Large rotor forge piece and anti-cracking heat treatment method thereof

Abstract

The invention discloses a large rotor forge piece and an anti-cracking heat treatment method thereof, belongs to the technical field of rotor preparation, and solves the problems that an existing large rotor forge piece is prone to cracking in the heat treatment process after forging, and the flaw detection sensitivity is often unqualified. The heat treatment method comprises the steps that first-time complete austenitizing normalizing, second-time complete austenitizing normalizing and third-time complete austenitizing normalizing are sequentially carried out, and normalizing heat preservation temperatures of the three times of normalizing are sequentially reduced. According to the heat treatment method, the forge piece can be prevented from cracking, and grain refinement can be achieved.

Description

Technical Field

[0001] The present invention relates to the technical field of rotor preparation, and particularly relates to a large rotor forging and a heat treatment method for preventing cracking thereof. Background Art

[0002] The rotor is 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 increasing continuously. Currently, the proportion of 600MW - 1000MW units has an absolute advantage in steam turbine power generation. The shaft body cross-section of its rotor forging is between φ1700mm - φ2500mm, and the shaft diameter size is between φ600 - φ900mm. Due to the increase in the shaft body size of the forging, the cross-section difference at adjacent diameter positions becomes larger, resulting in a larger temperature difference between the inside and outside of the forging during the heat treatment process, and the structure stress and thermal stress become larger. In addition, the as-forged grain size and structure after forging are coarse, and the strength and toughness reserve are insufficient. During the post-forging heat treatment process after forging, the phenomenon of longitudinal cracking of the forging frequently occurs, bringing great losses to the manufacturer. Summary of the Invention

[0003] In view of the above situation, the present invention aims to provide a large rotor forging and a heat treatment method for preventing cracking thereof, which can at least solve one of the following technical problems: existing large rotor forgings are prone to cracking during the post-forging heat treatment process, and the flaw detection sensitivity often fails to meet the standard.

[0004] The object of the present invention is mainly achieved through the following technical solutions:

[0005] The present invention provides a heat treatment method for preventing cracking of a large rotor forging. The heat treatment method includes: first full austenitizing normalizing, second full austenitizing normalizing, and third full austenitizing normalizing carried out in sequence, and the normalizing holding temperatures of the three normalizings decrease in sequence.

[0006] Further, the holding temperature of the first full austenitizing normalizing is AC3+(90 - 120°C), the holding temperature of the second full austenitizing normalizing is AC3+(60 - 90°C), and the holding temperature of the third full austenitizing normalizing is AC3+(50 - 70°C).

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

[0008] 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 and hold the temperature.

[0009] S2. Heat up to AC3+(90 - 120°C) at a speed of v1 and hold the temperature.

[0010] S3. Air cool the forging blank.

[0011] S4. Load the forging blank into the heat treatment furnace and perform low-temperature heat preservation in the range of 100 - 130 °C below the Ms point.

[0012] S5. Heat up to AC1 - 20 °C to AC1 at a slow speed v2.

[0013] S6. Then heat up to AC3+(60 - 90 °C) at a faster speed v3, hold for heat preservation, and then air cool.

[0014] S7. Load the forging blank into the heat treatment furnace and perform low-temperature heat preservation in the range of 120 - 150 °C below the Ms point; among them, the temperature of the low-temperature heat preservation in S7 is lower than that in S4.

[0015] S8. The same as S5.

[0016] S9. Then heat up to AC3+(50 - 70 °C) at a faster speed v4, hold for heat preservation, and then air cool; continue tempering after air cooling.

[0017] Furthermore, V4 > v3 > v1.

[0018] Furthermore, the time of the low-temperature heat preservation in S7 is less than that in S4.

[0019] Furthermore, in S3, when air cooling to the journal area within the range of 500 - 550 °C, put a slow-cooling cover on the shaft diameter area of the forging blank, and air cool until the shaft body is 100 - 150 °C below the Ms point.

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

[0021] Furthermore, v2 < v1.

[0022] Furthermore, in S2, control the heat preservation time 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 heat preservation time is h.

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

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

[0025] a) In the method of the present invention, three normalizing processes combined with stepped cooling are used to obtain a uniform and fine austenite grain size in large-section forgings. It should be noted that in the first normalizing process, most of the carbides in the structure are fully dissolved into austenite, improving the strength and toughness of the matrix. The second and third normalizing processes adopt a stepped cooling form to make the grains and structure uniform and refined. After three consecutive normalizing processes, the crystal orientation of the original coarse austenite structure is completely disrupted, the tissue heredity is cut off, and uniform and fine grain boundaries, phase boundaries and dispersed carbides are formed. After tempering, a uniform bainite tempered structure is formed, effectively refining the structure and grains of the forging and fully preparing the structure for the subsequent quenching and tempering treatment.

[0026] b) In the method of the present invention, during the forging process of large rotor forging blanks, first forge the journal with a small cross-section (if there is a flange, it is the journal and flange here), 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.

[0027] c) In the method of the present invention, the structure after forging is relatively coarse. Without waiting for the forging to cool to the temperature at which non-equilibrium structure transformation occurs, that is, without the bainite transformation, it directly enters the post-forging heat treatment furnace and waits for material insulation near Ar1, making it undergo partial isothermal transformation. While cutting off the tissue heredity, it prevents the large-section forging with a coarse post-forging structure from cracking due to inconsistent internal and external tissue stresses and thermal stresses.

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

[0029] e) In the method of the present invention, there is a stepped cooling in the holding temperature of each normalizing process, making the grain size of the forging gradually decrease, which significantly improves the sensitivity of the forging. The low-temperature holding temperature before the start of each normalizing heating gradually decreases, which is beneficial to the full transformation of the core structure of the forging and significantly promotes the recrystallization refinement of the forging material, being beneficial to improving the flaw detection sensitivity of the forging.

[0030] f) In the method of the present invention, by precisely controlling the cooling method after forging, process parameters such as the normalizing temperature at different steps, the heating rate, holding temperature, holding time, and cooling rate at different stages; the cracking of the forging is avoided, and grain refinement is achieved, and it can fully prepare the structure for the subsequent quenching and tempering treatment.

[0031] g) The microstructure of the large rotor forging of the present invention is tempered bainite structure, with fine and uniform grains. For example, the grain size is 3.5 - 5 grades. The flaw detection sensitivity of the above large rotor forging has reached a good level. For example, the central limit sensitivity is Φ2 - 2.5 mm.

[0032] Other features and advantages of the present invention will be described in the subsequent description, and some of them will be obvious from the description or can 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 description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0033] The drawings are only for the purpose of showing specific embodiments and are not considered as limitations of the present invention. Throughout the drawings, the same reference signs represent the same components.

[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 air cooling of the slow cooling cover sleeve forging;

[0037] Figure 4 is a grain diagram of the forging blank after the S3 step of Example 1;

[0038] Figure 5 is a grain diagram of the forging blank after the S6 step of Example 1;

[0039] Figure 6 is a grain diagram of the forging blank after the S9 step of Example 1.

[0040] REFERENCE SIGNS

[0041] 1 - shaft body, 2 - shaft neck, 3 - flange, 4 - slow cooling cover, 41 - inner lining, 42 - outer skin, 43 - support frame. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0042] The following will specifically describe the preferred embodiments of the present invention in conjunction with the drawings, where the drawings form a part of the present invention and are used together with the embodiments of the present invention to explain the principles of the present invention.

[0043] As shown in the attached Figure 1As shown in the figure, the present invention provides a large rotor forging. The large rotor forging includes a shaft body 1 and shaft necks 2 connected to both sides of the shaft body 1. The diameter D1 of the shaft body 1 and the diameter D2 of the shaft necks 2 satisfy the following relationship: D1 > D2, and the difference between D1 and D2 is more than 1000 mm. For example, the difference between D1 and D2 is 1000 - 1800 mm, such as 1100 mm, 1200 mm, 1300 mm, 1400 mm, 1500 mm, 1600 mm, 1700 mm. The cross-sectional difference at the connection position between the shaft body 1 and the shaft necks 2 of the large rotor forging is large, resulting in a large temperature difference between the inside and outside during the heat treatment process of the forging, and the structure stress and thermal stress become large. Coupled with the large and coarse crystal grain and structure in the as-forged state after forging, and insufficient strength and toughness reserve, the forging is prone to cracking during the post-forging heat treatment process after forging.

[0044] Specifically, the large rotor forging may further include a flange 3 connected to the outside of the shaft neck 2. The diameter D2 of the shaft neck 2 and the diameter D3 of the flange 3 satisfy the following relationship: D3 > D2, and the difference between D3 and D2 is 100 - 700 mm, such as 100 mm, 200 mm, 300 mm, 400 mm, 500 mm, 600 mm.

[0045] Specifically, the diameter D1 of the shaft body 1 is 1700 mm or more, for example, 1700 - 2500 mm, such as 1800 mm, 1900 mm, 2000 mm, 2100 mm, 2200 mm, 2300 mm, 2400 mm.

[0046] Specifically, the diameter D2 of the shaft neck 2 is 600 mm or more, for example, 600 - 900 mm, such as 650 mm, 700 mm, 750 mm, 800 mm, 850 mm.

[0047] Specifically, the diameter D3 of the flange 3 is 700 mm or more, for example, 700 - 1600 mm, such as 800 mm, 900 mm, 1000 mm, 1100 mm, 1200 mm, 1300 mm, 1400 mm, 1500 mm.

[0048] Specifically, the component of the large rotor forging of the present invention is a C-Cr-Ni-Mo-V steel with low carbon and tissue inheritance characteristics, such as 30Cr2Ni4MoV steel, 35Cr2Ni4MoV steel, and 25Cr2Ni4MoV steel.

[0049] Specifically, the components of the above-mentioned large rotor forging include, by mass percentage: C: 0.2% - 0.35%, Si: 0.04% - 0.1%, Mn: 0.2% - 0.6%, Cr: 1.7% - 4.0%, Mo: 0.3% - 0.5%, Ni: 3.0% - 4.0%, V: 0.05% - 0.15%, Al: 0.003% - 0.01%, S: ≤0.004%, P: ≤0.004%, and the balance is Fe and unavoidable impurities.

[0050] The present invention provides a heat treatment method for preventing cracking of large rotor forgings, including first full austenitizing normalizing, second full austenitizing normalizing, and third full austenitizing normalizing carried out in sequence, and the normalizing holding temperatures of the above three normalizings decrease in sequence.

[0051] Specifically, the holding temperature of the first full austenitizing normalizing is AC3 + (90 - 120°C), the holding temperature of the second full austenitizing normalizing is AC3 + (60 - 90°C), and the holding temperature of the third full austenitizing normalizing is AC3 + (50 - 70°C).

[0052] Specifically, the holding temperature of the first full austenitizing normalizing is 890 - 920°C, such as 900°C, 910°C; the holding temperature of the second full austenitizing normalizing is 860 - 890°C, such as 870°C, 880°C; the holding temperature of the third full austenitizing normalizing is 850 - 870°C, such as 855°C, 860°C, 865°C.

[0053] Specifically, the heat treatment method for preventing cracking of the above-mentioned large rotor forgings includes the following steps:

[0054] 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 and hold the temperature.

[0055] S2. Heat up to AC3 + (90 - 120°C) at a speed of v1 and hold the temperature.

[0056] S3. Air-cool the forging blank.

[0057] S4. After removing the slow cooling cover, load the forging blank into the heat treatment furnace and carry out low-temperature holding in the range of 100 - 130°C below the Ms point.

[0058] S5. Heat up to AC1 - 20°C to AC1 at a slow speed of v2.

[0059] S6. Then heat up to AC3 + (60 - 90°C) at a faster speed of v3, hold the temperature, and then air-cool.

[0060] S7. After removing the slow cooling cover, load the forging blank into the heat treatment furnace and perform low-temperature heat preservation within the range of 120 - 150 °C below the Ms point; among them, the low-temperature heat preservation temperature in S7 is lower than the low-temperature heat preservation temperature in S4.

[0061] S8. The same as S5.

[0062] S9. Then, heat up to AC3 + (50 - 70 °C) at a relatively fast speed v4, hold for heat preservation, and then air cool.

[0063] Specifically, V4 > v3 > v1.

[0064] Specifically, before the above-mentioned S1, it includes the forging of large rotor forgings. The forging of large rotor forgings 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 forgings.

[0065] Specifically, in the above-mentioned S1, after the forging of the large rotor forging blank is completed, it is not air cooled to room temperature. After the forging is completed, it is loaded into the heat treatment furnace as quickly as possible for heat preservation, and partial pearlite transformation from grain boundaries to grain interiors (volume percentage 20% - 35%) can occur. The mixed structure of pearlite isothermal structure + untransformed austenite can prevent the forging blank from cracking.

[0066] Specifically, in the above-mentioned S1, considering that if the incubation period of pearlite transformation is too long at too high or too low temperatures, the pearlite content will be reduced and the refinement effect will be reduced. Therefore, control the heat preservation temperature in S1 to be Ar1 ± 20 °C, such as 610 - 640 °C, such as 620 °C, 630 °C.

[0067] Specifically, in the above-mentioned S1, the role of heat preservation 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 heat preservation time will increase the heat treatment cycle, and too short heat preservation time will result in the core and surface temperatures not reaching uniform consistency and less pearlite structure content. Therefore, control the heat preservation time in 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 heat preservation time is h. Preferably, the heat preservation time is (1.5 - 2)D1 / 100.

[0068] Specifically, in the above-mentioned S2, in the plastic deformation stage, while ensuring that the instantaneous internal stress is controllable, relatively quickly reach AC3 + (90 - 120 °C). There are many nucleation points, which fully disrupt the original austenite grains and achieve the effect of refining the grains. Therefore, use a heating rate v1 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.

[0069] Specifically, in the above S2, 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 austenite, improving the strength and toughness of the matrix. Control the holding time 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, (2.5 - 3.5)D1 / 100h. For example, the holding time can be 2.5D1 / 100, 2.7D1 / 100, 3D1 / 100, 3.2D1 / 100, 3.5D1 / 100.

[0070] Specifically, in the above S3, lift the forging blank onto the spacer block 1000 mm high for air cooling.

[0071] Specifically, in the above S3, when air cooling to the range of 500 - 550 °C in the journal area (if there is a flange, this is the shaft diameter and the flange), put a slow cooling cover 4 on the shaft diameter area (if there is a flange, this is the shaft diameter and the flange) of the forging blank, and air cool 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 shaft diameter 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 with 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, making the core temperature of the forging blank lower and enabling more sufficient tissue transformation, achieving the effect of refining the tissue and grains.

[0072] Specifically, in the above 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 asbestos felt with good heat insulation effect, and the outer skin 42 is 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.

[0073] Specifically, as Figure 3 shown is a schematic diagram of air cooling the forging with the slow cooling cover sleeved.

[0074] Specifically, in the above S4, hold for a long time below the Ms point to make the core of the forging blank fully undergo tissue transformation as much as possible, which is beneficial to adjusting and refining the grain structure of the steel through subsequent phase transformation recrystallization. Therefore, control the holding time to be (1.2 - 1.6)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, for example, 1.3D1 / 100, 1.4D1 / 100, 1.5D1 / 100, 1.6D1 / 100.

[0075] Specifically, in the above-mentioned S5, this stage is a speed-limited 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 to reduce the cracking risk, and at the same time, it prepares for the phase transformation of the two-phase region. 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.

[0076] Specifically, in the above-mentioned S5, the temperature of AC1 - 20 °C to AC1 is preferably 680 - 700 °C, such as 680 °C, 690 °C, 700 °C.

[0077] Specifically, in the above-mentioned S6, in the plastic deformation stage, while ensuring that the instantaneous internal stress is controllable, it reaches AC3+(60 - 90 °C) relatively quickly. There are many nucleation points, which fully disrupt the original austenite grains and achieve the effect of grain refinement. This stage is a speed-limited heating stage. In the plastic deformation zone of the forging and after the forging has undergone a normalizing treatment, the structure is relatively fine and the plastic and toughness reserves have increased. Therefore, v3 > v1 is controlled. For example, v3 is below 30 °C / h, preferably, v3 is 20 - 30 °C / h, such as 22 °C / h, 24 °C / h, 26 °C / h, 28 °C / h.

[0078] Specifically, in the above-mentioned 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, therefore, 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. The holding time is the same as that in the holding stage of S2.

[0079] Specifically, in the above-mentioned S6, the operating steps of air cooling are the same as those in S3. The difference is that this time, the air cooling is carried out until the surface temperature of the shaft body is 30 - 50 °C lower than the final temperature of S3, so that the structure transformation of the forging core is more complete.

[0080] Specifically, in the above-mentioned S7, considering that in S6, the air cooling makes the temperature of the shaft body lower than that of S3, and 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 less than the low-temperature holding time in S4. For example, the holding time is controlled to be 1 - 1.5D1 / 100mm, where D1 is the diameter of the shaft body, the unit of D1 is mm, and the unit of the holding time is h, such as 1.1D1 / 100mm, 1.2D1 / 100mm, 1.2D1 / 100mm, 1.4D1 / 100mm.

[0081] Specifically, in the above S9, v4 > v3. Compared with the previous two normalizations, the plastic and toughness reserves of the forging are more sufficient, and the anti-cracking ability is stronger. Further increasing the heating rate in the two-phase region can effectively refine the grains and microstructure. Preferably, v4 is 30 - 40 °C / h, such as 32 °C / h, 34 °C / h, 36 °C / h, 38 °C / h.

[0082] Specifically, the holding time in the above S9 is less than that in S6.

[0083] Specifically, in the above S9, it is held at AC3 + (50 - 70 °C) to fully form equiaxed austenite small grains in the microstructure. Since two normalizations have been carried out before, both the microstructure and grain size are already very fine, and the time required for the austenitization to make the grains uniform is shorter than the previous two times. Therefore, the holding time in S9 is (1.8 - 3.5) D1 / 100, preferably 2 - 3 D1 / 100 mm, such as 2.2 D1 / 100 mm, 2.4 D1 / 100 mm, 2.6 D1 / 100 mm, 2.8 D1 / 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.

[0084] Specifically, in the above S9, the air cooling operation is the same as that in S6 and will not be elaborated here.

[0085] Specifically, the heat treatment method for preventing cracking of the above large rotor forging further includes a tempering process, and the tempering process includes the following steps:

[0086] S10: The same as S7;

[0087] S11: Heat up to AC1 - (50 - 80 °C) and hold;

[0088] S12: Cool down to 250 °C at a cooling rate of less than 10 °C / h and then air cool to room temperature.

[0089] Specifically, in S11, the heating rate is 10 - 25 °C / h, such as 13 °C / h, 15 °C / h, 18 °C / h, 20 °C / h, 23 °C / h.

[0090] Specifically, in S11, the temperature range of AC1 - (50 - 80 °C) can be 630 - 660 °C.

[0091] Specifically, in S11, the heat preservation time is (2.5 - 4)D1 / 100. For example, 2.6D1 / 100 mm, 2.8D1 / 100 mm, 3D1 / 100 mm, 3.2D1 / 100 mm, 3.4D1 / 100 mm, 3.6D1 / 100 mm, 3.8D1 / 100 mm. 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.

[0092] 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 machining. Therefore, the cooling rate is controlled below 10 °C / h. For example, 5 - 10 °C / h, such as 6 °C / h, 7 °C / h, 8 °C / h, 9 °C / h.

[0093] The present invention also provides a large rotor forging prepared by using the above heat treatment method.

[0094] 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 3.5 - 5 levels. The flaw detection sensitivity of the above large rotor forging reaches a good level. For example, the central limit sensitivity is Φ2 - 2.5 mm.

[0095] Compared with the prior art, in the method of the present invention, combined with the three normalizations of stepped cooling, a large cross-section forging obtains uniform and fine austenite grain size. It should be noted that in the first normalization, most of the carbides in the structure are fully dissolved into austenite, improving the strength and toughness of the matrix. The second and third normalizations adopt a stepped cooling form to make the grains and structure uniform and refined. After three consecutive normalizations, the crystal orientation of the original coarse austenite structure is completely disrupted, the tissue heredity is cut off, and uniform and fine grain boundaries, phase boundaries and dispersed carbides are formed. After tempering, a uniform bainite tempered structure is formed, effectively refining the structure and grains of the forging, and can fully prepare the structure for the subsequent quenching and tempering treatment.

[0096] In the method of the present invention, during the forging process of the large rotor forging blank, first forge the shaft neck with a small cross-section (if there is a flange, here it is the shaft neck 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 beneficial to the refinement of the grains and structure of the forging.

[0097] In the method of the present invention, the structure after forging is relatively coarse. Without waiting for the forgings to cool to the temperature at which non-equilibrium structure transformation occurs, that is, without the bainite transformation occurring, they are directly put into the post-forging heat treatment furnace and held for heat preservation near Ar1, causing partial isothermal transformation. While cutting off the inheritance of the structure, it prevents large-section forgings with coarse post-forging structures from cracking due to inconsistent internal and external structure stresses and thermal stresses.

[0098] In the method of the present invention, when large-section forgings are air-cooled, slow-cooling covers are put on the positions with smaller cross-sections to reduce the stress concentration at the transition positions of cross-section changes caused by inconsistent cooling rates and structure transformations due to cross-section differences during normalizing air-cooling, greatly reducing the cracking risk.

[0099] In the method of the present invention, there is a stepwise temperature drop in the holding temperature during each normalizing process, gradually reducing the grain size of the forgings, which significantly improves the sensitivity of the forgings. The low-temperature holding temperature before the start of each normalizing heating gradually decreases, which is beneficial to the full transformation of the structure in the core of the forgings, significantly promoting the recrystallization refinement of the forging materials and being beneficial to improving the flaw detection sensitivity of the forgings.

[0100] In the method of the present invention, by precisely controlling the cooling method after forging, process parameters such as the normalizing temperature at different steps, the heating rate, holding temperature, holding time, and cooling rate at different stages; the cracking of forgings is avoided, and grain refinement is achieved, which can fully prepare the structure for the subsequent quenching and tempering treatment.

[0101] The structure of the large rotor forgings of the present invention is bainite tempered structure, with fine and uniform grains. For example, the grain size is 3.5 - 5 grades. The flaw detection sensitivity of the above large rotor forgings reaches a good level. For example, the central limit sensitivity is Φ2 - 2.5mm.

[0102] The following uses specific examples and comparative examples to demonstrate the advantages of precise control of the process parameters of the present invention.

[0103] Example 1

[0104] This example provides a large motor rotor forging and its heat treatment method. The large rotor forging of this example includes a shaft body 1 and shaft necks 2 connected to both sides of the shaft body 1. The cross-sectional diameter of the shaft body is φ1850mm, and the cross-sectional diameter of the shaft neck 2 is 700mm; the material of the rotor is 25Cr2Ni4MoV steel, with the 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 is Fe and unavoidable impurities.

[0105] The heat treatment method of the large rotor forging of this example includes the following steps:

[0106] S0. First, forge a 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 1.25;

[0107] S1. Heat the heat treatment furnace to 610 °C, and quickly load the forged blank of the large rotor after forging into the heat treatment furnace, and keep it warm for 30 h;

[0108] S2. Raise the temperature to 900 °C at a speed of 15 °C / h, and keep it warm for 50 h;

[0109] S3. Air-cool the forged blank. After the journal is air-cooled to 500 - 550 °C, add a slow-cooling cover to the journal and air-cool it until the surface temperature of the shaft body is 250 °C;

[0110] S4. After removing the slow-cooling cover, load the forgings into the heat treatment furnace and keep it warm at 200 °C for 25 h;

[0111] S5. Raise the temperature to 680 °C at a slow speed of 8 °C / h;

[0112] S6. Then raise the temperature to 870 °C at a faster speed of 25 °C / h, keep it warm for 50 h, and then air-cool. The journal is air-cooled to 500 - 550 °C, and the journal is air-cooled with a slow-cooling cover until the surface temperature of the shaft body is 200 °C;

[0113] S7. After removing the slow-cooling cover, load the forgings into the heat treatment furnace and keep it warm at 180 °C for 22 h;

[0114] S8. The same as S5;

[0115] S9. Then raise the temperature to 860 °C at a faster speed of 34 °C / h, keep it warm for 45 h, and then air-cool. The journal is air-cooled to 500 - 550 °C, and the journal is air-cooled with a slow-cooling cover until the surface temperature of the shaft body is 200 °C;

[0116] S10. The same as S7;

[0117] S11. Raise the temperature to 640 °C and keep it warm for 55 h;

[0118] S12: Cool down at a cooling rate of 10 °C / h to 250 °C, and then air-cool to room temperature.

[0119] The grains of the forged blank after the treatment in step S3 of this embodiment are as Figure 4 shown, the grains of the forged blank after the treatment in step S6 are as Figure 5 shown, the grains of the forged blank after the treatment in step S9 are as Figure 6 shown. The flaw detection sensitivity and grain size of the forgings in this embodiment have reached a good level. The central limit sensitivity is Φ2.0 mm, and the grain size is 4 - 5 grades. And the forgings did not crack during the preparation process of this embodiment.

[0120] Example 2

[0121] This embodiment provides a large-scale low-pressure rotor forging and its heat treatment method. The shape of the large-scale 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 φ1950mm and the journal size is 800mm; the material of the rotor is 30Cr2Ni4MoV steel, and its 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%, and the balance is Fe and inevitable impurities.

[0122] The heat treatment method of the large-scale rotor forging in this embodiment is generally the same as that in Embodiment 1, including:

[0123] 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;

[0124] S1. Heat the heat treatment furnace to 620°C, and quickly load the large-scale rotor forging blank after forging into the heat treatment furnace and keep it warm for 32h;

[0125] S2. Raise the temperature to 910°C at a speed of 15°C / h and keep it warm for 55h;

[0126] S3. The same as that in Embodiment 1;

[0127] S4. After removing the slow cooling cover, load the forging into the heat treatment furnace and keep it warm at 210°C for 28h;

[0128] S5. Raise the temperature to 700°C at a slow speed of 8°C / h;

[0129] S6. Then raise the temperature to 870°C at a faster speed of 23°C / h, keep it warm for 55h, and then air cool. The journal is air cooled to 500 - 550°C, and the journal is air cooled with a slow cooling cover until the surface temperature of the shaft body is 200°C;

[0130] S7. After removing the slow cooling cover, load the forging into the heat treatment furnace and keep it warm at 180°C for 25h;

[0131] S8. The same as S5;

[0132] S9. Then raise the temperature to 850°C at a faster speed of 31°C / h, keep it warm for 50h, and then air cool. The journal is air cooled to 500 - 550°C, and the journal is air cooled with a slow cooling cover until the surface temperature of the shaft body is 200°C;

[0133] S10. The same as S7;

[0134] S11. Raise the temperature to 640°C and keep it warm for 60h;

[0135] S12: The same as in Embodiment 1.

[0136] For the forgings in this embodiment, both the flaw detection sensitivity and the grain size have reached a good level. The center limit sensitivity is Φ2.0mm, and the grain size is Grade 4.5. Moreover, no cracks occurred to the forgings during the preparation process of this embodiment.

[0137] Embodiment 3

[0138] 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 φ2200mm, and the journal size is 800mm; the material of the rotor is 35Cr2Ni4MoV steel, and its 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%, and the balance is Fe and inevitable impurities.

[0139] The heat treatment method of the large rotor forging in this embodiment is substantially the same as that in Embodiment 1, including:

[0140] 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 1.27;

[0141] S1: Heat the heat treatment furnace to 630°C, and quickly load the large rotor forging blank after forging into the heat treatment furnace, and keep it warm for 43h;

[0142] S2: Heat it to 920°C at a rate of 16°C / h and keep it warm for 65h;

[0143] S3: The same as in Embodiment 1;

[0144] S4: After removing the slow cooling cover, load the forging into the heat treatment furnace and keep it warm at 250°C for 35h;

[0145] S5: Heat it to 700°C at a slow rate of 6°C / h;

[0146] S6: Then heat it to 880°C at a faster rate of 25°C / h, keep it warm for 65h, and then air cool. Air cool the journal to 500 - 550°C, and add a slow cooling cover to the journal and air cool until the surface temperature of the shaft body is 200°C;

[0147] S7: After removing the slow cooling cover, load the forging into the heat treatment furnace and keep it warm at 200°C for 32h;

[0148] S8: The same as S5;

[0149] S9. Then, heat it up to 850°C at a relatively fast rate of 33°C / h, hold for 60 h, and then air cool. Air cool the journal to 500 - 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;

[0150] S10. It is the same as S7;

[0151] S11. Heat it up to 660°C and hold for 80 h;

[0152] S12: Cool it at a rate of 8°C / h to 250°C, and then air cool to room temperature.

[0153] For the forgings in this embodiment, both the flaw detection sensitivity and the grain size have reached a good level. The central limit sensitivity is Φ2.5 mm, and the grain size is grade 3.5. And during the preparation process of this embodiment, the forgings did not crack.

[0154] During the research process, the inventor has conducted a large number of studies. Now, some solutions with poor performance are taken as comparative examples.

[0155] Comparative Example 1

[0156] 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.

[0157] The preparation method of this comparative example is as follows:

[0158] In S1, cool the forging blank in the furnace. The process is to cool it in the furnace at a cooling rate of 8°C / h, cool it to 250°C and hold for 20 h. After holding, slowly heat it up to 610°C at a rate of 5°C / h, and then it is the same as S2 in Example 1.

[0159] When the forging in this comparative example was air cooled in S2, the furnace door was opened and it was found that the forging had longitudinal cracks.

[0160] Comparative Example 2

[0161] 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.

[0162] The preparation method of this comparative example is as follows: When performing air cooling in S3, a slow cooling cover was not used to protect the journal diameter, resulting in circumferential cracks being found at the transition position between the journal diameter and the shaft body when the forging was air cooled to about 250°C.

[0163] Comparative Example 3

[0164] 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.

[0165] The preparation method of this comparative example is substantially the same as that of Example 3, except that the form of stepwise cooling of the holding temperature during the three normalizing processes is not adopted. For the three normalizing processes, the holding temperature is 910 °C for 60 h, which ultimately results in relatively poor flaw detection sensitivity and grain size before the forging is quenched and tempered. The central limit sensitivity is Φ4 mm, and the grain size is Grade 1.5.

[0166] Comparative Example 4

[0167] 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.

[0168] In the preparation method of this comparative example, in S0, the shaft body was forged first with a forging ratio of 1.27, and the shaft neck had not been forged yet. The temperature of the forging had dropped below the process range and was unable to be forged, so the forging was returned to the heating furnace for forging heating. After the heating was completed, the shaft neck was forged, and the shaft body was not deformed, and the finished product was obtained.

[0169] The remaining steps are substantially the same as those in Example 3.

[0170] 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 Φ4 mm, showing a grass-like wave, and it is impossible to carry out the subsequent quenching and tempering.

[0171] As mentioned above, the above are only the preferred specific embodiments 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 by the protection scope of the present invention.

Claims

1. A heat treatment method for preventing cracking of large rotor forgings, characterized in that: The heat treatment method comprises: a first complete austenitizing normalizing, a second complete austenitizing normalizing and a third complete austenitizing normalizing are performed in sequence, and the normalizing holding temperatures of the three normalizing processes are successively reduced.

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

3. The heat treatment method according to claim 2, characterized in that: The heat treatment method comprises the following steps: S1. Heat the heat treatment furnace to Ar1±20℃, load the large rotor forging blank after forging into the heat treatment furnace as soon as possible and keep it warm; S2, heating to AC3+ (90-120°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 130°C below the Ms point; S5, slowly raise the temperature to AC1-20℃~AC1 at a speed v2; S6, then raise the temperature to AC3+ (60-90°C) at a faster speed v3, keep warm, and then air cool; S7, loading the forging billet into a heat treatment furnace, and performing low temperature insulation in the range of 120 to 150° C. below the Ms point; wherein the temperature of the low temperature insulation in S7 is lower than the temperature of the low temperature insulation in S4; S8, same as S5; S9. Then heat up to AC3+ (50-70℃) at a faster speed v4, keep warm, and then air cool; continue tempering after air cooling.

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 shorter than the low temperature insulation time in S4.

6. The heat treatment method according to claim 3, characterized in that: In the above S3, when the temperature of the journal region is within the range of 500-550°C by air cooling, the shaft diameter region of the forging blank is covered with a slow cooling cover and air cooled until the shaft body is 100-150°C lower than the Ms point.

7. The heat treatment method according to claim 3, characterized in that: The heat preservation time in S9 is shorter 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 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.

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

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