Preparation method of low-energy-consumption large rotor forge piece and large rotor forge piece

Abstract

The invention discloses a preparation method of a low-energy-consumption large rotor forge piece and the large rotor forge piece, belongs to the technical field of rotor preparation, and solves the problems that an existing post-forging heat treatment method of the large rotor forge piece is long in production period and high in energy consumption, and the forge piece is prone to cracking. The preparation method comprises the steps that a heat treatment furnace is heated to Ar1 + / -20 DEG C, and a forged forging stock is put into the heat treatment furnace for heat preservation; raising the temperature to AC3 + (80-120 DEG C), and keeping the temperature; performing air cooling on the forging stock; the forging stock is put into a heat treatment furnace, and heat preservation is conducted within the temperature of 100-150 DEG C lower than the Ms point; raising the temperature to T1, keeping the temperature, and then air-cooling; wherein T1 is more than or equal to AC1-30 DEG C and less than AC1; the forging stock is subjected to heat preservation at the temperature of 100-150 DEG C lower than the Ms point; raising the temperature to AC1-20 DEG C to AC1; and the temperature is raised to AC3 + (50-80 DEG C), heat preservation is conducted, and then air cooling is conducted. The method is low in energy consumption, the forge piece can be prevented from cracking, grain refinement can be achieved, and tissue preparation can be fully made for subsequent thermal refining.

Description

Technical Field

[0001] The present invention relates to the technical field of rotor preparation, and particularly to a preparation method and a large rotor forging of a large rotor forging with low energy consumption. Background Art

[0002] As a key core component of a steam turbine generator set, with the development trend of increasing parameters and capacity of the steam turbine unit, the size of the core component rotor forging is also continuously increasing. At present, the proportion of 600MW - 1000MW units has an absolute advantage in steam turbine power generation, and the shaft body cross-section of its rotor forging is between φ1000mm and φ2200mm. Due to the increase in the shaft body size of the forging, the heat treatment cycle for refining the forging after heat treatment to ensure the UT flaw detection sensitivity before quenching and tempering is relatively long, bringing greater pressure to production.

[0003] After decades of development, the post-forging heat treatment method for rotors made of 30Cr2Ni4MoV and 25Cr2Ni4MoV materials has always followed the traditional process method: three normalizations + one tempering. The cycle of the above process is generally about 50 days, and repeated high-temperature heat treatment results in high fuel costs, long production cycles, and low efficiency. Summary of the Invention

[0004] In view of the above situation, the present invention aims to provide a preparation method and a large rotor forging of a large rotor forging with low energy consumption, which can at least solve one of the following technical problems: the existing post-forging heat treatment method for large rotor forgings has a long production cycle, high energy consumption, and the forgings are prone to cracking.

[0005] The object of the present invention is mainly achieved through the following technical solutions:

[0006] The present invention provides a preparation method for a large rotor forging with low energy consumption, and the preparation method includes the following steps:

[0007] S1. Heat the heat treatment furnace to Ar1 ± 20°C, load the forged large rotor forging blank into the heat treatment furnace, and keep it warm;

[0008] S2. Heat it to AC3 + (80 - 120°C) at a speed of v1, and keep it warm;

[0009] S3. Air-cool the forging blank;

[0010] S4. Load the forging blank into the heat treatment furnace and perform low-temperature heat preservation in the range of 100 - 150°C below the Ms point;

[0011] S5. Heat it to T1 at a speed of v2, where AC1 - 30°C ≤ T1 < AC1;

[0012] S6. Keep it warm at T1, and then air-cool it;

[0013] S7. Load the forging blank into the heat treatment furnace and perform low-temperature heat preservation in the range of 100 - 150 °C below the Ms point.

[0014] S8. Heat up to AC1 - 20 °C to AC1 at a speed of v3.

[0015] S9. Then heat up to AC3 + (50 - 80 °C) at a speed of v4, hold for heat preservation, and then air cool; after air cooling, perform secondary tempering.

[0016] Furthermore, the low-temperature heat preservation time in S7 is less than that in S4.

[0017] Furthermore, in S3, when air cooling to 500 - 550 °C of the journal, cover the journal of the forging blank with a slow cooling cover and continue air cooling.

[0018] Furthermore, in S9, the air cooling step is the same as that in S3.

[0019] Furthermore, v1 > v2, v4 > v3.

[0020] Furthermore, v3 is 5 - 10 °C / h, and v4 is below 50 °C / h.

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

[0022] Furthermore, in S4, the heat preservation time is not less than 0.85D1 / 100, where D1 is the diameter of the shaft body, the unit of D1 is mm, and the unit of the heat preservation time is h.

[0023] Furthermore, before S1, it includes the forging of a large rotor forging blank, and the forging of the large rotor forging blank includes: first forging the journal area with a small cross-section, and then forging with a large forging ratio to deform the shaft body with a large cross-section.

[0024] The present invention also provides a large rotor forging prepared by the above method.

[0025] Compared with the prior art, the present invention can at least achieve one of the following beneficial effects:

[0026] a) In the preparation method of the large rotor forging with low energy consumption of the present invention, one tempering is adopted between two normalizations. By precisely controlling process parameters such as the temperature of different steps, the heating rate at different stages, the heat preservation temperature, the heat preservation time, and the cooling rate, etc.; it avoids the cracking of the forging and realizes the refinement of grains, and can fully prepare the microstructure for the subsequent quenching and tempering treatment.

[0027] b) In the method of the present invention, during the forging process of the large rotor forging blank, first forge the journal area with a small cross-section (if there is a flange, the journal area here includes the journal and the flange), and then deform the shaft body by large forging ratio. There is a large deformation in the shaft body, forming a large driving force for austenite nucleation, which is beneficial to subsequent static recrystallization and the refinement of the grains and structure of the forging.

[0028] c) In the method of the present invention, when the forging is normalized and air-cooled, a slow-cooling cover is put on the position with a small cross-section to reduce the stress concentration at the transition position of the cross-section change caused by the inconsistent cooling rate and phase transformation due to the cross-section difference during the normalized air-cooling process. While greatly reducing the cracking risk, the temperature of the forging can be lowered below Ms for a long time, making the phase transformation more sufficient and beneficial to the refinement of the forging grains.

[0029] d) In the method of the present invention, two normalizations are adopted. Compared with the existing process of three normalizations, the time is greatly reduced, the energy consumption cost is lowered, and the production efficiency is improved.

[0030] e) The structure of the large rotor forging of the present invention is a bainite tempered structure, with fine and uniform grains. For example, the grain size is grade 4 - 5. The flaw detection sensitivity of the above large rotor forging reaches a good level. For example, the central limit sensitivity is Φ1.6 - 2.5 mm.

[0031] Other features and advantages of the present invention will be described in the subsequent specification, and some of them will become obvious from the specification or be understood by implementing the present invention. The objectives and other advantages of the present invention can be achieved and obtained through the content specifically pointed out in the written specification and the drawings. Brief Description of the Drawings

[0032] The drawings are only for the purpose of showing specific embodiments and are not considered as a limitation to the present invention. Throughout the drawings, the same reference signs represent the same components.

[0033] Figure 1 is a schematic diagram of the large rotor forging of the present invention;

[0034] Figure 2 is a schematic diagram of the slow-cooling cover of the present invention;

[0035] Figure 3 is a schematic diagram of the slow-cooling cover sleeved on the forging for air-cooling;

[0036] Figure 4 is a grain diagram of the forging blank after the treatment of step S3 in Example 1;

[0037] Figure 5 is a grain diagram of the forging blank after the treatment of step S6 in Example 1;

[0038] Figure 6The grain diagram of the forged blank after the treatment of step S9 in Embodiment 1.

[0039] Reference numerals

[0040] 1 - shaft body, 2 - journal, 3 - flange, 4 - slow cooling cover, 41 - inner lining, 42 - outer skin, 43 - support frame. Detailed implementation manners

[0041] The preferred embodiments of the present invention will be specifically described below with reference to the accompanying drawings, in which the accompanying drawings form a part of the present invention and are used together with the embodiments of the present invention to explain the principle of the present invention.

[0042] As shown in the attached Figure 1 drawing, the present invention provides a large - scale rotor forging. The large - scale rotor forging includes a shaft body 1, journals 2 connected to both sides of the shaft body 1. The diameter D1 of the shaft body 1 and the maximum diameter D2 of the journals 2 satisfy the following relationship: D1 > D2, and the difference between D1 and D2 is more than 600 mm. For example, the difference between D1 and D2 is 600 - 1200 mm, such as 700 mm, 800 mm, 900 mm, 1000 mm, 1100 mm. The cross - section difference at the connection position between the shaft body 1 and the journals 2 of the large - scale rotor forging is large, resulting in a large temperature difference between the inside and outside during the heat treatment process of the forging, an increase in tissue stress and thermal stress. In addition, the as - forged grain size and structure after forging are coarse, and the strength and toughness reserve are insufficient, so the forging is prone to cracking during the post - forging heat treatment process after forging.

[0043] Specifically, the large - scale rotor forging may further 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 satisfy the following relationship: D3 > D2, and the difference between D3 and D2 is 50 - 700 mm, such as 100 mm, 200 mm, 300 mm, 400 mm, 500 mm, 600 mm.

[0044] Specifically, the diameter D1 of the shaft body 1 is 1000 mm or more, such as 1000 - 2200 mm, such as 1200 mm, 1400 mm, 1600 mm, 1800 mm, 2000 mm.

[0045] Specifically, D2 is 600 mm or more, such as 600 - 900 mm, such as 650 mm, 700 mm, 750 mm, 800 mm, 850 mm.

[0046] Specifically, the diameter D3 of the flange 3 is 650 mm or more, such as 650 - 1400 mm, such as 800 mm, 900 mm, 1000 mm, 1100 mm, 1200 mm, 1300 mm.

[0047] Specifically, the large rotor forging of the present invention is made of C-Cr-Ni-Mo-V steel with low carbon and tissue heredity characteristics, such as 25Cr2Ni4MoV and 30Cr2Ni4MoV.

[0048] Specifically, the components of 25Cr2Ni4MoV by mass percentage include C: ≤0.28%, Si: ≤0.1%, Mn: 0.15% - 0.40%, Cr: 1.35% - 2.00%, Mo: 0.22% - 0.62%, Ni: 3.25% - 3.75%, V: 0.06% - 0.16%, Al: ≤0.01%, S: ≤0.01%, P: ≤0.01%, and the balance is Fe and inevitable impurities.

[0049] Specifically, the components of 30Cr2Ni4MoV by mass percentage include C: ≤0.32%, Si: ≤0.1%, Mn: 0.15% - 0.35%, Cr: 1.45% - 2.00%, Mo: 0.22% - 0.62%, Ni: 3.25% - 4.25%, V: 0.06% - 0.16%, Al: ≤0.01%, S: ≤0.01%, P: ≤0.01%, and the balance is Fe and inevitable impurities.

[0050] The present invention provides a method for preparing a large rotor forging with low energy consumption, including the following steps:

[0051] S1. Heat the heat treatment furnace to Ar1 ± 20°C, and quickly load the large rotor forging blank after forging into the heat treatment furnace for heat preservation.

[0052] S2. Heat at a rate v1 of more than 15°C / h to AC3 + (80 - 120°C) for heat preservation.

[0053] S3. Air-cool the forging blank.

[0054] S4. After removing the slow-cooling cover, load the forging blank into the heat treatment furnace and perform low-temperature heat preservation in the range of 100 - 150°C below the Ms point.

[0055] S5. Heat at a slow rate v2 to T1, where AC1 - 30°C ≤ T1 < AC1 (this type of range can be abbreviated as AC1 - 30°C to <AC1 in the following text).

[0056] S6. Insulate at T1 and then air-cool.

[0057] S7. Load the forging blank into the heat treatment furnace and perform low-temperature heat preservation in the range of 100 - 150°C below the Ms point.

[0058] S8. Heat at a slow rate v3 to AC1 - 20°C to AC1.

[0059] S9. Then, heat it up to AC3+(50 - 80°C) at a relatively fast speed v4, hold for insulation, and then air cool.

[0060] Specifically, the above S1 - S3 is the first normalizing stage, S4 - S6 is the first tempering stage, and S7 - S9 is the second normalizing stage. Specifically, the holding temperature in S2 is higher than that in S9. For example, the holding temperature in S2 is 880 - 910°C, such as 890°C, 900°C; the holding temperature in S9 is 840 - 870°C, such as 850°C, 860°C, 870°C.

[0061] Specifically, before the above S1, it includes the forging of a large - scale rotor forging blank. The forging of the large - scale rotor forging blank includes: first forging the journal area with a small cross - section (if there is a flange, the journal area here includes the journal and the flange), and then forging and deforming the shaft body with a large forging ratio. Specifically, the range of the large forging ratio is 1.2 - 1.3; by making the shaft body have a large deformation, a large driving force for austenite nucleation is formed, which is beneficial to subsequent static recrystallization and beneficial to the refinement of the grains and structure of the forging.

[0062] Specifically, in the above S1, the large - scale rotor forging blank after forging is not air - cooled to room temperature. After forging, it is loaded into the heat treatment furnace as quickly as possible for insulation, and partial pearlite transformation from grain boundary to grain interior distribution (volume percentage 20% - 35%) can occur. The mixed structure of pearlite isothermal structure + untransformed austenite can prevent the forging blank from cracking.

[0063] Specifically, in the above S1, considering that if the temperature is too high or too low, the incubation period of pearlite transformation is too long, which will reduce the pearlite content and the refinement effect. Therefore, control the holding temperature of S1 to be Ar1±20°C. Specifically, Ar1±20°C can be 590 - 630°C, such as 600°C, 610°C, 620°C.

[0064] Specifically, in the above S1, the function of insulation is to ensure that the internal and external temperatures of the forging blank are consistent, reduce the temperature difference during the heating process of the forging blank, and reduce the thermal stress. Considering that too long insulation time will increase the heat treatment cycle, and too short insulation time will result in the core and surface temperatures not reaching uniform consistency and less pearlite structure content. Therefore, control the insulation time of S1 to be not less than D1 / 100, where D1 is the diameter of the shaft body, the unit of D1 is mm, and the unit of the insulation time is h. Preferably, the insulation time is (1.4 - 2)D1 / 100.

[0065] Specifically, in the above-mentioned S2, during the plastic deformation stage, while ensuring that the instantaneous internal stress is controllable, it reaches AC3+(80-120°C) relatively quickly, with many nucleation sites, fully disrupting the original austenite grains and achieving the effect of grain refinement. Therefore, the heating rate v1 is above 15°C / h. Preferably, v1 is 15-20°C / h, such as 16°C / h, 17°C / h, 18°C / h, 19°C / h.

[0066] Specifically, in the above-mentioned S2, in order to ensure that the core of the forging is thoroughly heated and the internal and external temperatures are consistent, 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 (2-4)D1 / 100, where D1 is the diameter of the shaft body, the unit of D1 is mm, and the unit of the holding time is h. Preferably, the holding time is (2.5-3.5)D1 / 100 h, such as the holding time being 2.5D1 / 100, 2.7D1 / 100, 3D1 / 100, 3.2D1 / 100, 3.5D1 / 100.

[0067] Specifically, in the above-mentioned S3, the forging blank is lifted onto a bolster 1000 mm high for air cooling.

[0068] Specifically, in the above-mentioned S3, when air cooling to a range of 500-550°C for the journal (if there is a flange, it is the journal and the flange here), the journal (if there is a flange, it is the journal and the flange here) of the forging blank is sleeved with a slow cooling cover 4, and air cooled until the shaft body is 100-150°C below the Ms point. It should be noted that the slow cooling cover can reduce the cooling rate of the journal and the flange (for example, the cooling rate reaches 5-10°C / h), thereby achieving the effect of reducing the thermal stress and tissue stress at the transition between the journal and the large cross-section of the shaft body and reducing the cracking risk of the forging. At the same time, it can ensure that the forging blank is air cooled to a lower temperature, enabling the core temperature of the forging blank to be lower and more sufficient tissue transformation to occur, achieving the effect of refining the tissue and grains.

[0069] Specifically, in the above-mentioned S3, the structure of the slow cooling cover 4 is as Figure 2 shown. The inner diameter of the slow cooling cover 4 is equivalent to the diameter of the shaft body. The slow cooling cover 4 includes a lining 41 and an outer skin 42. The lining 41 is an asbestos felt with good heat insulation effect, and the outer skin 42 is an iron sheet for support; the slow cooling cover 4 also includes a support frame 43 with adjustable height, and the support frame 43 is connected to the outer skin 42 and can adjust the position of the slow cooling cover 4.

[0070] Specifically, as Figure 3 shown is a schematic diagram of the slow cooling cover sleeving the forging blank for air cooling.

[0071] Specifically, in the above S4, when holding at a temperature below the Ms point for a long time, the microstructure transformation in the core of the forging blank is fully promoted as much as possible, which is beneficial to the subsequent tempering process to transform into a uniform tempered microstructure. Therefore, the holding time is controlled to be not less than 0.85D1 / 100, where D1 is the diameter of the shaft body, the unit of D1 is mm, and the unit of the holding time is h. Preferably, the holding time is (1.2 - 1.6)D1 / 100.

[0072] Specifically, in the above S5, this stage is a speed-limiting heating stage. 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 as much as possible to reduce the cracking risk. At the same time, it is ensured that the forging enters the tempering holding stage as uniformly as possible inside and outside, which is beneficial to the uniform microstructure. Therefore, v2 < v1 is controlled. For example, v2 is below 10 °C / h. Preferably, v2 is 5 - 10 °C / h, such as 6 °C / h, 7 °C / h, 8 °C / h, 9 °C / h.

[0073] Specifically, in the above S5, the temperature range of AC1 - 30 °C to < AC1 can be 670 - 700 °C, such as 680 °C, 685 °C, 690 °C, 695 °C.

[0074] Specifically, in the above S6, considering that the cross-section of the large forging is relatively large and there is a certain temperature difference between the inside and outside of the forging during the heating process. In addition, in this tempering holding stage, the forging needs to be fully tempered inside and outside to temper the bainite structure transformed from austenite into tempered bainite, and to fully decompose the retained austenite that did not transform in S3. This disrupts the orientation relationship between the austenite after re-nucleation and the original austenite and cuts off the microstructure heredity. Therefore, the holding temperature is the tempering temperature as high as possible before the phase transformation, AC1 - 30 °C to < AC1, and the holding time is controlled to be (2 - 4)D1 / 100, where D1 is the diameter of the shaft body, the unit of D1 is mm, and the unit of the holding time is h.

[0075] Specifically, for the air cooling in the above S6, considering that the tempered microstructure will not undergo microstructure transformation during the cooling process and there is no microstructure stress, and the tempering temperature is relatively low and the thermal stress is relatively small. At the same time, the microstructure after tempering has strong strength and toughness. Therefore, after being taken out of the furnace, the forging is lifted to a 1000 - mm - high bolster and directly air - cooled until the temperature of the shaft body is ≤ 150 °C. The purpose of this stage is to air - cool the microstructure that has been fully recovered in the holding stage to form a stable tempered microstructure, retaining more nucleation sites, which is beneficial to increasing the nucleation sites in the subsequent process and achieving the purpose of effectively refining the grains.

[0076] Specifically, in the above-mentioned S7, considering that the temperature of the shaft body after air cooling in S6 is relatively low and it is a tempered structure, and the microstructure transformation of the forging core has been completely completed, there is no need for further microstructure transformation during the S7 process. S7 only serves to ensure uniform internal and external temperature differences. Therefore, the holding time in the low-temperature holding stage is relatively shorter than that in S4, shortened to not less than 0.6D1 / 100 mm. Preferably, the holding time is 0.9 - 1.3D1 / 100 mm, such as D1 / 100 mm, 1.1D1 / 100 mm, 1.2D1 / 100 mm. D1 is the diameter of the shaft body, the unit of D1 is mm, and the unit of the holding time is h.

[0077] Specifically, in the above-mentioned S8, this stage is a speed-limiting heating stage. In the elastic deformation zone of the forging, the internal and external temperature differences and instantaneous internal stresses in the large rotor forging are minimized to reduce the cracking risk. At the same time, it is ensured that the forging enters the tempering holding stage as uniformly as possible inside and outside, which is beneficial to uniform microstructure. Therefore, v3 < v1 is controlled. For example, v3 is below 10 °C / h. Preferably, v3 is 5 - 10 °C / h, such as 6 °C / h, 7 °C / h, 8 °C / h, 9 °C / h.

[0078] Specifically, in the above-mentioned S8, the temperature range of AC1 - 20 °C to AC1 can be 680 - 700 °C, such as 685 °C, 690 °C, 695 °C.

[0079] Specifically, in the above-mentioned S9, v4 > v3. Compared with the first normalizing + the first tempering, the plastic and toughness reserves of the forging are very sufficient at this time, and the anti-cracking ability is stronger. Further increasing the heating rate in the two-phase region can effectively refine the grains and microstructure. Therefore, v4 > v3 is controlled. For example, v4 is below 50 °C / h. Preferably, v4 is 35 - 45 °C / h, such as 37 °C / h, 39 °C / h, 41 °C / h, 43 °C / h.

[0080] Specifically, in the above-mentioned S9, it is held at AC3 + (50 - 80 °C) to completely form equiaxed austenite grains in the microstructure. Since it has been normalized once + tempered once before, the austenite recrystallized this time has no orientation relationship of microstructure inheritance with the original austenite, and the grains can be fully and uniformly refined at this stage. Also, since this temperature range is not the temperature range for grain growth, sufficient holding can make the internal and external microstructure and grain size of the forging more uniform, which is beneficial to improving the flaw detection sensitivity. Therefore, the holding time required in this process is (2.5 - 4)D1 / 100, preferably 2.5 - 3.5D1 / 100 mm, such as 2.7D1 / 100 mm, 2.9D1 / 100 mm, 3.1D1 / 100 mm, 3.3D1 / 100 mm. D1 is the diameter of the shaft body, the unit of D1 is mm, and the unit of the holding time is h.

[0081] Specifically, in the above S9, the air cooling operation is the same as that in S3. The difference is that the air cooling is carried out until the temperature is ≤200°C here, which will not be elaborated further.

[0082] Specifically, the heat treatment method of the above large rotor forging further includes a second tempering process, and the tempering process includes the following steps:

[0083] S10: The same as S4;

[0084] S11: Heat up to AC1-(50-80°C) at a speed of v5 and hold for heat preservation;

[0085] S12: After cooling down to below 250°C at a cooling rate of less than 10°C / h, air cool to room temperature.

[0086] Specifically, in S11, v5 is 10-25°C / h, such as 13°C / h, 15°C / h, 18°C / h, 20°C / h, 23°C / h.

[0087] Specifically, in the above S11, the heat preservation time is (2.5-4)D1 / 100, where D1 is the diameter of the shaft body, the unit of D1 is mm, and the unit of the heat preservation time is h. This step forms a uniform bainite tempered structure through sufficient tempering, effectively refines the structure and grains of the forging, and can fully prepare the structure for the subsequent process.

[0088] Specifically, in S12, considering that too large a cooling rate will cause excessive thermal stress at different cross-sections, resulting in deformation of the forging, and in addition, too high a cooling rate will make the hardness of the forging with forging black skin too high, causing the risk of difficult processing. Therefore, the cooling rate is controlled below 10°C / h. For example, 5-10°C / h.

[0089] The present invention also provides a large rotor forging prepared by the above method.

[0090] Specifically, the structure of the above large rotor forging is a bainite tempered structure, with fine and uniform grains. For example, the grain size is 4-5 levels. The flaw detection sensitivity of the above large rotor forging reaches a good level. For example, the central limit sensitivity is Φ1.6-2.5mm.

[0091] Compared with the prior art, in the method for preparing a low-energy-consumption large rotor forging of the present invention, one tempering is carried out between two normalizations. By precisely controlling process parameters such as the temperature of different steps, the heating rate at different stages, the heat preservation temperature, the heat preservation time, and the cooling rate, etc.; the cracking of the forging is avoided, and the grain refinement is achieved, and the structure can be fully prepared for the subsequent quenching and tempering treatment.

[0092] In the method of the present invention, during the forging process of a large rotor forging blank, first forge a journal with a small cross-section (if there is a flange, it is the journal and the flange here), and then deform the shaft body by large reduction forging. There is a large deformation in the shaft body, forming a large driving force for austenite nucleation, which is beneficial to subsequent static recrystallization and the refinement of the grains and structure of the forging.

[0093] In the method of the present invention, when the forging is normalized and air-cooled, a slow-cooling cover is put on the position with a small cross-section to reduce the stress concentration at the transition position of the cross-section change caused by inconsistent cooling rates and tissue transformation due to the cross-section difference during the normalized air-cooling process. While greatly reducing the cracking risk, the temperature of the forging can be lowered below Ms for a long time, making the tissue transformation more sufficient and beneficial to the refinement of the forging grains.

[0094] In the method of the present invention, two normalizations are adopted. Compared with the existing process of three normalizations, the time is greatly reduced, the energy consumption cost is reduced, and the production efficiency is improved.

[0095] The structure of the large rotor forging of the present invention is a bainite tempered structure, with fine and uniform grains. For example, the grain size is 4-5 levels. The flaw detection sensitivity of the above large rotor forging reaches a good level. For example, the central limit sensitivity is Φ1.6-2.5mm.

[0096] The following shows the advantages of precise control of the process parameters of the present invention with specific examples and comparative examples.

[0097] Example 1

[0098] This example provides a large motor rotor forging and its preparation method. The large rotor forging of this example 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 1200mm, and the maximum cross-sectional diameter of the journal 2 is 600mm; the material of the rotor is 25Cr2Ni4MoV steel, and the composition is 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 is Fe and unavoidable impurities.

[0099] The preparation method of the large rotor forging of this example includes the following steps:

[0100] S0. First forge a journal with a small cross-section and then deform the shaft body by large reduction forging. The value of the large reduction ratio is 1.25;

[0101] S1. Heat the heat treatment furnace to 610°C, and quickly load the large rotor forging blank after forging into the heat treatment furnace and keep it warm for 18h;

[0102] S2. Heat up to 900 °C at a rate of 18 °C / h and hold for 30 h;

[0103] S3. Air-cool the forging blank. After the journal is air-cooled to 530 °C, add a slow-cooling cover to the journal and continue air-cooling until the surface temperature of the shaft body reaches 250 °C;

[0104] S4. After removing the slow-cooling cover, load the forging into the heat treatment furnace and hold at 200 °C for 15 h;

[0105] S5. Heat up to 690 °C at a slow rate of 10 °C / h and hold for 40 h;

[0106] S6. Then air-cool to the surface temperature of the shaft body being 150 °C;

[0107] S7. Load the forging into the heat treatment furnace and hold at 220 °C for 12 h;

[0108] S8. Heat up to 700 °C at a slow rate of 10 °C / h;

[0109] S9. Then heat up to 860 °C at a faster rate of 38 °C / h, hold for 40 h, and then air-cool. After the journal is air-cooled to 530 °C, add a slow-cooling cover to the journal and continue air-cooling until the surface temperature of the shaft body reaches 200 °C;

[0110] S10. The same as S4;

[0111] S11. Heat up the forging to 640 °C at a faster rate of 25 °C / h and hold for 40 h;

[0112] S12: Cool down at a rate of 10 °C / h to 250 °C and then air-cool to room temperature.

[0113] The grains of the forging blank after the treatment in step S3 of this embodiment are as Figure 4 shown, the grains of the forging blank after the treatment in step S6 are as Figure 5 shown, the grains of the forging blank after the treatment in step S9 are as Figure 6 shown. The flaw detection sensitivity and grain size of the forging in this embodiment have reached a good level. The central limit sensitivity is Φ1.6 mm and the grain size is grade 5. And the forging did not crack during the preparation process of this embodiment.

[0114] Example 2

[0115] This embodiment is basically the same as Embodiment 1, except that the cross-sectional diameter of the shaft body is φ1700 mm and the cross-sectional diameter of journal 2 is 800 mm; The preparation method of the large rotor forging in this embodiment includes the following steps:

[0116] S0. First forge the journal with a small cross-section and then deform the shaft body by forging with a large forging ratio. The value of the large forging ratio is 1.26;

[0117] S1. Heat the heat treatment furnace to 610 °C, and quickly load the large rotor forging blank after forging into the heat treatment furnace, and keep it warm for 25 h;

[0118] S2. Heat it up to 900 °C at a rate of 15 °C / h and keep it warm for 43 h;

[0119] S3. Air-cool the forging blank. After the journal is air-cooled to 520 °C, add a slow-cooling cover to the journal and continue air-cooling until the surface temperature of the shaft body is 250 °C;

[0120] S4. After removing the slow-cooling cover, load the forging into the heat treatment furnace and keep it warm at 200 °C for 22 h;

[0121] S5. Heat it up to 690 °C at a slow rate of 8 °C / h and keep it warm for 50 h;

[0122] S6. Then air-cool it until the surface temperature of the shaft body is 150 °C;

[0123] S7. Load the forging into the heat treatment furnace and keep it warm at 220 °C for 20 h;

[0124] S8. Heat it up to 700 °C at a slow rate of 8 °C / h;

[0125] S9. Then heat it up to 860 °C at a faster rate of 32 °C / h, keep it warm for 50 h, and then air-cool it. After the journal is air-cooled to 520 °C, add a slow-cooling cover to the journal and continue air-cooling until the surface temperature of the shaft body is 200 °C;

[0126] S10. The same as S4;

[0127] S11. Heat the forging to 640 °C at a faster rate of 22 °C / h and keep it warm for 50 h;

[0128] S12: Cool it at a rate of 8 °C / h until it reaches 250 °C, and then air-cool it to room temperature.

[0129] The flaw detection sensitivity and grain size of the forging in this embodiment have reached a good level. The center limit sensitivity is Φ2.2 mm and the grain size is grade 4. And the forging did not crack during the preparation process of this embodiment.

[0130] Example 3

[0131] 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 Example 2, except that the material of the rotor forging 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%.

[0132] The flaw detection sensitivity and grain size of the forgings in this embodiment have reached a good level. The central limit sensitivity is Φ2.3mm, and the grain size is grade 4. And during the preparation process of this embodiment, the forgings did not crack.

[0133] During the research process, the inventors conducted a large number of studies and now use some solutions with poor performance as comparative examples.

[0134] Comparative Example 1

[0135] This comparative example provides a large rotor forging and its heat treatment method. The components and shape of the large rotor forging in this comparative example are the same as those in Example 1 and will not be elaborated here.

[0136] The preparation method of this comparative example is roughly the same as that of Example 1, with the difference being:

[0137] The holding temperature of S5 is 640°C.

[0138] After the post-forging heat treatment of the large rotor forging in this comparative example, serious mixed grains appeared in the grain size: 25% of grade 1.0, 30% of grade 2.5, and 45% of grade 4. And the flaw detection sensitivity reached Φ4.5mm, showing a grassy wave, and it was impossible to carry out the subsequent quenching and tempering.

[0139] Comparative Example 2

[0140] This comparative example provides a large rotor forging and its heat treatment method. The components and shape of the large rotor forging in this comparative example are the same as those in Example 2 and will not be elaborated here.

[0141] The preparation method of this comparative example is roughly the same as that of Example 2, with the difference being:

[0142] No slow cooling cover was added in S3. When air-cooled to 250°C of the shaft body, it was found that the shaft body and shaft neck of the forging cracked longitudinally as a whole, resulting in the scrapping of the forging and it was impossible to carry out subsequent heat treatment.

[0143] Comparative Example 3

[0144] This comparative example provides a large rotor forging and its heat treatment method. The components and shape of the large rotor forging in this comparative example are the same as those in Example 3 and will not be elaborated here.

[0145] The preparation method of this comparative example is roughly the same as that of Example 3, with the difference being:

[0146] The heating rate of S2 is 3°C / h, and the heating rate of S9 is 10°C / h.

[0147] After the post-forging heat treatment of the large rotor forging in this comparative example, the grain size shows coarse grains of grade 2.0 - 2.5, and the flaw detection sensitivity reaches Φ3.5mm, with grass-shaped waves appearing, making it impossible to carry out the subsequent quenching and tempering.

[0148] Comparative Example 4

[0149] 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 elaborated here.

[0150] In the preparation method of this comparative example, in S0, first forge the shaft body without forging the shaft neck according to a forging ratio of 1.25. The temperature of the forging has dropped below the process range and cannot be forged, so the forging is returned to the heating furnace for forging heating. After the heating is completed, forge the shaft neck without deforming the shaft body, and obtain the finished product.

[0151] The remaining steps are substantially the same as those in Example 2.

[0152] After the post-forging heat treatment of the large rotor forging in this comparative example, the grain size shows coarse grains of grade 2.0 - 2.5, and the flaw detection sensitivity reaches Φ4mm, with grass-shaped waves appearing, making it impossible to carry out the subsequent quenching and tempering.

[0153] The above is only the preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by those skilled in the art within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing a large rotor forging with low energy consumption, characterized in that: The preparation method comprises the following steps: S1. Heat the heat treatment furnace to Ar1±20℃, load the forged large rotor forging blank into the heat treatment furnace and keep it warm; S2, heating to AC3+ (80-120°C) at speed v1, keeping warm; S3, air cooling of forging blank; S4, loading the forging billet into a heat treatment furnace and performing low temperature insulation in the range of 100 to 150°C below the Ms point; S5, heating to T1 at speed v2, where AC1-30℃≤T1<AC1; S6, keep warm at T1, then air cool; S7, loading the forging billet into a heat treatment furnace, and performing low temperature insulation in the range of 100 to 150°C below the Ms point; S8, heating at speed v3 to AC1-20℃~AC1; S9, then heat up to AC3+ (50-80°C) at speed v4, keep warm, and then air cool; after air cooling, perform the second tempering.

2. The preparation method according to claim 1, characterized in that: The low temperature insulation time in S7 is shorter than the low temperature insulation time in S4.

3. The preparation method according to claim 1, characterized in that: In S3, when the journal temperature is cooled to 500-550°C by air, the journal of the forging blank is covered with a slow cooling cover and air cooling is continued.

4. The preparation method according to claim 3, characterized in that: In S9, the air cooling step is the same as that in S3.

5. The preparation method according to claim 1, characterized in that: v1>v2, v4>v3.

6. The preparation method according to claim 5, characterized in that: The v3 is 5 to 10°C / h, and the v4 is below 50°C / h.

7. The preparation method according to claim 1, characterized in that: The insulation time of S2 is (2-4)D1 / 100, D1 is the diameter of the shaft body, the unit of D1 is mm, and the unit of insulation time is h.

8. The preparation method according to claim 1, characterized in that: In the above S4, the holding time is greater than 0.85D1 / 100, D1 is the diameter of the shaft body, the unit of D1 is mm, and the unit of the holding time is h.

9. The preparation method according to any one of claims 1 to 8, characterized in that: The forging of a large rotor forging blank is included before S1, and the forging of the large rotor forging blank includes: first forging a journal area with a small cross-section, and then forging and deforming the shaft body with a large cross-section.

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

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