of the 2Cr13 stainless steel blade after the heat treatment method are: tensile strength Rm ≥ 700 MPa, yield strength Rp0.2 ≥ 500 MPa, hardness HB 260 to 320, and room temperature impact energy Akv ≥ 30 J.

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