Large rotor forging and heat treatment method therefor

Abstract

The present invention relates to the technical field of rotor preparation, and provides a large rotor forging and a heat treatment method therefor. The heat treatment method for the large rotor forging comprises the following steps: a heat treatment method for a large rotor forging. The method of the present invention prevents forging cracking, achieves grain refinement, and is capable of fully preparing a microstructure for subsequent quenching and tempering treatment.

Description

A large rotor forging and its heat treatment method Technical Field

[0001] This invention relates to the field of rotor manufacturing technology, and more specifically, to a large rotor forging and its heat treatment method. Background Technology

[0002] As a key core component of steam turbine generator sets, the rotor's dimensions are continuously increasing with the inevitable trend of expanding the parameters and capacity of steam turbine units. Currently, the cross-sectional area of ​​forged rotors, including large flywheel rotors, low-pressure rotors, generator rotors, and welded rotors, has increased to between φ1600mm and φ3000mm. Due to the increased size of the forged shaft, the heat treatment cycle for achieving a uniform and refined microstructure after forging, thus ensuring the sensitivity of UT (Ultra-Temperature Testing) before tempering, is lengthy. Furthermore, this process suffers from high energy consumption and a tendency for forgings to crack, placing significant pressure on production. Summary of the Invention

[0003] The problem solved by this invention is: how to shorten the heat treatment cycle of large rotor forgings, reduce energy consumption, and avoid cracking of forgings.

[0004] To address the above problems, this invention provides a heat treatment method for large rotor forgings, the heat treatment method comprising the following steps:

[0005] The first two-phase zone normalizing, the second two-phase zone normalizing, and the third complete austenitizing normalizing.

[0006] Optionally, the holding temperature for the first two-phase region normalizing is T1, where AC3-20℃≤T1<AC3; the holding temperature for the second two-phase region normalizing is T2, where AC3-20℃≤T2<AC3; and the holding temperature for the third complete austenitizing normalizing is T3, where T3 is AC3+(50 to 80℃).

[0007] Optionally, the heat treatment method includes the following steps:

[0008] S1. Heat the heat treatment furnace to Ar1±20℃, and load the large rotor forging billet that has been forged into the heat treatment furnace as soon as possible and keep it at the temperature.

[0009] S2. Heat to T1 at a rate of v1 and maintain the temperature.

[0010] S3, air cooling of forging blank;

[0011] S4. After removing the slow cooling cover, load the forging billet into the heat treatment furnace and keep it at a low temperature within the range of 100 to 150°C below Ms point.

[0012] S5. Increase the temperature slowly at a rate of v2 to AC1-20℃ to AC1;

[0013] S6, then heat up to T2 at a relatively fast rate using v3, hold at that temperature, and then air cool;

[0014] S7. After removing the slow cooling cover, load the forging billet into the heat treatment furnace and keep it at a low temperature within the range of 100 to 150°C below Ms point.

[0015] S8 is the same as S5;

[0016] S9. Heat to T3 at a relatively fast rate using v4, hold at that temperature, and then air cool.

[0017] Optionally, speed v3 > speed v1.

[0018] Optionally, in S3, when the journal is air-cooled to 500 to 550°C, a slow cooling cover is fitted onto the journal of the forged billet, and air-cooling continues until the shaft body is 120 to 150°C below Ms point.

[0019] Optionally, the heat preservation time in S1 shall not be less than D1 / 100, where D1 is the diameter of the shaft body, the unit of D1 is mm, and the unit of heat preservation time is h.

[0020] Optionally, in S2, the speed v1 is 15 to 20 °C / h.

[0021] Optionally, in S2, the heat preservation time is (2 to 4)D1 / 100, where D1 is the diameter of the shaft body, the unit of D1 is mm, and the unit of heat preservation time is h.

[0022] Optionally, in S5, velocity v2 < velocity v1.

[0023] To address the aforementioned problems, the present invention also provides a large rotor forging, which is prepared using the above method.

[0024] Compared with the prior art, the present invention has the following beneficial effects:

[0025] In this invention, two-phase region normalizing processes are employed to hold the large-section forging at the highest possible temperature in the two-phase region (above AC3-20℃ but below AC3, hereinafter abbreviated as AC3-20℃ to <AC3). This causes most of the forging's internal and external microstructure (70% to 90% by volume) to transform into a uniform and fine austenitic structure, while a small portion retains the bainitic high-temperature tempered structure. Notably, the bainitic high-temperature tempered structure is segmented by the transformed austenitic structure, with the two phases interpenetrating and mixing together, followed by air cooling. After two consecutive two-phase region normalizing processes at temperatures close to AC3, the original coarse austenitic structure's crystalline phases are completely disrupted, and the original coarse austenitic grains are broken by the two incompletely austenitized two-phase structures, forming uniform and fine grain boundaries, phase boundaries, and dispersed carbides. At this point, a third complete austenitizing normalizing process is performed to form equiaxed austenite grains. After normalizing, tempering forms a uniform bainitic tempered structure, which effectively refines the forging structure and grains, and fully prepares the structure for subsequent quenching and tempering treatment.

[0026] In this invention, by precisely controlling the cooling method after forging, the normalizing temperature of different steps, the heating rate of different stages, the holding temperature, the holding time, the cooling rate, and other process parameters, cracking of the forging is avoided, and grain refinement is achieved, which can fully prepare the microstructure for subsequent quenching and tempering treatment.

[0027] In this invention, the first two normalizing temperatures are relatively low. Combined with precise control of each process step and key process parameters, the microstructure of the forging can be guaranteed to meet the requirements, greatly reducing time, lowering energy consumption costs, and improving production efficiency.

[0028] The microstructure of the large rotor forging of the present invention is a bainitic tempered microstructure with fine and uniform grains, for example, a grain size of 3.5 to 5. The flaw detection sensitivity of the above-mentioned large rotor forging has reached a good level, for example, a center limit sensitivity of Φ2.0 to 2.5 mm. Attached Figure Description

[0029] Figure 1 is a schematic diagram of a large rotor forging in an embodiment of the present invention;

[0030] Figure 2 is a schematic diagram of the slow cooling cover in an embodiment of the present invention;

[0031] Figure 3 is a schematic diagram of the slow-cooling cover forging air cooling in an embodiment of the present invention;

[0032] Figure 4 is a grain diagram of the forging billet after step S3 of Embodiment 1 of the present invention;

[0033] Figure 5 is a grain diagram of the forging billet after step S6 of Embodiment 1 of the present invention;

[0034] Figure 6 is a grain diagram of the forging billet after step S9 of Embodiment 1 of the present invention.

[0035] Explanation of reference numerals in the attached drawings: 1. Shaft body; 2. Journal; 3. Flange; 4. Cooling cover; 41. Inner liner; 42. Outer skin; 43. Support frame. Detailed Implementation

[0036] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0037] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate so that the embodiments of the invention described herein can be implemented in sequences other than those illustrated or described herein.

[0038] As shown in Figure 1, this embodiment of the invention provides a large rotor forging, which includes a shaft body 1 and journals 2 connected to both sides of the shaft body 1. The diameter D1 of the shaft body 1 and the diameter D2 of the journals 2 conform to the following relationship: D1 > D2, and the difference between D1 and D2 is greater than 1000 mm, for example, the difference between D1 and D2 is 1100 to 2000 mm, such as 1200 mm, 1300 mm, 1400 mm, 1500 mm, 1600 mm, 1700 mm, 1800 mm, or 1900 mm. The large cross-sectional difference at the connection point between the shaft body 1 and the journals 2 of the large rotor forging leads to a larger internal and external temperature difference during the heat treatment process, resulting in increased structural stress and thermal stress. In addition, the forged grain size and microstructure are coarse after forging, and the strength and toughness reserves are insufficient, making the forging prone to cracking during the post-forging heat treatment process.

[0039] Specifically, large rotor forgings may also include a flange 3 connected to the outside of the journal 2. The diameter D2 of the journal 2 and the diameter D3 of the flange 3 meet the following relationship: D3 > D2, and the difference between D3 and D2 is 100 to 700 mm, for example, 100 mm, 200 mm, 300 mm, 400 mm, 500 mm, and 600 mm.

[0040] Specifically, the diameter D1 of the shaft body 1 is 1700mm or more, for example, 1700 to 3000mm, such as 1800mm, 2000mm, 2200mm, 2400mm, 2600mm, 2800mm.

[0041] Specifically, the diameter D2 of journal 2 is 600mm or more, for example, 600 to 900mm, such as 650mm, 700mm, 750mm, 800mm, or 850mm.

[0042] Specifically, the diameter D3 of flange 3 is 700mm or more, for example, 700 to 1600mm, such as 800mm, 900mm, 1000mm, 1100mm, 1200mm, 1300mm, 1400mm, 1500mm.

[0043] Specifically, the large rotor forging of the present invention is composed of low-carbon C-Cr-Ni-Mo-V steel with microstructure inheritance characteristics, such as 30Cr2Ni4MoV steel, 35Cr2Ni4MoV steel, 25Cr2Ni4MoV steel and 22Cr2Ni3MoV steel.

[0044] For example, the composition of the large rotor forging of the present invention comprises, by mass percentage, C: 0.2% to 0.35%, Si: 0.04% to 0.1%, Mn: 0.2% to 0.6%, Cr: 1.7% to 4.0%, Mo: 0.3% to 0.5%, Ni: 3.0% to 4.0%, V: 0.05% to 0.15%, Al: 0.003% to 0.01%, S: ≤0.004%, P: ≤0.004%, with the balance being Fe and unavoidable impurities.

[0045] This invention provides a heat treatment method for large rotor forgings, including a first two-phase region normalizing, a second two-phase region normalizing, and a third complete austenitizing normalizing.

[0046] Specifically, the holding temperature for the first two-phase region normalizing is T1, where AC3-20℃≤T1<AC3; the holding temperature for the second two-phase region normalizing is T2, where AC3-20℃≤T2<AC3; and the holding temperature for the third complete austenitizing normalizing is T3, where T3 is AC3+(50 to 80℃).

[0047] Compared with existing technologies, the heat treatment method for large rotor forgings in this invention employs two-phase region normalizing processes. This allows the large-section forging to be held at the highest possible temperature in the two-phase region (AC3-20℃ to <AC3), transforming most of the forging's internal and external microstructure (70% to 90% by volume) into a uniform and fine austenitic structure, while a small portion retains the bainitic high-temperature tempered structure. Notably, the bainitic high-temperature tempered structure is segmented by the transformed austenitic structure, with the two phases interpenetrating and mixing together, followed by air cooling. Through two consecutive two-phase region normalizing processes at near AC3 temperatures, the original coarse austenitic structure's crystalline phases are completely disrupted, and the original coarse austenitic grains are broken up by the two incompletely austenitized two-phase structures, forming uniform and fine grain boundaries, phase boundaries, and dispersed carbides. At this point, a third complete austenitizing normalizing process is performed to form equiaxed austenite grains. After normalizing, tempering forms a uniform bainitic tempered structure, which effectively refines the forging structure and grains, and fully prepares the structure for subsequent quenching and tempering treatment.

[0048] Specifically, the heat treatment method for the aforementioned large rotor forgings includes the following steps:

[0049] S1. Heat the heat treatment furnace to Ar1±20℃, and load the large rotor forging billet that has been forged into the heat treatment furnace as soon as possible and keep it at the temperature.

[0050] S2. Increase the temperature of v1 to AC3-20℃ to <AC3 at a rate of 15℃ / h or higher, and hold at that temperature.

[0051] S3, air cooling of forging blank;

[0052] S4. After removing the slow cooling cover, load the forging billet into the heat treatment furnace and keep it at a low temperature within the range of 100 to 150°C below Ms point.

[0053] S5. Increase the temperature slowly at a rate of v2 to AC1-20℃ to AC1;

[0054] S6. Then, heat the temperature to AC3-20℃ to <AC3 at a relatively fast rate v3, hold the temperature, and then air cool; the rate v3 > the rate v1.

[0055] S7. After removing the slow cooling cover, load the forging billet into the heat treatment furnace and keep it at a low temperature within the range of 100 to 150°C below Ms point.

[0056] S8 is the same as S5;

[0057] S9. Then heat up to AC3+ (50 to 80°C) at a faster rate v4, keep warm, and then air cool; rate v4 > rate v3.

[0058] Specifically, the process preceding S1 includes the forging of a large rotor forging blank. The forging of the large rotor forging blank involves: first forging a journal region with a small cross-section (if a flange is present, this journal region includes both the journal and the flange); then, forging the shaft body using a large forging ratio. Specifically, the large forging ratio ranges from 1.2 to 1.3. By allowing for significant deformation of the shaft body, a larger austenite nucleation driving force is formed, which is beneficial for subsequent static recrystallization and for refining the grain size and microstructure of the forging.

[0059] Specifically, in S1 above, the large rotor forging blank after forging is not air-cooled to room temperature. After forging, it is placed in a heat treatment furnace as quickly as possible for heat treatment and kept at a certain temperature. This allows for a partial transformation of pearlite distributed from grain boundaries to grain interior (volume percentage 20% to 35%). At this time, the microstructure of the forging blank is a mixture of isothermal pearlite and untransformed austenite, which can prevent the forging blank from cracking.

[0060] Specifically, in S1 above, considering that excessively high or low temperatures would prolong the incubation period for pearlite transformation, leading to a decrease in pearlite content and a reduction in the refining effect, the holding temperature of S1 is controlled at Ar1±20℃, for example, 600 to 630℃, such as 610℃, 620℃, or 630℃.

[0061] Specifically, in S1 above, the purpose of heat preservation is to ensure uniform temperature inside and outside the forging billet, reduce the temperature difference during the heating process, and lower thermal stress. Considering that excessive heat preservation time would increase the heat treatment cycle, while insufficient heat preservation time would result in uneven core and surface temperatures and a low pearlite content, the heat preservation time in S1 is controlled to be no less than D1 / 100, where D1 is the diameter of the shaft (in mm) and the heat preservation time is in hours (h). Preferably, the heat preservation time is (1.5 to 2)D1 / 100.

[0062] Specifically, in S2 above, the plastic deformation stage, while ensuring controllable instantaneous internal stress, reaches a temperature close to AC3 (e.g., AC3-20℃ to <AC3, e.g., 780 to 800℃, e.g., 785℃, 790℃, 795℃) relatively quickly. This results in numerous nucleation points, effectively disrupting the original austenite grains and refining the grain size. Therefore, a heating rate v1 of 15℃ / h or higher is used. Preferably, the heating rate v1 is 15 to 20℃ / h, e.g., 16℃ / h, 17℃ / h, 18℃ / h, or 19℃ / h.

[0063] Specifically, in S2 above, to ensure thorough heating of the forged billet core and uniform internal and external temperatures, allowing for sufficient nucleation of the two-phase region within the core, the holding time is controlled to be (2 to 4)D1 / 100, where D1 is the diameter of the shaft body in mm, and the holding time is in hours. Preferably, the holding time is (2.3 to 3.5)D1 / 100h, for example, 2.5D1 / 100, 2.7D1 / 100, 3D1 / 100, 3.2D1 / 100, or 3.5D1 / 100.

[0064] Specifically, in S3 above, the forging billet is hoisted onto a 1000mm high pad for air cooling.

[0065] Specifically, in S3 above, when the journal (or both journal and flange if present) is air-cooled to within the range of 500 to 550°C, a slow cooling cover 4 is placed over the journal (or both journal and flange if present) of the forging billet, and the shaft body is air-cooled to 120 to 150°C below Ms point, for example, 220 to 250°C. It should be noted that the slow cooling cover can reduce the cooling rate of the journal and flange (e.g., to 5 to 10°C / h), thereby reducing the thermal and structural stresses at the large-section transition between the journal and shaft body, and reducing the risk of forging cracking. Simultaneously, it ensures that the forging billet is air-cooled to a lower temperature, resulting in a lower core temperature and more complete microstructural transformation, achieving the effect of refining the microstructure and grain size.

[0066] Specifically, in the above S3, the structure of the slow cooling cover 4 is shown in Figure 2. The inner diameter of the slow cooling cover 4 is equivalent to the diameter of the shaft body. The slow cooling cover 4 includes an inner lining 41 and an outer skin 42. The inner lining 41 is asbestos felt with good heat insulation effect, and the outer skin 42 is iron sheet that provides support. The slow cooling cover 4 also includes a height-adjustable support frame 43, which is connected to the outer skin 42 and can adjust the position of the slow cooling cover 4.

[0067] Specifically, Figure 3 shows a schematic diagram of the slow-cooling cover forging air cooling.

[0068] Specifically, in S4 above, holding the steel at a temperature below Ms for an extended period allows for sufficient microstructural transformation in the core of the forging billet, which is beneficial for adjusting and refining the steel's grain structure through subsequent phase transformation recrystallization. Therefore, the holding time is controlled to be (1.2 to 1.6)D1 / 100, where D1 is the diameter of the shaft (in mm) and the holding time is in hours (h). Preferably, for example, 1.3D1 / 100, 1.4D1 / 100, 1.5D1 / 100, or 1.6D1 / 100.

[0069] Specifically, in the above-mentioned S5 stage, the heating is limited to minimize the internal and external temperature difference and instantaneous internal stress in the elastic deformation zone of the forging, thereby reducing the risk of cracking and preparing for the transformation of the microstructure in the two-phase region. Therefore, the speed v2 is controlled to be less than the speed v1, for example, the speed v2 is below 10℃ / h, preferably, the speed v2 is 5 to 10℃ / h, for example, 6℃ / h, 7℃ / h, 8℃ / h, 9℃ / h.

[0070] Specifically, in S5 above, the temperature of AC1-20℃ to AC1 is preferably 670 to 700℃, for example 680℃, 690℃, or 700℃.

[0071] Specifically, in S6 above, during the plastic deformation stage, while ensuring controllable instantaneous internal stress, the temperature reaches close to AC3 relatively quickly, resulting in numerous nucleation points and effectively disrupting the original austenite grains, thus refining the grain size. This stage involves rate-limited heating. Since the forging has undergone normalizing treatment in the plastic deformation zone, its microstructure is relatively fine, and its plasticity and toughness reserves have increased. Therefore, the speed v3 is controlled to be greater than the speed v1. For example, the speed v3 is below 30℃ / h. Preferably, the speed v3 is 20 to 30℃ / h, such as 22℃ / h, 24℃ / h, 26℃ / h, or 28℃ / h.

[0072] Specifically, in S6 above, considering that the cross-section of the large forging is large and there is a certain temperature difference between the inside and outside of the forging during the heating process, the holding time is controlled to be (2 to 4)D1 / 100, where D1 is the diameter of the shaft body, the unit of D1 is mm, the unit of holding time is h, and the holding time is the same as the holding stage in S2.

[0073] Specifically, in S6 above, the air cooling operation steps are the same as in S3, except that the surface temperature of the shaft is 30 to 50°C lower than the final temperature in S3.

[0074] Specifically, in S7 above, considering that in S6, the air cooling to the shaft temperature is lower than in S3, the core of the forging will also decrease accordingly. Therefore, the holding time during the low-temperature holding stage is relatively shortened. For example, the holding time is controlled to be 1 to 1.5D1 / 100mm, where D1 is the diameter of the shaft in mm and the holding time is in hours, such as 1.1D1 / 100mm, 1.2D1 / 100mm, 1.2D1 / 100mm, and 1.4D1 / 100mm.

[0075] Specifically, in S9 above, the speed v4 > speed v3. Compared with the previous two normalizing processes, the forging has a more sufficient reserve of plasticity and toughness, and stronger resistance to cracking. Further increasing the heating rate in the two-phase region can effectively refine the grains and microstructure. Preferably, the speed v4 is 30 to 40℃ / h, for example, 32℃ / h, 34℃ / h, 36℃ / h, or 38℃ / h.

[0076] Specifically, in S9 above, the temperature is maintained at AC3+ (50 to 80°C) (e.g., 850 to 890°C) to allow the microstructure to fully form equiaxed austenitic grains. Since two normalizing processes have already been performed, the microstructure and grain size are already very fine, and the time required for austenitization to achieve uniform grain size is shorter than the previous two processes. Therefore, the required holding time is (1.8 to 3.5)D1 / 100, preferably 2 to 3D1 / 100mm, such as 2.2D1 / 100mm, 2.4D1 / 100mm, 2.6D1 / 100mm, or 2.8D1 / 100mm, where D1 is the diameter of the shaft, the unit of D1 is mm, and the unit of holding time is h.

[0077] Specifically, in S9 above, the air cooling operation is the same as that in S6, and will not be repeated here.

[0078] Specifically, the heat treatment method for the aforementioned large rotor forgings also includes a tempering process, which comprises the following steps:

[0079] S10 is the same as S7;

[0080] S11. Heat to AC1 (50 to 80℃) and maintain the temperature;

[0081] S12. After cooling to 250℃ at a rate of less than 10℃ / h, air cool to room temperature.

[0082] Specifically, in S11, the heating rate is 10 to 25℃ / h, for example 13℃ / h, 15℃ / h, 18℃ / h, 20℃ / h, 23℃ / h.

[0083] Specifically, in S11, the holding time is (2.5 to 4)D1 / 100, for example, 2.6D1 / 100mm, 2.8D1 / 100mm, 3D1 / 100mm, 3.2D1 / 100mm, 3.4D1 / 100mm, 3.6D1 / 100mm, and 3.8D1 / 100mm, where D1 is the diameter of the shaft, and the unit of D1 is mm. The unit of holding time is h. This step, through sufficient tempering, forms a uniform bainitic tempered structure, effectively refining the forging structure and grains, and fully preparing the microstructure for subsequent processes.

[0084] Specifically, in S12, considering that excessive cooling rate can lead to excessive thermal stress at different cross-sections, causing forging deformation, and that excessive cooling rate can result in excessively high hardness of forgings with forging scale, creating a risk of difficulty in machining, a cooling rate of less than 10℃ / h is controlled. For example, 5 to 10℃ / h, such as 6℃ / h, 7℃ / h, 8℃ / h, or 9℃ / h.

[0085] The present invention also provides a large rotor forging, which is prepared by the above-described heat treatment method.

[0086] Specifically, the microstructure of the aforementioned large rotor forging is a bainitic tempered microstructure with fine and uniform grains, for example, a grain size of 3.5 to 5. The flaw detection sensitivity of the aforementioned large rotor forging has reached a good level, for example, a center limit sensitivity of Φ2.0 to 2.5 mm.

[0087] Compared with existing technologies, this invention employs two-phase region normalizing processes. The large-section forging is held at the highest possible temperature in the two-phase region (AC3-20℃ to <AC3), transforming most of the forging's internal and external microstructure (70% to 90% by volume) into a uniform and fine austenitic structure, while a small portion retains a bainitic high-temperature tempered structure. Notably, the bainitic high-temperature tempered structure is segmented by the transformed austenitic structure, with the two phases interpenetrating and mixing together, followed by air cooling. After two consecutive two-phase region normalizing processes at near AC3 temperatures, the original coarse austenitic structure's crystalline phases are completely disrupted, and the original coarse austenitic grains are broken up by the two incomplete austenitization processes, forming uniform and fine grain boundaries, phase boundaries, and dispersed carbides. A third complete austenitizing normalizing process then completes the formation of equiaxed austenitic grains. Tempering after normalizing forms a uniform bainitic tempered structure, effectively refining the forging's microstructure and grains, and adequately preparing the microstructure for subsequent quenching and tempering treatment.

[0088] In this embodiment of the invention, during the forging process of the large rotor forging billet, a small cross-section journal area (if there is a flange, this is the journal and the flange) is forged first, and then the shaft body is forged and deformed with a large forging ratio. The shaft body has a large deformation, which forms a large austenite nucleation driving force, which is beneficial to subsequent static recrystallization and to the refinement of the grains and structure of the forging.

[0089] In this embodiment of the invention, by precisely controlling the cooling method after forging, the normalizing temperature of different steps, the heating rate of different stages, the holding temperature, the holding time, the cooling rate, and other process parameters, the cracking of the forging is avoided, and the grain refinement is achieved, which can fully prepare the microstructure for subsequent quenching and tempering treatment.

[0090] In this embodiment of the invention, the temperature of the first two normalizing processes is relatively low. Combined with precise control of each process step and the control of key process parameters, it can ensure that the microstructure of the forging meets the requirements, greatly reducing time, lowering energy consumption costs, and improving production efficiency.

[0091] The advantages of precise control of process parameters of the present invention will be demonstrated below with specific embodiments and comparative examples.

[0092] Example 1

[0093] This embodiment provides a large motor rotor forging and its heat treatment method. The large rotor forging of this embodiment includes a shaft body 1 and journals 2 connected on both sides of the shaft body 1. The cross-sectional diameter of the shaft body is φ1800mm, and the cross-sectional diameter of the journals 2 is 700mm. The rotor is made of 25Cr2Ni4MoV steel with the following composition: C: 0.24%, Si: 0.05%, Mn: 0.25%, Cr: 1.75%, Mo: 0.38%, Ni: 3.35%, V: 0.1%, Al: 0.005%, S: 0.002%, P: 0.002%, and the balance being Fe and unavoidable impurities.

[0094] The heat treatment method for the large rotor forging in this embodiment includes the following steps:

[0095] S0. First, forge a journal with a small cross-section, then forge and deform the shaft body with a large forging ratio. The value of the large forging ratio is 1.25.

[0096] S1. Heat the heat treatment furnace to 610℃, and load the large rotor forging billet that has been forged into the heat treatment furnace as soon as possible, and keep it at that temperature for 30 hours.

[0097] S2. Heat to 795℃ at a rate of 15℃ / h and hold for 50h;

[0098] S3. After the forging billet is air-cooled and the journal is air-cooled to 500 to 550°C, the journal is air-cooled with a slow cooling cover until the surface temperature of the shaft body is 250°C.

[0099] S4. After removing the slow cooling cover, put the forging into the heat treatment furnace and keep it at 200℃ for 25 hours.

[0100] S5. Increase the temperature to 680℃ at a slow rate of 8℃ / h;

[0101] S6. Then heat up to 795℃ at a relatively fast rate of 25℃ / h, keep it at that temperature for 50h, and then air cool it. Air cool the journal to 500 to 550℃, and then add a slow cooling cover to the journal and air cool it until the surface temperature of the shaft body is 200℃.

[0102] S7. After removing the slow cooling cover, put the forging into the heat treatment furnace and keep it at 180℃ for 22 hours.

[0103] S8 is the same as S5;

[0104] S9. Then, heat the journal to 860°C at a relatively fast rate of 34°C / h, hold for 45 hours, and then air cool it. Air cool the journal to 500 to 550°C, and then add a slow cooling cover to the journal and air cool it until the surface temperature of the shaft body is 200°C.

[0105] S10 is the same as S7;

[0106] S11. Heat to 640℃ and keep warm for 55 hours;

[0107] S12: Cool down to 250℃ at a rate of 10℃ / h, then air cool to room temperature.

[0108] The grain size of the forging after step S3 in this embodiment is shown in Figure 4, the grain size of the forging after step S6 is shown in Figure 5, and the grain size of the forging after step S9 is shown in Figure 6. The forging in this embodiment exhibits good flaw detection sensitivity and grain size, with a center limit sensitivity of Φ2.0 mm and a grain size of grade 5. Furthermore, the forging did not crack during the preparation process in this embodiment.

[0109] Example 2

[0110] This embodiment provides a large low-pressure rotor forging and its heat treatment method. The shape of the large rotor forging in this embodiment is basically the same as that in Embodiment 1, except that the shaft cross-section of the rotor forging is φ2000mm and the journal size is 800mm. The rotor material is 30Cr2Ni4MoV steel, and the chemical composition includes C: 0.26%, Si: 0.06%, Mn: 0.26%, Cr: 3.65%, Mo: 0.39%, Ni: 3.75%, V: 0.087%, Al: 0.003%, S: 0.0021%, P: 0.004%, with the balance being Fe and unavoidable impurities.

[0111] The heat treatment method for the large rotor forging in this embodiment is largely the same as that in Embodiment 1, including:

[0112] S0. First, forge a journal with a small cross-section, then forge the shaft body with a large forging ratio. The value of the large forging ratio is 1.26.

[0113] S1. Heat the heat treatment furnace to 620℃, and load the large rotor forging billet that has been forged into the heat treatment furnace as soon as possible, and keep it at that temperature for 32 hours.

[0114] S2. Heat to 790℃ at a rate of 15℃ / h and hold for 55h;

[0115] S3, Same as in Example 1;

[0116] S4. After removing the slow cooling cover, put the forging into the heat treatment furnace and keep it at 210℃ for 28 hours.

[0117] S5. Increase the temperature to 700℃ at a slow rate of 8℃ / h;

[0118] S6. Then heat up to 790℃ at a relatively fast rate of 23℃ / h, hold for 55h, then air cool. Air cool the journal to 500-550℃, and add a slow cooling cover to the journal and air cool until the surface temperature of the shaft body is 200℃.

[0119] S7. After removing the slow cooling cover, put the forging into the heat treatment furnace and keep it at 180℃ for 25 hours.

[0120] S8 is the same as S5;

[0121] S9. Then, heat the journal to 870℃ at a relatively fast rate of 31℃ / h, hold for 50h, and then air cool it. Air cool the journal to 500-550℃, and then add a slow cooling cover to the journal and air cool it until the surface temperature of the shaft body is 200℃.

[0122] S10 is the same as S7;

[0123] S11. Heat to 640℃ and keep warm for 60 hours;

[0124] S12: Same as in Example 1.

[0125] The forging in this embodiment achieved good levels of flaw detection sensitivity and grain size, with a center limit sensitivity of Φ2.5mm and a grain size of grade 4.5. Furthermore, the forging did not crack during the manufacturing process of this embodiment.

[0126] Example 3

[0127] This embodiment provides a large flywheel rotor forging and its heat treatment method. The shape of the large rotor forging in this embodiment is basically the same as that in Embodiment 1, except that the shaft body cross section of the rotor forging is φ2800mm and the journal size is 800mm. The rotor material is 35Cr2Ni4MoV steel, and the chemical composition includes C: 0.33%, Si: 0.1%, Mn: 0.28%, Cr: 1.8%, Mo: 0.42%, Ni: 3.75%, V: 0.09%, Al: 0.003%, S: 0.002%, P: 0.003%, with the balance being Fe and unavoidable impurities.

[0128] The heat treatment method for the large rotor forging in this embodiment is largely the same as that in Embodiment 1, including:

[0129] S0. First, forge a journal with a small cross-section, then forge the shaft body with a large forging ratio. The value of the large forging ratio is 1.28.

[0130] S1. Heat the heat treatment furnace to 630℃, and load the large rotor forging billet that has been forged into the heat treatment furnace as soon as possible, and keep it at that temperature for 45 hours.

[0131] S2. Heat to 800℃ at a rate of 16℃ / h and hold for 65h;

[0132] S3, Same as in Example 1;

[0133] S4. After removing the slow cooling cover, put the forging into the heat treatment furnace and keep it at 250℃ for 38 hours.

[0134] S5. Increase the temperature to 700℃ at a slow rate of 6℃ / h;

[0135] S6. Then heat up to 800℃ at a relatively fast rate of 25℃ / h, keep it at that temperature for 65h, and then air cool it. Air cool the journal to 500 to 550℃, and then add a slow cooling cover to the journal and air cool it until the surface temperature of the shaft body is 200℃.

[0136] S7. After removing the slow cooling cover, put the forging into the heat treatment furnace and keep it at 200℃ for 35 hours.

[0137] S8 is the same as S5;

[0138] S9. Then, heat the journal to 880°C at a relatively fast rate of 33°C / h, hold for 60 hours, and then air cool it. Air cool the journal to 500 to 550°C, and then add a slow cooling cover to the journal and air cool it until the surface temperature of the shaft body is 200°C.

[0139] S10 is the same as S7;

[0140] S11. Heat to 660℃ and keep warm for 80 hours;

[0141] S12: Cool down to 250℃ at a rate of 8℃ / h, then air cool to room temperature.

[0142] The forging in this embodiment achieved good levels of flaw detection sensitivity and grain size, with a center-limiting sensitivity of Φ2.5mm and a grain size of level 4. Furthermore, the forging did not crack during the manufacturing process of this embodiment.

[0143] Example 4

[0144] This embodiment provides a large welded rotor forging and its heat treatment method. The shape of the large rotor forging in this embodiment is basically the same as that in Embodiment 1, except that the shaft cross-section of the rotor forging is φ2500mm and the journal size is 700mm. The rotor material is 22Cr2Ni3MoV steel, and the chemical composition includes C: 0.22%, Si: 0.04%, Mn: 0.58%, Cr: 1.7%, Mo: 0.45%, Ni: 3.15%, V: 0.095%, Al: 0.003%, S: 0.002%, P: 0.003%, with the balance being Fe and unavoidable impurities.

[0145] The heat treatment method for the large rotor forging in this embodiment is largely the same as that in Embodiment 1, including:

[0146] S0. First, forge a journal with a small cross-section, then forge and deform the shaft body with a large forging ratio. The value of the large forging ratio is 1.27.

[0147] S1. Heat the heat treatment furnace to 600℃, and load the large rotor forging billet that has been forged into the heat treatment furnace as soon as possible, and keep it at that temperature for 43 hours.

[0148] S2. Heat to 780℃ at a rate of 16℃ / h and hold for 63h;

[0149] S3, Same as in Example 1;

[0150] S4. After removing the slow cooling cover, put the forging into the heat treatment furnace and keep it at 230℃ for 35 hours.

[0151] S5. Increase the temperature to 670℃ at a slow rate of 7℃ / h;

[0152] S6. Then heat up to 780℃ at a relatively fast rate of 22℃ / h, keep it at that temperature for 63h, and then air cool it. Air cool the journal to 500 to 550℃, and then add a slow cooling cover to the journal and air cool it until the surface temperature of the shaft body is 200℃.

[0153] S7. After removing the slow cooling cover, put the forging into the heat treatment furnace and keep it at 180℃ for 32 hours.

[0154] S8 is the same as S5;

[0155] S9. Then heat up to 850℃ at a relatively fast rate of 33℃ / h, hold for 60h, and then air cool. Air cool the journal to 500 to 550℃, and then add a slow cooling cover to the journal and air cool until the surface temperature of the shaft body is 200℃.

[0156] S10 is the same as S7;

[0157] S11. Heat to 640℃ and keep warm for 75 hours;

[0158] S12: Same as in Example 3.

[0159] The forging in this embodiment achieved good levels of flaw detection sensitivity and grain size, with a center-limiting sensitivity of Φ2.2mm and a grain size of grade 3.5. Furthermore, the forging did not crack during the manufacturing process of this embodiment.

[0160] The inventors conducted extensive research during the research process, and some poorly performing solutions are now presented as comparative examples.

[0161] Comparative Example 1

[0162] This comparative example provides a large rotor forging and its heat treatment method. The composition and shape of the large rotor forging in this comparative example are the same as those in Example 1, and will not be repeated here.

[0163] The preparation method of the large rotor forging in this comparative example is roughly the same as that in Example 1, except that:

[0164] Without a slow cooling shroud, when the shaft body was air-cooled to 250°C, longitudinal cracks were found in the forging shaft body and journal, resulting in the scrapping of the forging and making it impossible to carry out subsequent heat treatment.

[0165] Comparative Example 2

[0166] This comparative example provides a large rotor forging and its heat treatment method. The composition and shape of the large rotor forging in this comparative example are the same as those in Example 2, and will not be repeated here.

[0167] The preparation method of the large rotor forging in this comparative example is roughly the same as that in Example 2, except that:

[0168] The holding temperature reached by S2 and S6 is 760℃.

[0169] The large rotor forgings in this comparative example exhibited severe mixed grain size after heat treatment following forging: 25% at grade 1.0, 30% at grade 2.5, and 45% at grade 4. Furthermore, the flaw detection sensitivity reached Φ4.5mm, showing grass-like waves, making subsequent tempering impossible.

[0170] Comparative Example 3

[0171] This comparative example provides a large rotor forging and its heat treatment method. The composition and shape of the large rotor forging in this comparative example are the same as those in Example 3, and will not be repeated here.

[0172] The preparation method of the large rotor forging in this comparative example is roughly the same as that in Example 3, except that:

[0173] The high-temperature insulation time for S2, S6 and S9 is 35 hours.

[0174] The large rotor forgings in this comparative example exhibited severe mixed grain size after heat treatment following forging: 35% of grade 1.0, 30% of grade 2.0, and 35% of grade 3.0. Furthermore, the flaw detection sensitivity reached Φ5.5mm, and grass-like waves were observed, making subsequent tempering impossible.

[0175] Comparative Example 4

[0176] This comparative example provides a large rotor forging and its heat treatment method. The composition and shape of the large rotor forging in this comparative example are the same as those in Example 1, and will not be repeated here.

[0177] In the preparation method of this comparative example, in S0, the shaft is first forged according to a forging ratio of 1.25, but the journal is not yet forged. The temperature of the forging has dropped below the process range and cannot be forged. The forging is then returned to the heating furnace for forging reheating. After heating, the journal is forged, and the shaft body is not deformed, resulting in the finished product.

[0178] The remaining steps are largely the same as in Example 1.

[0179] The large rotor forgings in this comparative example exhibited coarse grains (grades 2.0 to 2.5) after heat treatment following forging, and the flaw detection sensitivity reached Φ4mm, showing grass-like waves, making subsequent tempering impossible.

[0180] While the present invention has been disclosed above, its scope of protection is not limited thereto. Those skilled in the art can make various changes and modifications without departing from the spirit and scope of the present invention, and all such changes and modifications will fall within the scope of protection of the present invention.

Claims

1. A method of heat treating a large rotor forging, characterized by, The heat treatment method includes: a first two-phase region normalizing, a second two-phase region normalizing, and a third complete austenitizing normalizing.

2. The heat treatment method of large rotor forgings according to claim 1, characterized in that, The holding temperature for the first two-phase region normalizing is T1, where AC3-20℃≤T1<AC3; the holding temperature for the second two-phase region normalizing is T2, where AC3-20℃≤T2<AC3; and the holding temperature for the third complete austenitizing normalizing is T3, where T3 is AC3+(50 to 80℃).

3. The heat treatment method of large rotor forgings according to claim 2, characterized in that, The heat treatment method includes the following steps: S1. Heat the heat treatment furnace to Ar1±20℃, and load the large rotor forging billet that has been forged into the heat treatment furnace as soon as possible and keep it at the temperature. S2. Heat to T1 at a rate of v1 and maintain the temperature. S3, air cooling of forging blank; S4. The forging billet is placed into a heat treatment furnace and kept at a low temperature within a range of 100 to 150°C below Ms point. S5. Increase the temperature slowly at a rate of v2 to AC1-20℃ to AC1; S6, then heat up to T2 at a relatively fast rate using v3, hold at that temperature, and then air cool; S7. Load the forging billet into the heat treatment furnace and keep it at a low temperature within the range of 100 to 150°C below Ms point; S8 is the same as S5; S9. Heat to T3 at a relatively fast rate using v4, hold at that temperature, and then air cool.

4. The heat treatment method of large rotor forgings according to claim 3, characterized in that, Speed ​​v3 > Speed ​​v1.

5. The heat treatment method of large rotor forgings according to claim 3, characterized in that, In step S3, when the journal is air-cooled to 500 to 550°C, a slow cooling cover is fitted onto the journal of the forged billet, and air-cooling continues until the shaft body is 120 to 150°C below Ms point.

6. The heat treatment method of large rotor forgings according to claim 3, characterized by The heat preservation time of S1 shall not be less than D1 / 100, where D1 is the diameter of the shaft body, the unit of D1 is mm, and the unit of heat preservation time is h.

7. The heat treatment method of large rotor forgings according to claim 3, characterized by In S2, the speed v1 is 15 to 20 °C / h.

8. The heat treatment method of large rotor forgings according to claim 3, characterized by, In S2, the heat preservation time is (2 to 4)D1 / 100, where D1 is the diameter of the shaft body, the unit of D1 is mm, and the unit of heat preservation time is h.

9. The heat treatment method of large rotor forgings according to any one of claims 3 to 8, characterized in that, The heat treatment method further includes a tempering process, which comprises the following steps: S10 is the same as S7; S11. Heat to AC1 (50 to 80℃) and maintain the temperature; S12. After cooling to 250℃, air cool to room temperature.

10. A large rotor forging characterized by, The large rotor forging is obtained by heat treatment using the heat treatment method for large rotor forgings as described in any one of claims 1 to 9.

Images