A repair heat treatment method for 30Cr2Ni4MoV steel and its application

Abstract

The present invention relates to a repair heat treatment method and application of 30Cr2Ni4MoV steel, which belongs to the field of heat treatment and is used to solve the problem that the surface grain size of rotor forgings is too fine, resulting in extremely high sensitivity of flaw detection. The repair heat treatment method comprises heating the 30Cr2Ni4MoV steel after pre-treatment, starting from 220°C, and heating it to 650-700°C at a heating rate of 10-25°C / h; then heating it to 970-990°C at a heating rate of 25-60°C / h; keeping it at 970-990°C for 10-30h; then cooling it to 850-940°C at a cooling rate of 8-15°C / h, and keeping it for 2-5h, and then air cooling it to room temperature after keeping it warm. The surface grain size of the obtained rotor forging is coarsened, and the grain size of the surface and the core is kept at the same level, both reaching level 3.0-4.5, meeting the requirements of the flaw detection standard, and ensuring that the performance of the rotor forging meets the requirements.

Description

Technical Field

[0001] The present invention relates to the technical field of heat treatment, in particular to a repair heat treatment method for 30Cr2Ni4MoV steel and its application. Background Art

[0002] The integrally forged high-pressure and low-pressure rotors in the conventional island of a nuclear power plant are core components of nuclear power generation and play an irreplaceable role in the plant. Therefore, their performance directly impacts the service life of the plant.

[0003] For over a decade, the heat treatment process for integrally forged nuclear power conventional island rotor forgings has relied on traditional heat treatment techniques: post-forging heat treatment with multiple positive tempering and tempering heat treatment with quenching and high-temperature tempering. However, due to the large cross-section of the rotor forgings, traditional heat treatment resulted in significant discrepancies in the heating and cooling rates between the surface and core of the rotor forgings. This led to differences in grain size near the surface and core of the rotor forgings, ultimately resulting in significant differences in the detection sensitivity limits between the rotor shaft surface and core, making it impossible to meet the nuclear power standard for ultrasonic flaw detection, which requires no lower limit for defect detection.

[0004] Therefore, there is an urgent need for a heat treatment method to repair rotor forgings that have failed the traditional heat treatment due to minor defects in the flaw detection, so that the grain size of the surface and core of the rotor forgings remain at the same level, reduce the excessive flaw detection sensitivity of the forging surface, and meet the nuclear power standard for acceptance with no lower limit requirement for ultrasonic flaw detection. Summary of the Invention

[0005] In view of the above analysis, the present invention aims to provide a repair heat treatment method and application of 30Cr2Ni4MoV steel to solve the problem that the surface grain of rotor forgings using traditional heat treatment is too fine, resulting in extremely high surface flaw detection sensitivity, so that the grain size of the surface and core of the rotor forging remains at the same level.

[0006] In one aspect, the present invention provides a repair heat treatment method for 30Cr2Ni4MoV steel, comprising the following steps:

[0007] S1: heating the pre-treated 30Cr2Ni4MoV steel, starting from 220°C, slowly heating it to 650-700°C at a heating rate of 10-25°C / h; then rapidly heating it to 970-990°C at a heating rate of 25-60°C / h;

[0008] S2: Keep the temperature within the range of 970-990℃ for 10-30h;

[0009] S3: Cool down to 850-940°C at a cooling rate of 8-15°C / h and keep warm for 2-5h. After keeping warm, cool to room temperature by air.

[0010] Furthermore, during the slow heating stage, the heating rate is 10-15°C / h.

[0011] Furthermore, during the rapid heating stage, the heating rate is 25-50° C. / h.

[0012] Preferably, during the rapid heating stage, the heating rate is 40-50°C / h.

[0013] Furthermore, the temperature is kept within the range of 970-990° C. for 10-20 hours.

[0014] Preferably, the temperature is kept within the range of 970-990° C. for 15-20 hours.

[0015] Furthermore, the chemical composition of the 30Cr2Ni4MoV steel includes, by weight percentage, C: ≤0.37%, Si: ≤0.12%, Mn: 0.17-0.43%, Cr: 1.45-2.05%, Mo: 0.22-0.62%, Ni: 3.18-3.82%, V: 0.06-0.16%, Al: ≤0.012%, S: ≤0.015%, and P: ≤0.015%.

[0016] On the other hand, the present invention provides an application of a repair heat treatment method for 30Cr2Ni4MoV steel, wherein the repair heat treatment method is used to prepare a rotor forging, wherein the diameter of the rotor forging is φ1300-φ2000 mm.

[0017] Furthermore, the preparation of the rotor forging includes the following steps:

[0018] S1: Smelting: Alloy raw materials are smelted to produce ingots;

[0019] S2: Forging: Forging the ingot to obtain a forging;

[0020] S3: heat treatment: performing post-forging heat treatment and quenching and tempering heat treatment on the forging;

[0021] S4: Repair heat treatment: The rotor forging is produced by adopting the repair heat treatment method of 30Cr2Ni4MoV steel described in the present invention.

[0022] Furthermore, the post-forging heat treatment includes multiple normalizing and post-forging tempering.

[0023] Furthermore, the tempering heat treatment includes quenching and tempering.

[0024] Furthermore, the grain sizes of the rotor forging near the surface and the core are both 3.0-4.5, and the core can meet the detection sensitivity requirements of the standard.

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

[0026] 1. The present invention provides a repair heat treatment method for 30Cr2Ni4MoV steel, which comprises first slowly heating the temperature from 220°C to a temperature range of 650-700°C at a heating rate of 10-25°C / h, then rapidly heating the temperature to 970-990°C at a heating rate of 25-60°C / h, holding the temperature for a period of time, then cooling the temperature to 850-940°C at a cooling rate of 8-15°C / h, cooling to room temperature after holding the temperature, and finally performing a quenching and tempering treatment. The repair heat treatment method provided by the present invention is used after a pre-treatment combining post-forging heat treatment and quenching and tempering heat treatment, so that the grain size of the surface and core of the rotor forging reaches 3.0-4.5, meeting the requirements of the flaw detection standard, and ensuring that the mechanical properties of the rotor forging meet the standard requirements.

[0027] 2. The repair heat treatment method provided by the present invention is applicable to rotor forgings with a diameter of more than 1300 mm. After the repair treatment, the grain size of the surface of the rotor forging is coarsened, which not only keeps the grain size of the core and the surface at the same level, but also does not affect the overall mechanical properties of the rotor forging.

[0028] 3. After the repair heat treatment of the present invention, during ultrasonic testing, tiny defects with a surface equivalent of ≤Φ0.4 are effectively shielded, and the center limit sensitivity is ≤Φ1.6, meeting the standard requirements.

[0029] In the present invention, the above-mentioned technical solutions can be combined with each other to achieve more preferred combinations. Other features and advantages of the present invention will be described in the following description, and some advantages will become apparent from the description or be learned through practice of the present invention. The objectives and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the description and drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0030] The accompanying drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like parts throughout the drawings.

[0031] Figure 1 is the heat treatment process curve of the test block;

[0032] Figure 2 This is a metallographic photograph of the surface of the rotor forging obtained in Example 3;

[0033] Figure 3This is a metallographic photograph of the core of the rotor forging obtained in Example 3;

[0034] Figure 4 This is a metallographic photograph of the surface of an existing rotor forging;

[0035] Figure 5 This is a metallographic photograph of the core of an existing rotor forging. DETAILED DESCRIPTION

[0036] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of this application and are used together with the embodiments of the present invention to illustrate the principles of the present invention, and are not used to limit the scope of the present invention.

[0037] Currently, conventional heat treatment is used to adjust the grain size of rotor forgings made of 30Cr2Ni4MoV. This includes post-forging heat treatment and quenching and tempering. However, this treatment results in finer grains on the surface and coarser grains in the core of the rotor forgings, failing to meet the nuclear power acceptance standard requiring no lower limit for ultrasonic flaw detection.

[0038] Therefore, the present invention provides a repair heat treatment method for 30Cr2Ni4MoV steel, comprising the following steps:

[0039] S1: heating the pre-treated 30Cr2Ni4MoV steel, starting from 220°C, slowly heating it to 650-700°C at a heating rate of 10-25°C / h; then rapidly heating it to 970-990°C at a heating rate of 25-60°C / h;

[0040] S2: Keep the temperature within the range of 970-990℃ for 10-30h;

[0041] S3: Cool down to 850-940°C at a cooling rate of 8-15°C / h and keep warm for 2-5h. After keeping warm, cool to room temperature by air.

[0042] Compared with the existing technology, the 30Cr2Ni4MoV steel after pre-treatment is repaired, and goes through the heating, insulation and cooling stages, and finally undergoes a tempering treatment, so that the near-surface grain size of the 30Cr2Ni4MoV steel is coarsened to reduce the sensitivity of surface ultrasonic flaw detection, and the near-surface and internal grains of the 30Cr2Ni4MoV steel are kept at the same level, meeting the nuclear power standard for acceptance of ultrasonic flaw detection with no lower limit requirement.

[0043] In the present invention, pretreatment includes smelting, forging, and heat treatment, with heat treatment comprising post-forging heat treatment and tempering heat treatment. However, during pretreatment, the surface of the 30Cr2Ni4MoV steel rotor forging heats up and cools faster than the core, resulting in a finer grain size at the surface than at the core, even far finer than the standard requirements. Consequently, the grain size distribution inside and outside the pretreated 30Cr2Ni4MoV steel is uneven, leading to significant differences in ultrasonic testing sensitivity between the surface and core.

[0044] The present invention repairs 30Cr2Ni4MoV steel after pretreatment, and by adjusting the heating, insulation and cooling processes, the surface grain size of the 30Cr2Ni4MoV steel forging after pretreatment is coarsened as much as possible while meeting standard requirements without affecting the mechanical properties of the 30Cr2Ni4MoV steel.

[0045] During the heating stage, a combination of slow and rapid heating is used for heating. During the slow heating stage, slow heating is chosen to avoid cracking of large cross-sections due to increased thermal stress caused by rapid heating during the elastic deformation stage of the forging. The Ac1 temperature of 30Cr2Ni4MoV steel is 705°C, and the Ac3 temperature is 800°C. After the rotor forging is slowly heated to 650-700°C, the forging enters the plastic deformation stage. Rapid heating does not pose a risk of cracking. Furthermore, rapid heating in the two-phase region can effectively prevent the core, which has a slower heating rate, from becoming severely coarsened during the subsequent insulation stage.

[0046] Illustratively, in the slow heating stage, the heating rate may be: 25°C / h, 22°C / h, 21°C / h, 20°C / h, 18°C / h, 15°C / h, 14°C / h, 13°C / h, 12°C / h, 11°C / h or 10°C / h.

[0047] Preferably, during the slow heating stage, the heating rate is 10-15°C / h.

[0048] During the rapid heating stage, in order to ensure that there is a temperature difference between the inside and outside of the same cross-section of the forging in the high temperature stage, the surface is heated to the coarsening temperature as quickly as possible and kept warm, and the core does not reach a higher coarsening temperature, thereby achieving the effect of obvious coarsening of the surface grain size while not obvious coarsening of the core.

[0049] Illustratively, the heating rate in the rapid heating stage can be: 25°C / h, 28°C / h, 30°C / h, 33°C / h, 35°C / h, 38°C / h, 40°C / h, 42°C / h, 45°C / h, 48°C / h, 50°C / h, 52°C / h, 55°C / h, 57°C / h, 59°C / h or 60°C / h.

[0050] Preferably, during the rapid heating stage, the heating rate is 25-50°C / h.

[0051] Preferably, during the rapid heating stage, the heating rate is 40-50°C / h.

[0052] During the insulation stage, insulation is carried out in the range of 970-990℃. The insulation time is 10-30h according to the actual cross-section size of the forging. The larger the cross-section, the longer the insulation time, so as to ensure that the temperature near the surface of the forging reaches above 960℃ for 10h, so as to achieve grain coarsening in the near-surface range of the forging.

[0053] Illustratively, the temperature is kept within the range of 970-990° C., and the temperature keeping time can be: 10 h, 12 h, 15 h, 17 h, 18 h, 20 h, 22 h, 24 h, 26 h, 28 h or 30 h.

[0054] Preferably, the temperature is kept within the range of 970-990° C. for 10-20 hours.

[0055] Preferably, the temperature is kept within the range of 970-990° C. for 15-20 hours.

[0056] During the cooling phase, the temperature is first lowered to 850-940°C at a rate of 8-15°C / h and then held for a period of time. This cooling and holding process is to prevent the forging surface from excessive coarsening due to long-term high-temperature holding, while ensuring that the core temperature of the forging can reach 930-950°C, a temperature where no significant coarsening occurs.

[0057] It should be noted that the chemical composition of the 30Cr2Ni4MoV steel in the present invention, by weight percentage, includes C: ≤0.37%, Si: ≤0.12%, Mn: 0.17-0.43%, Cr: 1.45-2.05%, Mo: 0.22-0.62%, Ni: 3.18-3.82%, V: 0.06-0.16%, Al: ≤0.012%, S: ≤0.015%, and P: ≤0.015%.

[0058] The invention provides an application of a repair heat treatment method for 30Cr2Ni4MoV steel in preparing a rotor forging. The diameter of the rotor forging is φ1300-φ2000 mm.

[0059] The preparation of the rotor forging comprises the following steps:

[0060] S1: Smelting: Alloy raw materials are smelted to produce ingots;

[0061] S2: Forging: Forging the ingot to obtain a forging;

[0062] S3: heat treatment: performing post-forging heat treatment and quenching and tempering heat treatment on the forging;

[0063] S4: Repair heat treatment: The rotor forging is produced by adopting the repair heat treatment method of 30Cr2Ni4MoV steel.

[0064] Compared to existing technologies, the preparation of rotor forgings involves smelting, forging, heat treatment, and repair heat treatment. Specifically, the overall heat treatment of the rotor forgings includes post-forging heat treatment, tempering heat treatment, repair heat treatment, and secondary tempering heat treatment. The addition of the repair heat treatment coarsens the grains near the surface of the rotor forgings, bringing them closer to those in the core. It also reduces the flaw detection sensitivity near the surface of the rotor forgings, meeting the nuclear power standard for ultrasonic flaw detection, which requires no lower limit for acceptance. After the repair heat treatment, the mechanical properties and flaw detection performance are both acceptable, and the forgings are qualified for use.

[0065] Preferably, during the smelting process, molten steel is cruded in an alkaline electric furnace, and an appropriate slag-making system, ladle refining, and vacuum carbon deoxidation (LVCD+VCD) are employed. A vacuum treatment is performed before pouring and during the ingot pouring process, with a vacuum degree of ≤266 Pa, a tapping temperature of 1620-1640°C, and a pouring temperature of 1575-1595°C to produce the ingot.

[0066] Preferably, during the forging process, the steel ingot is forged to produce forgings using the process of "pressing the jaws + first upsetting + second upsetting + drawing out the finished product", the starting forging temperature is 1250°C, the final forging temperature is 750°C, the process number of fires is four, and the total deformation is ≥3.5%.

[0067] Furthermore, the post-forging heat treatment includes two normalizing steps and one post-forging tempering step.

[0068] Preferably, the normalizing process is as follows:

[0069] S1: Heat the forging and keep it at a low temperature within the range of 170-270℃ for 10-20h;

[0070] S2: slowly increase the temperature to 650-700°C at a heating rate of 6-15°C / h; then increase the temperature to 860-910°C at a heating rate of 5-10°C / h;

[0071] S3: Keep warm in the range of 860-910℃ for 30-50h;

[0072] S4: After keeping warm, take out of the furnace and air cool to 170-270℃.

[0073] Preferably, the post-forging tempering process is as follows:

[0074] S1: Heat the forgings that have been air-cooled to warm temperature and keep them at a low temperature within the range of 170-210℃ for 10-20h;

[0075] S2: slowly increase the temperature to 640-660°C at a heating rate of 10-25°C / h;

[0076] S3: Keep the temperature within the range of 640-660℃ for 40-70h;

[0077] S4: After keeping warm, slowly cool the temperature to 250°C at a cooling rate of ≤10°C / h, and then air cool to room temperature.

[0078] Furthermore, the tempering heat treatment includes quenching and tempering.

[0079] Preferably, the quenching heat treatment process is as follows:

[0080] S1: heating the forging after post-forging heat treatment, slowly heating the temperature to 650-700°C at a heating rate of 10-25°C / h; then rapidly heating the temperature to 830-850°C at a heating rate of 25-50°C / h;

[0081] S2: Keep the temperature within the range of 830-850℃ for 30-50h;

[0082] S4: After keeping warm, spray water or immerse in water to cool to ≤50℃.

[0083] Preferably, the quenching and tempering process is as follows:

[0084] S1: Heat the forgings that have been air-cooled to warm temperature and keep them at a low temperature within the range of 170-210℃ for 10-20h;

[0085] S2: slowly increase the temperature to 640-660°C at a heating rate of 10-25°C / h;

[0086] S3: Keep the temperature within the range of 590-630℃ for 40-60h;

[0087] S4: After keeping warm, slowly cool the temperature to 250°C at a cooling rate of ≤10°C / h, and then air cool to room temperature.

[0088] After the heat treatment, the repair heat treatment provided by the present invention is added, so that the grain size near the surface of the rotor forging is coarsened, reducing its flaw detection sensitivity, and the grain size close to the core is adjusted so that the grain size near the surface and the core are both 3.0-4.5 levels, and the core can meet the flaw detection sensitivity requirements of the standard.

[0089] In order to more clearly understand the present invention, the following examples and comparative examples are provided for illustration.

[0090] The existing rotor forging is made of 30Cr2Ni4MoV steel, which specifically includes smelting, forging, and heat treatment, wherein the heat treatment includes post-forging heat treatment and quenching and tempering heat treatment. The diameter of the rotor forging is φ1500mm.

[0091] Smelting: The alloy raw materials are smelted at a vacuum degree of 266 Pa, a tapping temperature of 1630°C, and a pouring temperature of 1585°C to obtain steel ingots;

[0092] Forging: Forging the steel ingot, the starting forging temperature is 1250℃, the final forging temperature is 750℃, the process number of fires is four, and the total deformation is 5%;

[0093] Heat treatment after forging: two normalizing + one tempering after forging,

[0094] The normalizing process is as follows:

[0095] S1: Heat the forging and keep it at 270℃ for 15h;

[0096] S2: slowly heating the temperature to 700°C at a heating rate of 8°C / h; then heating the temperature to 900°C at a heating rate of 7°C / h;

[0097] S3: Keep warm at 900℃ for 35h;

[0098] S4: After keeping warm, take it out of the furnace and air-cool it to 200℃.

[0099] The tempering process after forging is as follows:

[0100] S1: Heat the forgings that have been air-cooled to warm temperature and keep them at 200°C for 10 hours;

[0101] S2: heating up to 650°C at a rate of 15°C / h;

[0102] S3: Keep warm at 650°C for 50h;

[0103] S4: After keeping warm, slowly cool the temperature to 250°C at a cooling rate of 10°C / h, and then air-cool to room temperature.

[0104] Quenching and tempering heat treatment: including quenching and tempering,

[0105] The quenching process is as follows:

[0106] S1: heating the forging after post-forging heat treatment, slowly heating it to 650°C at a heating rate of 15°C / h; then rapidly heating it to 840°C at a heating rate of 35°C / h;

[0107] S2: Keep warm at 840℃ for 40h;

[0108] S4: After keeping warm, immerse in water and cool to ≤50℃.

[0109] The quenching and tempering process is as follows:

[0110] S1: Heat the forgings that have been air-cooled to warm temperature and keep them at 190°C for 15 hours;

[0111] S2: heating up to 650°C at a rate of 20°C / h;

[0112] S3: Keep warm at 600℃ for 40h;

[0113] S4: After heat preservation, the temperature is slowly lowered to 250° C. at a cooling rate of 10° C. / h, and then air-cooled to room temperature to obtain a rotor forging.

[0114] The grain size and mechanical properties of the rotor forgings were tested, and the test results are shown in Table 1.

[0115] Table 1 Test results of existing rotor forgings

[0116]

[0117] Example 1

[0118] Sampling was carried out on the test area of ​​the existing rotor forging (the test area with a radial depth of 150 mm on the forging shaft), the size of the test block was 20 mm × 20 mm × 20 mm, and the relationship between grain size and heating temperature and holding time was explored.

[0119] Reference Figure 1 , the test block is heat treated, and the heating process is as follows:

[0120] (1) After the furnace is heated to the holding temperature, two metallographic test blocks are placed in the furnace and kept at this temperature for 2 hours and 10 hours respectively, and then taken out and air-cooled;

[0121] (2) The holding temperatures T are set to 960°C, 970°C, 980°C, 990°C, 1000°C, 1010°C, 1020°C, 1030°C, and 1050°C, respectively.

[0122] The grain size of the test blocks after heat treatment was tested, and the specific test results are shown in Table 2.

[0123] Table 2 Test block test results (grain size / grade)

[0124]

[0125] Conclusion: According to the results in Table 2, within the range of 960℃-1050℃, as the holding temperature increases, the grain size gradually coarsens (the larger the value, the finer the grain). At the same temperature, the grain size also coarsens as the holding time increases.

[0126] Example 2

[0127] NDT test blocks measuring 40 mm x 150 mm x 170 mm were taken from the sample area of ​​an existing rotor forging (a 150 mm deep radial area along the forging shaft). The relationship between grain size and sensitivity was observed. The test blocks were heated in the same manner as in Example 1, at temperatures of 970°C, 990°C, 1020°C, and 1050°C, with a holding time of 10 hours.

[0128] The flaw detection method adopts the test block method required by nuclear power standards. The flaw detector adopts HS511 / 51208 / 51184, the probe model is 2.25MHz / 1.0 (17E01U2P), and the probe frequency is 2.25MHz. The obtained grain size and flaw detection sensitivity are shown in Table 3.

[0129] Table 3 Relationship between grain size and flaw detection sensitivity

[0130] Group Heating temperature / ℃ Grain size / grade of flaw detection test piece Sensitivity of flaw detection test block / mm 1 1050 00 Φ0.45 2 1020 1.0 Φ0.42 3 990 2.0 Φ0.40 4 970 2.5 Φ0.38 5 Original tempering state 6.5 Φ0.36

[0131] *The original quenched and tempered state refers to the existing rotor forgings.

[0132] As can be seen from Table 3, as the heating temperature gradually increases, the grain size of the test piece gradually coarsens (the larger the value, the finer the grain). As the grain size of the test piece gradually coarsens, the detection sensitivity gradually decreases (the larger the value, the lower the sensitivity).

[0133] Example 3

[0134] The existing rotor forging is made of 30Cr2Ni4MoV steel, which specifically includes smelting, forging, and heat treatment, wherein the heat treatment includes post-forging heat treatment and quenching and tempering heat treatment. The diameter of the rotor forging is φ1500mm.

[0135] The repair process of the existing rotor forging after heat treatment includes the following steps:

[0136] S1: heating the heat-treated forgings, starting at 220°C, slowly heating to 650°C at a heating rate of 15°C / h; then rapidly heating to 980°C at a heating rate of 40°C / h;

[0137] S2: Keep warm at 980℃ for 15h;

[0138] S3: Cooling the sample to 930°C at a cooling rate of 10°C / h and keeping the temperature for 3 hours. After keeping the temperature, air cooling the sample to room temperature was performed.

[0139] Finally, a tempering heat treatment is performed to obtain the rotor forging.

[0140] Example 4

[0141] The repair treatment method of Example 4 is substantially the same as that of Example 3, except that the diameter of the forging in Example 4 is Φ1900 ​​mm and the holding time in S2 is 20 h.

[0142] Example 5

[0143] The repair method of Example 5 is substantially the same as that of Example 3, except that in S1, starting from 220°C, the temperature is slowly increased to 650°C at a heating rate of 10°C / h; and then the temperature is rapidly increased to 980°C at a heating rate of 25°C / h.

[0144] Example 6

[0145] The repair processing method of Example 6 is substantially the same as that of Example 3, except that in S1, starting from 220°C, the temperature is slowly increased to 650°C at a heating rate of 25°C / h; and then the temperature is rapidly increased to 980°C at a heating rate of 60°C / h.

[0146] Comparative Example 1

[0147] The repair process of Comparative Example 1 is substantially the same as that of Example 3, except that the S2 holding temperature in the repair process of Comparative Example 1 is 950°C.

[0148] The rotor forgings obtained after repair were tested for grain size and performance. The specific test results are shown in Table 4. The metallographic structure photos are shown in Table 4. Figure 2-5 shown.

[0149] Table 4 Test results

[0150]

[0151] Combining Examples 3-6 and Comparative Examples 1-2 and Table 4, it can be seen that Figure 2-5 The grain size is coarsened from level 6.5 to about level 3.0-4.5, which can effectively reduce the sensitivity of UT flaw detection and effectively shield the display of UT tiny defects; within this range of grain size coarsening, it can still meet the performance requirements of rotor forgings.

[0152] The above description is only a preferred specific 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 thought of by any technician familiar with this technical field within the technical scope disclosed in the present invention should be covered by the scope of protection of the present invention.

Claims

1. A repair heat treatment method for 30Cr2Ni4MoV steel, characterized in that: The following steps are included: S1: heating the pre-treated 30Cr2Ni4MoV steel, starting from 220°C, slowly heating it to 650-700°C at a heating rate of 10-25°C / h; then rapidly heating it to 970-990°C at a heating rate of 25-60°C / h; S2: Keep the temperature within the range of 970-990℃ for 10-30h; S3: Cool down to 850-940°C at a rate of 8-15°C / h and keep warm for 2-5h. After keeping warm, cool to room temperature by air cooling. The repair heat treatment method is used to prepare a rotor forging, the diameter of the rotor forging is φ1300-2000 mm; the grain size near the surface and the core of the rotor forging are both 3.0-4.

5.

2. The repair heat treatment method for 30Cr2Ni4MoV steel according to claim 1, characterized in that: During the slow heating stage, the heating rate is 10-15°C / h.

3. The repair heat treatment method for 30Cr2Ni4MoV steel according to claim 1, characterized in that: During the rapid heating stage, the heating rate is 25-50° C. / h.

4. The repair heat treatment method for 30Cr2Ni4MoV steel according to claim 3, characterized in that: The heating rate is 40-50°C / h.

5. The repair heat treatment method for 30Cr2Ni4MoV steel according to claim 1, characterized in that: Keep warm in the range of 970-990℃ for 10-20h.

6. The repair heat treatment method for 30Cr2Ni4MoV steel according to claim 5, characterized in that: The insulation time is 15-20 hours.

7. The repair heat treatment method for 30Cr2Ni4MoV steel according to claim 1, characterized in that: In terms of weight percentage, the chemical composition of the 30Cr2Ni4MoV steel includes C: ≤0.37%, Si: ≤0.12%, Mn: 0.17-0.43%, Cr: 1.45-2.05%, Mo: 0.22-0.62%, Ni: 3.18-3.82%, V: 0.06-0.16%, Al: ≤0.012%, S: ≤0.015%, and P: ≤0.015%.

8. An application of the repair heat treatment method for 30Cr2Ni4MoV steel according to any one of claims 1 to 7, characterized in that: The repair heat treatment method is used to prepare a rotor forging, and the diameter of the rotor forging is φ1300-2000 mm.

9. Application of the repair heat treatment method for 30Cr2Ni4MoV steel according to claim 8, characterized in that: Preparation of the rotor forging The following steps are involved: S1: Smelting: Alloy raw materials are smelted to produce ingots; S2: Forging: Forging the ingot to obtain a forging; S3: heat treatment: performing post-forging heat treatment and quenching and tempering heat treatment on the forging; S4: The rotor forging is obtained after rework heat treatment.

10. Application of the repair heat treatment method for 30Cr2Ni4MoV steel according to claim 9, characterized in that: The post-forging heat treatment includes multiple normalizing and post-forging tempering.

11. Application of the repair heat treatment method for 30Cr2Ni4MoV steel according to claim 9, characterized in that: The quenching and tempering heat treatment includes quenching and quenching and tempering.

12. Application of the repair heat treatment method for 30Cr2Ni4MoV steel according to claim 9, characterized in that: The grain sizes of the rotor forging near the surface and the core are both 3.0-4.5, and the core can meet the flaw detection sensitivity required by the standard.

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