Large rotor forging, and heat treatment method for preventing cracking thereof

Abstract

The present invention belongs to the technical field of rotor preparation. Disclosed are a large rotor forging and a heat treatment method for preventing cracking thereof. The present invention solves the problems that existing large rotor forgings are prone to cracking during a post-forging heat treatment and often do not comply with the standards for flaw detection sensitivity. The heat treatment method comprises: sequentially performing a first full austenitizing normalizing treatment, a second full austenitizing normalizing treatment and a third full austenitizing normalizing treatment, wherein the holding temperatures of the three normalizing treatments decrease successively. The heat treatment method of the present invention can prevent the forging from cracking, and can realize grain refinement.

Description

A large rotor forging and its crack-preventing heat treatment method Technical Field

[0001] This invention relates to the field of rotor manufacturing technology, and in particular to a large rotor forging and its crack-preventing heat treatment method. Background Technology

[0002] The rotor is a key core component of a steam turbine generator set. With the trend of increasing the parameters and capacity of steam turbine units, the size of its core component, the rotor forging, is also continuously increasing. Currently, 600MW to 1000MW units account for an absolute majority of steam turbine power generation, and the shaft cross-section of their rotor forgings is all within [specific dimensions missing]. to Between, the shaft diameter dimensions are all within to As the size of the forging shaft increases, the difference in cross-section between adjacent diameter positions becomes larger, leading to a greater internal and external temperature difference during the heat treatment process. This results in increased structural and thermal stress. In addition, the forged grain size and microstructure are coarse after forging, and the strength and toughness reserves are insufficient. Consequently, longitudinal cracking of the forging frequently occurs during the post-forging heat treatment process, causing significant losses to the manufacturer. Summary of the Invention

[0003] In view of the above, the present invention aims to provide a large rotor forging and its anti-cracking heat treatment method, which can at least solve one of the following technical problems: existing large rotor forgings are prone to cracking during post-forging heat treatment, and the flaw detection sensitivity is often unqualified.

[0004] The objective of this invention is mainly achieved through the following technical solutions:

[0005] This invention provides a heat treatment method for preventing cracking in large rotor forgings. The heat treatment method includes: a first full austenitizing normalizing, a second full austenitizing normalizing, and a third full austenitizing normalizing, with the normalizing holding temperature decreasing sequentially for the three normalizing processes.

[0006] Furthermore, the holding temperature for the first complete austenitizing normalizing is AC3+ (90℃ to 120℃), the holding temperature for the second complete austenitizing normalizing is AC3+ (60℃ to 90℃), and the holding temperature for the third complete austenitizing normalizing is AC3+ (50℃ to 70℃).

[0007] Furthermore, 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 AC3+ (90℃ to 120℃) at a rate of v1, and maintain the temperature.

[0010] S3, air cooling of forging blank;

[0011] S4. The forging billet is placed into a heat treatment furnace and kept at a low temperature within the range of 100°C to 130°C below Ms point.

[0012] S5. Increase the temperature to AC1 at a slow rate of v2 (20°C to AC1);

[0013] S6. Then, heat up to AC3+ (60℃ to 90℃) at a relatively fast rate using v3, hold at that temperature, and then air cool.

[0014] S7. The forging billet is placed into a heat treatment furnace and held at a low temperature within the range of 120°C to 150°C below Ms point; wherein the low temperature of S7 is lower than the low temperature of S4.

[0015] S8 is the same as S5;

[0016] S9. Then, heat the temperature to AC3+ (50℃ to 70℃) at a relatively fast rate using v4, hold the temperature, and then air cool; after air cooling, continue tempering.

[0017] Furthermore, V4 > v3 > v1.

[0018] Furthermore, the low-temperature insulation time in S7 is shorter than the low-temperature insulation time in S4.

[0019] Furthermore, in S3, when the journal area is air-cooled to a temperature range of 500°C to 550°C, a slow cooling cover is placed over the shaft diameter area of ​​the forging billet, and the shaft body is air-cooled to a temperature 100°C to 150°C below Ms point.

[0020] Furthermore, the heat preservation time in S9 is shorter than that in S6.

[0021] Furthermore, v2 < v1.

[0022] Furthermore, in S2, the heat preservation 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, and the unit of heat preservation time is h.

[0023] The present invention also provides a large rotor forging, which is prepared by the above method.

[0024] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0025] a) In the method of this invention, the three-stage normalizing process with stepped cooling is combined to obtain a uniform and fine austenitic grain size in large-section forgings. It is worth noting that the first normalizing process fully dissolves most of the carbides in the microstructure into the austenite, improving the strength and toughness of the matrix. The second and third normalizing processes use stepped cooling to refine the grains and microstructure. After three consecutive normalizing processes, the crystalline phases of the original coarse austenitic microstructure are completely disrupted, the microstructure inheritance is cut off, and uniform and fine grain boundaries, phase boundaries, and dispersed carbides are formed. After tempering, a uniform bainitic tempered microstructure is formed, effectively refining the microstructure and grains of the forging, and fully preparing the microstructure for subsequent quenching and tempering treatment.

[0026] b) In the method of the present invention, during the forging process of the large rotor forging blank, a journal with a small cross section is forged first (if there is a flange, this is the journal and the flange), and then the shaft body is forged and deformed with a large forging ratio. The shaft body has a large deformation, forming 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.

[0027] c) In the method of the present invention, the microstructure after forging is relatively coarse. The forging is directly put into the post-forging heat treatment furnace without waiting for it to cool to the temperature at which non-equilibrium microstructure transformation occurs, i.e., without bainite transformation. It is held near Ar1 to allow it to undergo partial isothermal transformation, thereby cutting off the microstructure inheritance and preventing large-section forgings with coarse post-forging microstructure from cracking due to inconsistent internal and external microstructure stress and thermal stress.

[0028] d) In the method of the present invention, when the large cross-section forging is air-cooled, a slow cooling cover is put on the position of the smaller cross-section to reduce the stress concentration at the transition position of the cross-section change caused by the difference in cooling rate and the inconsistency of microstructure transformation due to the difference in cross-section during the normalizing and air-cooling process, thereby greatly reducing the risk of cracking.

[0029] e) In the method of the present invention, the normalizing holding temperature decreases in stages, causing the grain size of the forging to gradually decrease, which significantly improves the sensitivity of the forging. The gradual decrease in the low-temperature holding temperature before each normalizing heating is conducive to the full transformation of the core structure of the forging, which significantly promotes the recrystallization and refinement of the forging material and improves the flaw detection sensitivity of the forging.

[0030] f) In the method of the present 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 the subsequent quenching and tempering treatment.

[0031] g) 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 Φ2mm to Φ2.5mm.

[0032] Other features and advantages of the invention will be set forth in the description which follows, and will be apparent in part from the description, or may be learned by practicing the invention. The objects and other advantages of the invention may be realized and obtained by means of what is particularly pointed out in the written description and the accompanying drawings. Attached Figure Description

[0033] The accompanying drawings are for illustrative purposes only and are not intended to limit the invention. Throughout the drawings, the same reference numerals denote the same parts.

[0034] Figure 1 is a schematic diagram of the large rotor forging of the present invention;

[0035] Figure 2 is a schematic diagram of the slow cooling cover of the present invention;

[0036] Figure 3 shows a schematic diagram of the slow-cooling cover forging air cooling;

[0037] Figure 4 shows the grain diagram of the forging billet after step S3 of Example 1;

[0038] Figure 5 shows the grain diagram of the forging billet after step S6 of Example 1;

[0039] Figure 6 shows the grain diagram of the forging billet after step S9 of Example 1.

[0040] The attached diagram is labeled as follows: 1. Shaft body; 2. Journal; 3. Flange; 4. Cooling cover; 41. Liner; 42. Outer skin; 43. Support frame. Detailed Implementation

[0041] The preferred embodiments of the present invention will now be described in detail with reference to the accompanying drawings, which form part of the present invention and, together with the embodiments of the present invention, serve to illustrate the principles of the present invention.

[0042] As shown in Figure 1, the present 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 1000 mm to 1800 mm, such as 1100 mm, 1200 mm, 1300 mm, 1400 mm, 1500 mm, 1600 mm, and 1700 mm. The large cross-sectional difference at the connection position between the shaft body 1 and the journals 2 of the large rotor forging leads to a large 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.

[0043] 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 100mm to 700mm, for example, 100mm, 200mm, 300mm, 400mm, 500mm, 600mm.

[0044] Specifically, the diameter D1 of the shaft body 1 is 1700mm or more, for example, 1700mm to 2500mm, such as 1800mm, 1900mm, 2000mm, 2100mm, 2200mm, 2300mm, 2400mm.

[0045] Specifically, the diameter D2 of journal 2 is 600mm or more, for example, 600mm to 900mm, such as 650mm, 700mm, 750mm, 800mm, 850mm.

[0046] Specifically, the diameter D3 of flange 3 is 700mm or more, for example, 700mm to 1600mm, such as 800mm, 900mm, 1000mm, 1100mm, 1200mm, 1300mm, 1400mm, 1500mm.

[0047] 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 and 25Cr2Ni4MoV steel.

[0048] Specifically, the composition of the aforementioned large rotor forgings 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.

[0049] This invention provides a heat treatment method for preventing cracking of large rotor forgings, comprising a first full austenitizing normalizing, a second full austenitizing normalizing, and a third full austenitizing normalizing performed sequentially, wherein the normalizing holding temperature of the above three normalizing processes decreases sequentially.

[0050] Specifically, the holding temperature for the first complete austenitizing normalizing is AC3+ (90℃ to 120℃), the holding temperature for the second complete austenitizing normalizing is AC3+ (60℃ to 90℃), and the holding temperature for the third complete austenitizing normalizing is AC3+ (50℃ to 70℃).

[0051] Specifically, the holding temperature for the first full austenitizing normalizing is 890℃ to 920℃, for example, 900℃ or 910℃; the holding temperature for the second full austenitizing normalizing is 860℃ to 890℃, for example, 870℃ or 880℃; and the holding temperature for the third full austenitizing normalizing is 850℃ to 870℃, for example, 855℃, 860℃ or 865℃.

[0052] Specifically, the heat treatment method for preventing cracking of the above-mentioned large rotor forgings includes the following steps:

[0053] 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.

[0054] S2. Heat to AC3+ (90℃ to 120℃) at a rate of v1, and maintain the temperature.

[0055] S3, air cooling of forging blank;

[0056] 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°C to 130°C below Ms point.

[0057] S5. Increase the temperature to AC1 at a slow rate of v2 (20°C to AC1);

[0058] S6. Then, heat up to AC3+ (60℃ to 90℃) at a relatively fast rate using v3, hold at that temperature, and then air cool.

[0059] S7. After removing the slow cooling cover, the forging billet is placed into the heat treatment furnace and kept at a low temperature within the range of 120°C to 150°C below Ms point; wherein, the low temperature in S7 is lower than the low temperature in S4.

[0060] S8 is the same as S5;

[0061] S9. Then, heat up to AC3+ (50℃ to 70℃) at a relatively fast rate using v4, keep warm, and then air cool.

[0062] Specifically, V4 > v3 > v1.

[0063] Specifically, the process preceding S1 includes the forging of a large rotor forging blank. This forging process 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.

[0064] Specifically, in S1 above, the large rotor forging blank, after forging, is not air-cooled to room temperature. Instead, it is placed in a heat treatment furnace as quickly as possible after forging and held at that temperature. This allows for a partial transformation of pearlite (20% to 35% by volume) from grain boundaries to the interior of the grains. The mixed structure of isothermal pearlite and untransformed austenite helps prevent the forging blank from cracking.

[0065] Specifically, in S1 above, considering that excessively high or low temperatures would prolong the incubation period for pearlite transformation, thus reducing the pearlite content and refining effect, the holding temperature of S1 is controlled at Ar1±20℃, for example, 610℃ to 640℃, such as 620℃ or 630℃.

[0066] 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 excessively long heat preservation time increases the heat treatment cycle, while excessively short time results in uneven core and surface temperatures and a lower 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.

[0067] Specifically, in S2 above, during the plastic deformation stage, while ensuring controllable instantaneous internal stress, the temperature reaches AC3+ (90℃ to 120℃) relatively quickly, resulting in numerous nucleation points and effectively disrupting the original austenite grains, thus refining the grain size. Therefore, a heating rate v1 of 15℃ / h or higher is adopted. Preferably, v1 is 15℃ / h to 20℃ / h, for example, 16℃ / h, 17℃ / h, 18℃ / h, or 19℃ / h.

[0068] Specifically, in S2 above, the core of the forging is ensured to be fully heated, with consistent internal and external temperatures. During this process, most of the carbides are fully dissolved into the austenite, improving the strength and toughness of the matrix. The holding time is controlled to be (2 to 4)D1 / 100, where D1 is the diameter of the shaft in mm, and the holding time is in hours. Preferably, it is (2.5 to 3.5)D1 / 100h, for example, holding times of 2.5D1 / 100, 2.7D1 / 100, 3D1 / 100, 3.2D1 / 100, and 3.5D1 / 100.

[0069] Specifically, in S3 above, the forging billet is hoisted onto a 1000mm high pad for air cooling.

[0070] Specifically, in S3 above, when the journal area (or shaft diameter and flange if present) is air-cooled to within the range of 500°C to 550°C, a slow cooling cover 4 is placed over the shaft diameter area (or shaft diameter and flange if present) of the forging blank, and the shaft body is air-cooled to 100°C to 150°C below Ms point. It should be noted that the slow cooling cover can reduce the cooling rate of the shaft diameter and flange (e.g., to 5°C / h to 10°C / h), thereby reducing the thermal and structural stresses at the transition point with the large cross-section of the shaft body, and reducing the risk of forging cracking. Simultaneously, it ensures that the forging blank is air-cooled to a lower temperature, allowing for a more complete microstructure transformation at a lower core temperature, thus refining the microstructure and grain size.

[0071] 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.

[0072] Specifically, Figure 3 shows a schematic diagram of the slow-cooling cover forging air cooling.

[0073] 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, values ​​are, for example, 1.3D1 / 100, 1.4D1 / 100, 1.5D1 / 100, or 1.6D1 / 100.

[0074] Specifically, in the above S5 stage, the heating rate is limited. In the elastic deformation zone of the forging, the internal and external temperature difference and instantaneous internal stress in the large rotor forging are minimized to reduce the risk of cracking, while preparing for the microstructure transformation in the two-phase region. Therefore, v2 is controlled to be less than v1. For example, v2 is below 10℃ / h. Preferably, v2 is between 5℃ / h and 10℃ / h, such as 6℃ / h, 7℃ / h, 8℃ / h, or 9℃ / h.

[0075] Specifically, in S5 above, the temperature of AC1-(20°C to AC1) is preferably 680°C to 700°C, for example 680°C, 690°C, or 700°C.

[0076] Specifically, in S6 above, during the plastic deformation stage, AC3+ (60℃ to 90℃) is reached relatively quickly while ensuring controllable instantaneous internal stress. This results in numerous nucleation points, effectively disrupting the original austenite grains and refining the grain size. This stage involves rate-limited heating. Since the forging has undergone normalizing in the plastic deformation zone, the microstructure is relatively fine, and the plasticity and toughness reserves have increased. Therefore, v3 is controlled to be greater than v1. For example, v3 is below 30℃ / h. Preferably, v3 is between 20℃ / h and 30℃ / h, such as 22℃ / h, 24℃ / h, 26℃ / h, or 28℃ / h.

[0077] 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.

[0078] Specifically, in S6 above, the air cooling operation steps are the same as in S3. The difference is that the surface temperature of the shaft is 30°C to 50°C lower than the final temperature in S3, which makes the core structure of the forging more fully transformed.

[0079] Specifically, in S7 above, considering that the air cooling to the shaft temperature is lower than in S3 in S6, the core of the forging will also decrease accordingly. Therefore, the holding time in the low-temperature holding stage is relatively shortened. That is, the low-temperature holding time in S7 is shorter than the low-temperature holding time in S4. For example, the holding time is controlled at (1 to 1.5)D1 / 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.

[0080] Specifically, in S9 above, v4 > v3. Compared to 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, v4 is 30℃ / h to 40℃ / h, for example, 32℃ / h, 34℃ / h, 36℃ / h, or 38℃ / h.

[0081] Specifically, the heat preservation time in S9 is shorter than the heat preservation time in S6.

[0082] Specifically, in S9 above, the temperature is maintained at AC3+ (50℃ to 70℃) to allow the microstructure to fully form equiaxed austenitic small 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 holding time required in S9 is (1.8 to 3.5)D1 / 100, preferably (2 to 3)D1 / 100mm, such as 2.2D1 / 100mm, 2.4D1 / 100mm, 2.6D1 / 100mm, and 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.

[0083] Specifically, in S9 above, the air cooling operation is the same as that in S6, and will not be repeated here.

[0084] Specifically, the heat treatment method for preventing cracking of the aforementioned large rotor forgings also includes a tempering process, which includes the following steps:

[0085] S10 is the same as S7;

[0086] S11. Heat to AC1 (50℃ to 80℃) and maintain the temperature;

[0087] S12. After cooling to 250℃ at a rate of less than 10℃ / h, air cool to room temperature.

[0088] Specifically, in S11, the heating rate is from 10℃ / h to 25℃ / h, for example 13℃ / h, 15℃ / h, 18℃ / h, 20℃ / h, 23℃ / h.

[0089] Specifically, in S11, the temperature range of AC1-(50℃ to 80℃) can be 630℃ to 660℃.

[0090] 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.

[0091] 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, the cooling rate is controlled to below 10℃ / h. For example, 5℃ / h to 10℃ / h, such as 6℃ / h, 7℃ / h, 8℃ / h, or 9℃ / h.

[0092] The present invention also provides a large rotor forging, which is prepared by the above-described heat treatment method.

[0093] 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 Φ2mm to Φ2.5mm.

[0094] Compared with existing technologies, the method of this invention combines three normalizing processes with stepped cooling to achieve a uniform and fine austenitic grain size in large-section forgings. Notably, the first normalizing process fully dissolves most of the carbides in the microstructure into the austenite, improving the strength and toughness of the matrix. The second and third normalizing processes employ stepped cooling to refine the grains and microstructure. Through three consecutive normalizing processes, the crystalline phases of the original coarse austenitic microstructure are completely disrupted, the microstructure inheritance is severed, and uniform and fine grain boundaries, phase boundaries, and dispersed carbides are formed. After tempering, a uniform bainitic tempered microstructure is formed, effectively refining the microstructure and grains of the forging and fully preparing the microstructure for subsequent quenching and tempering treatment.

[0095] In the method of the present invention, during the forging process of the large rotor forging billet, a journal with a small cross section is forged first (if there is a flange, this is the journal and the flange), 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.

[0096] In the method of the present invention, the microstructure after forging is relatively coarse. The forging is directly put into the post-forging heat treatment furnace without waiting for it to cool to the temperature at which non-equilibrium microstructure transformation occurs, i.e., without bainite transformation. It is held at a temperature near Ar1 to allow it to undergo partial isothermal transformation. This cuts off the inheritance of microstructure while preventing large-section forgings with coarse post-forging microstructure from cracking due to inconsistent internal and external microstructure stress and thermal stress.

[0097] In the method of the present invention, when the large cross-section forging is air-cooled, a slow cooling cover is placed on the position of the smaller cross-section to reduce the stress concentration at the transition position of the cross-section change caused by the difference in cooling rate and the inconsistency of microstructure transformation due to the difference in cross-section during the normalizing and air-cooling process, thereby greatly reducing the risk of cracking.

[0098] In the method of this invention, the normalizing holding temperature decreases in stages, causing the grain size of the forging to gradually decrease, which significantly improves the sensitivity of the forging. The gradually decreasing low-temperature holding temperature before each normalizing heating is beneficial to the full transformation of the core structure of the forging, which significantly promotes the recrystallization and refinement of the forging material, and helps to improve the flaw detection sensitivity of the forging.

[0099] In the method of the present 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.

[0100] 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 Φ2mm to Φ2.5mm.

[0101] The advantages of precise control of process parameters of the present invention will be demonstrated below with specific embodiments and comparative examples.

[0102] Example 1

[0103] This embodiment provides a large motor rotor forging and its heat treatment method. The large rotor forging in this embodiment includes a shaft body 1 and journals 2 connected to both sides of the shaft body 1. The cross-sectional diameter of the shaft body is [missing information]. The diameter of journal 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%, with the balance being Fe and unavoidable impurities.

[0104] The heat treatment method for the large rotor forging in this embodiment includes the following steps:

[0105] 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.

[0106] 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.

[0107] S2. Heat to 900℃ at a rate of 15℃ / h and hold for 50h;

[0108] S3. After the forging billet is air-cooled and the journal is air-cooled to 500°C to 550°C, the journal is air-cooled with a slow cooling cover until the surface temperature of the shaft body is 250°C.

[0109] S4. After removing the slow cooling cover, put the forging into the heat treatment furnace and keep it at 200℃ for 25 hours.

[0110] S5. Increase the temperature to 680℃ at a slow rate of 8℃ / h;

[0111] S6. Then heat up to 870℃ 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℃.

[0112] S7. After removing the slow cooling cover, put the forging into the heat treatment furnace and keep it at 180℃ for 22 hours.

[0113] S8 is the same as S5;

[0114] 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°C 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.

[0115] S10 is the same as S7;

[0116] S11. Heat to 640℃ and keep warm for 55 hours;

[0117] S12: Cool down to 250℃ at a rate of 10℃ / h, then air cool to room temperature.

[0118] 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-limiting sensitivity of Φ2.0 mm and a grain size of grade 4 to 5. Furthermore, the forging did not crack during the preparation process in this embodiment.

[0119] Example 2

[0120] 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... The journal diameter is 800mm; the rotor material is 30Cr2Ni4MoV steel, with a chemical composition including 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.

[0121] The heat treatment method for the large rotor forging in this embodiment is largely the same as that in Embodiment 1, including:

[0122] 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.

[0123] 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.

[0124] S2. Heat to 910℃ at a rate of 15℃ / h and hold for 55h;

[0125] S3, Same as in Example 1;

[0126] S4. After removing the slow cooling cover, put the forging into the heat treatment furnace and keep it at 210℃ for 28 hours.

[0127] S5. Increase the temperature to 700℃ at a slow rate of 8℃ / h;

[0128] S6. Then heat up to 870℃ 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℃.

[0129] S7. After removing the slow cooling cover, put the forging into the heat treatment furnace and keep it at 180℃ for 25 hours.

[0130] S8 is the same as S5;

[0131] S9. Then heat up to 850℃ at a relatively fast rate of 31℃ / h, hold for 50h, 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℃.

[0132] S10 is the same as S7;

[0133] S11. Heat to 640℃ and keep warm for 60 hours;

[0134] S12: Same as in Example 1.

[0135] The forging in this embodiment exhibits good levels of flaw detection sensitivity and grain size, with a center-limiting sensitivity of Φ2.0 mm and a grain size of grade 4.5. Furthermore, the forging did not crack during the manufacturing process of this embodiment.

[0136] Example 3

[0137] 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 cross-section of the rotor forging is... The journal size is 800mm; the rotor material is 35Cr2Ni4MoV steel, with a chemical composition including 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.

[0138] The heat treatment method for the large rotor forging in this embodiment is largely the same as that in Embodiment 1, including:

[0139] 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.

[0140] 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 43 hours.

[0141] S2. Heat to 920℃ at a rate of 16℃ / h and hold for 65h;

[0142] S3, Same as in Example 1;

[0143] S4. After removing the slow cooling cover, put the forging into the heat treatment furnace and keep it at 250℃ for 35 hours.

[0144] S5. Increase the temperature to 700℃ at a slow rate of 6℃ / h;

[0145] S6. Then heat up to 880℃ 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℃.

[0146] S7. After removing the slow cooling cover, put the forging into the heat treatment furnace and keep it at 200℃ for 32 hours.

[0147] S8 is the same as S5;

[0148] S9. Then heat up to 850℃ at a relatively fast rate of 33℃ / h, keep it at that temperature for 60h, 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℃.

[0149] S10 is the same as S7;

[0150] S11. Heat to 660℃ and keep warm for 80 hours;

[0151] S12: Cool down to 250℃ at a rate of 8℃ / h, then air cool to room temperature.

[0152] 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 grade 3.5. Furthermore, the forging did not crack during the manufacturing process of this embodiment.

[0153] The inventors conducted extensive research during the research process, and some poorly performing solutions are now presented as comparative examples.

[0154] Comparative Example 1

[0155] 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.

[0156] The preparation method for this comparative example is as follows:

[0157] In S1, the forging billet is cooled in the furnace at a rate of 8°C / h, and then held at 250°C for 20 hours. After holding, the temperature is slowly increased to 610°C at a rate of 5°C / h, and then the process is the same as S2 in Example 1.

[0158] In this comparative example, when the forging was air-cooled at S2 stage, longitudinal cracks were found in the forging when the furnace door was opened.

[0159] Comparative Example 2

[0160] 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.

[0161] The preparation method of this comparative example is as follows: When performing S3 air cooling, a slow cooling cover was not used to protect the shaft diameter, which resulted in a ring crack being found at the transition position between the shaft diameter and the shaft body when the forging was air cooled to about 250°C.

[0162] Comparative Example 3

[0163] 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.

[0164] The preparation method of this comparative example is roughly the same as that of Example 3. The difference is that the three-stage normalizing and holding temperature step-down method was not used. All three normalizing processes were held at 910℃ for 60 hours, which resulted in poor flaw detection sensitivity and grain size of the forging before tempering. The center limit sensitivity was Φ4mm and the grain size was 1.5 grade.

[0165] Comparative Example 4

[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 3, and will not be repeated here.

[0167] In the preparation method of this comparative example, in S0, the shaft is first forged according to a forging ratio of 1.27, 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.

[0168] The remaining steps are largely the same as in Example 3.

[0169] After heat treatment following forging, the large rotor forgings in this comparative example exhibited coarse grains of grade 2.0 to 2.5, and the flaw detection sensitivity reached Φ4mm, showing grass-like waves, making it impossible to proceed with the next heat treatment step.

[0170] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in the present invention should be included within the scope of protection of the present invention.

Claims

1. A heat treatment method for preventing cracking in large rotor forgings, characterized in that, The heat treatment method includes: a first full austenitizing normalizing, a second full austenitizing normalizing, and a third full austenitizing normalizing performed sequentially, with the normalizing holding temperature decreasing sequentially for each of the three normalizing processes.

2. The heat treatment method according to claim 1, characterized in that, The holding temperature for the first complete austenitizing normalizing is AC3+ (90℃ to 120℃), the holding temperature for the second complete austenitizing normalizing is AC3+ (60℃ to 90℃), and the holding temperature for the third complete austenitizing normalizing is AC3+ (50℃ to 70℃).

3. The heat treatment method 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 AC3+ (90℃ to 120℃) 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 the range of 100°C to 130°C below Ms point. S5. Increase the temperature to AC1 at a slow rate of v2 (20°C to AC1); S6. Then, heat up to AC3+ (60℃ to 90℃) at a relatively fast rate using v3, hold at that temperature, and then air cool. S7. The forging billet is placed into a heat treatment furnace and kept at a low temperature within the range of 120°C to 150°C below Ms point; wherein the low temperature in S7 is lower than the low temperature in S4. S8 is the same as S5; S9. Then, heat the temperature to AC3+ (50℃ to 70℃) at a relatively fast rate using v4, hold the temperature, and then air cool; after air cooling, continue tempering.

4. The heat treatment method according to claim 3, characterized in that, V4 > v3 > v1.

5. The heat treatment method according to claim 3, characterized in that, The low-temperature insulation time in S7 is less than the low-temperature insulation time in S4.

6. The heat treatment method according to claim 3, characterized in that, In S3, when the journal area is air-cooled to a temperature range of 500°C to 550°C, a slow cooling cover is placed over the shaft diameter area of ​​the forging blank, and the shaft body is air-cooled to a temperature 100°C to 150°C below Ms point.

7. The heat treatment method according to claim 3, characterized in that, The heat preservation time in S9 is less than the heat preservation time in S6.

8. The heat treatment method according to claim 3, characterized in that, v2 < v1.

9. The heat treatment method according to any one of claims 3 to 8, characterized in that, In S2, the heat preservation 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, and the unit of heat preservation time is h.

10. A large rotor forging, characterized in that, The large rotor forging is prepared by the method described in any one of claims 1 to 9.

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