Heat treatment method for improving ultrasonic flaw detection capability of large-outer-diameter shaft head forgings

Abstract

The invention discloses a heat treatment method for improving the ultrasonic flaw detection capability of large-outer-diameter shaft head forgings, belongs to the technical field of heat treatment of forgings, and solves the problem of poor ultrasonic detection sensitivity of shaft body sections caused by large and non-uniform grain structures of large-outer-diameter shaft head forgings in the prior art. The heat treatment method comprises the following steps: S1, carrying out isothermal transformation preheat treatment on a forge piece, in the isothermal transformation preheat treatment, austenitizing the forge piece, and then carrying out isothermal transformation treatment, so that the whole forge piece is fully subjected to pearlite transformation from the surface to the central part; s2, the forge piece is machined, so that the surface of the forge piece is machined to be smooth; and S3, the forge piece is subjected to performance heat treatment so as to refine structure grains of the forge piece. According to the method, through preheating treatment and performance heat treatment including isothermal transformation, structure refinement and homogenization of the forge piece are achieved, and the shaft body center limit sensitivity of the large-outer-diameter-size shaft head type forge piece during UT flaw detection can be effectively improved.

Description

Technical Field

[0001] The invention relates to the technical field of heat treatment of forgings, and in particular to a heat treatment method for improving the ultrasonic flaw detection capability of large outer diameter shaft head forgings. Background Art

[0002] Large outer diameter shaft head forgings are widely used in the manufacturing field of high-end energy equipment such as nuclear power turbine units. Their shape characteristics are that the diameter of the shaft body is super large, the shaft diameter and flange size are relatively small, and the cross-sectional difference is large. Due to the harsh service environment conditions, the detection requirements for metallurgical defects such as non-metallic inclusions inside them are very strict. Ultrasonic testing (UT for short) is usually used to detect and inspect harmful defects inside forgings.

[0003] However, the outer diameter of the shaft section of the shaft head forging is very large. When ultrasonic waves of a certain frequency are emitted from the outer circle to the center for flaw detection, the sound path is long. If the grain size of the forging is coarse and uneven, the ultrasonic waves will be severely scattered and absorbed, resulting in poor central limit sensitivity of UT detection in the center of the shaft section, that is, small equivalent size defects in the center of the shaft cannot be found, which is easy to cause missed detection, which will greatly increase the failure risk of the forging during service. Summary of the invention

[0004] In view of the above analysis, an embodiment of the present invention aims to provide a heat treatment method for improving the ultrasonic flaw detection capability of large outer diameter shaft head forgings, so as to solve the problem that the existing large outer diameter shaft head forgings have large and uneven grain structure, resulting in poor ultrasonic detection sensitivity of the shaft body and easy missed detection.

[0005] An embodiment of the present invention provides a heat treatment method for improving the ultrasonic flaw detection capability of large outer diameter shaft head forgings, including: step S1, performing isothermal transformation preparatory heat treatment on the forging, in which the forging is subjected to isothermal transformation treatment after austenitization, so that the forging undergoes pearlite transformation as a whole from the surface to the center; step S2, mechanically processing the forging to smooth the surface of the forging; step S3, performing performance heat treatment on the forging to refine the microstructure grains of the forging.

[0006] Furthermore, in step S1, the forging is subjected to isothermal transformation preparatory heat treatment, including: step S11, charging the forging into a furnace and subjecting the forging to material cooling treatment; step S12, subjecting the forging after the material cooling treatment to a first stage heating treatment, wherein in the first stage the forging is heated to a stage temperature and kept warm, and in the second stage the forging is further heated to a first austenitizing temperature and kept warm, so as to austenitize the forging; step S13, subjecting the forging to isothermal transformation treatment.

[0007] Furthermore, in step S13, the forging is subjected to isothermal transformation treatment, including: cooling the forging to an isothermal transformation temperature and keeping the temperature, wherein the isothermal transformation temperature is 620-670° C. and the isothermal transformation time is 100-300 h.

[0008] Furthermore, in step S12, the stage temperature is 630-670°C, and the first austenitizing temperature is 870-930°C.

[0009] Furthermore, in step S3, the forgings are subjected to performance heat treatment, including: step S31, charging the forgings into a furnace and subjecting them to low-temperature insulation treatment; step S32, subjecting the forgings after the low-temperature insulation treatment to a second stage heating treatment, wherein in the first stage, the forgings are heated to a stage temperature and are kept warm, and in the second stage, the forgings are further heated to a second austenitizing temperature and are kept warm; step S33, taking the forgings out of the furnace and quenching them; and step S34, subjecting the quenched forgings to tempering treatment.

[0010] Furthermore, in step S32, during the second stage heating treatment, the stage temperature is 630-670°C, and the second austenitizing temperature is 840-880°C.

[0011] Furthermore, in step S34, tempering the forging includes: heating the forging to a cryogenic temperature and keeping the temperature constant; after the temperature constant is reached, heating the forging to a tempering temperature and keeping the temperature constant.

[0012] Furthermore, in the tempering treatment, the low temperature is 100-300°C and the tempering temperature is 580-620°C.

[0013] Furthermore, when the forging is taken out of the furnace for quenching, the forging is immersed in a water tank for quenching, and the temperature of the core of the end face of the forging is cooled to below 50°C before the water is removed; after the forging is quenched and taken out of the water, the re-temperature of the core of the end face of the forging is measured after being placed for a predetermined time; when the re-temperature is greater than 50°C, the forging is continued to be cooled according to the re-temperature.

[0014] Furthermore, the method further includes: step S4, processing the surface to be inspected of the forging to be smooth.

[0015] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0016] 1. By adopting the heat treatment method of the present invention, after the forging of the shaft head forging is completed, the preliminary heat treatment including isothermal transformation and performance heat treatment are performed to achieve the refinement and homogenization of the structure of the shaft head forging, so that the ultrasonic penetration ability of the shaft body section of the shaft head forging is greatly enhanced, thereby effectively improving the shaft body center limit sensitivity of the shaft head forging with large outer diameter during UT flaw detection, comprehensively and accurately checking the harmful defects inside the forging, and improving the reliability of the forging product.

[0017] 2. In the preparatory heat treatment and performance heat treatment, the present invention heats in stages so that the shaft head forgings are fully kept warm at a low temperature before austenite transformation occurs, so that the temperature of the surface and the core of the forgings tend to be consistent. Then, after heating and entering the austenite transformation temperature range, the temperature difference between the surface and the core of the shaft head forgings can be reduced, and the synchronization of the organizational transformation is improved.

[0018] 3. The present invention divides the tempering heating in the performance heat treatment into two stages. In the first stage, low-temperature insulation is performed to reduce the temperature difference between the surface and the core of the forging during the subsequent heating process; in the second stage, long-term tempering and insulation are performed to make the core of the forging reach the same temperature as the surface through heat transfer, and finally the surface and the core of the forging obtain a very uniform microstructure grain state.

[0019] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the embodiments of the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The 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 components throughout the drawings.

[0021] Figure 1 Schematic diagram of the structure of shaft head forgings according to some embodiments of the present invention.

[0022] Figure 2 Schematic diagram of isothermal transformation preparatory heat treatment process curve for some embodiments of the present invention.

[0023] Figure 3 This is a schematic diagram of the structure of forgings after being loaded into a furnace according to some embodiments of the present invention.

[0024] Figure 4 Schematic diagram of performance heat treatment process curves of some embodiments of the present invention.

[0025] Figure 5 Schematic diagram of the structure of quenching forgings in some embodiments of the present invention.

[0026] Figure 6 This is a process curve diagram of isothermal transformation preparatory heat treatment of Example 1 of the present invention.

[0027] Figure 7 This is a performance heat treatment process curve diagram of Example 1 of the present invention.

[0028] Figure 8Schematic diagram of UT flaw detection of forgings according to some embodiments of the present invention.

[0029] Fig. 9 Schematic diagram of UT flaw detection for shaft head forgings.

[0030] Fig.10 This is a preliminary heat treatment process curve diagram of comparative example 1 of the present invention.

[0031] Fig.11 This is a performance heat treatment process curve diagram of comparative example 1 of the present invention.

[0032] Fig.12 This is a schematic diagram of UT flaw detection of the forging of comparative example 1 of the present invention. DETAILED DESCRIPTION

[0033] In order to make the purpose, technical solutions and advantages of the present invention clearer, exemplary embodiments of the present invention will be described below in conjunction with the accompanying drawings. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not intended to limit the present invention. For the sake of clarity and conciseness, not all features of the actual implementation are described in the specification.

[0034] An embodiment of the present invention provides a heat treatment method for improving the ultrasonic flaw detection capability of large outer diameter shaft head forgings, which comprises the following steps:

[0035] Step S1, performing isothermal transformation preparatory heat treatment on the forging, in which the forging is subjected to isothermal transformation treatment after austenitization, so that pearlite transformation fully occurs in the forging from the surface to the center;

[0036] Step S2, machining the forging to make the surface of the forging smooth;

[0037] Step S3, performing a performance heat treatment on the forging to refine the microstructure grains of the forging.

[0038] According to the heat treatment method of the present invention, after the forging of the shaft head forging is completed, the shaft head forging is subjected to isothermal transformation preparatory heat treatment. During the treatment process, the forging is subjected to high-temperature austenitization and then isothermal transformation, so that the forging is fully transformed from the surface to the center, and a fine and uniform microstructure grain state is obtained, so that the microstructure is prepared for the subsequent performance heat treatment. After the preparatory heat treatment is completed, the forging is subjected to mechanical processing and then to performance heat treatment, so that while improving the mechanical properties of the alloy, the microstructure grains of the forging are ensured to be fine and uniform, so that the ultrasonic penetration ability of the shaft body section of the shaft head forging is greatly enhanced, thereby effectively improving the shaft body center limit sensitivity of the shaft head forging with a large outer diameter during UT flaw detection, comprehensively and accurately checking harmful defects inside the forging, and improving the reliability of the forging product.

[0039] The heat treatment method of the present invention is suitable for shaft head forgings with large outer diameters, such as Figure 1 As shown, usually, the shaft body outer diameter D1 ≥ 2500mm, the shaft body thickness H ≥ 600mm, the shaft diameter size D2 < D1, and the flange size D3 < D1.

[0040] Among them, the shaft head forgings are welded parts. In some embodiments, the steel grade of the shaft head forgings is 25Cr2Ni2MoV, which belongs to the Ni-Cr-Mo-V series steel with a low carbon content. The chemical composition range of the material (mass percentage wt.%) is shown in Table 1.

[0041] Table 1 Chemical composition range of shaft head forgings

[0042]

[0043] It should be noted that this grade of steel has a strong organizational inheritance, that is, it is difficult to refine the organizational grains. In addition, due to the large size specifications of shaft head forgings, they are large forgings. During the heat treatment process, there are large temperature field unevenness and asynchrony of organizational transformation in the surface and core during the heating and cooling processes. It is difficult for general heat treatment processes to refine and homogenize the overall organization of large outer diameter shaft head forgings, especially for the shaft body section. The larger the outer diameter of the shaft body, the greater the difference in surface and core unevenness.

[0044] The invention realizes the microstructure refinement and homogenization of the shaft head type forging by means of preliminary heat treatment and performance heat treatment including isothermal transformation.

[0045] In some embodiments, in step S1, the forging is subjected to isothermal transformation preparatory heat treatment, including:

[0046] Step S11, loading the forging into the furnace and performing a material cooling treatment;

[0047] Step S12, performing a first stage heating treatment on the forging after the material cooling treatment, wherein in the first stage, the forging is heated to a stage temperature and kept warm, and in the second stage, the forging is further heated to a first austenitizing temperature and kept warm, so as to austenitize the forging;

[0048] Step S13, performing isothermal transformation treatment on the forging.

[0049] In the embodiment of the present invention, when performing preliminary heat treatment, austenitization of the forging is achieved by heating in stages, which can ensure the uniformity and synchronization of the structural transformation of each part of the forging.

[0050] Specifically, in step S11, Figure 2As shown, the forged forging is loaded into a heat treatment furnace for waiting, with a waiting temperature of T1 and a holding time of h1. After the waiting is completed, the forging is cooled to a low holding temperature of T2 and a holding time of h2.

[0051] It should be noted that when the forging process of the shaft head forging is completed, the surface temperature is low but the core temperature is still very high, and the temperature difference between the two is large. In order to make the temperature of each part of the forging tend to be consistent, the present invention reduces the core temperature of the forging to be consistent with the surface before cooling by keeping the forging at temperature T1 for a certain period of time, thereby reducing the temperature difference between the surface and the core during the subsequent cooling process. Then, by keeping the forging at a low temperature section at temperature T2, the overall temperature of the shaft head forging is further completely reduced, so that the organization is fully transformed.

[0052] Among them, for the shaft head forgings of the materials shown in Table 1, the waiting temperature T1 ranges from 660 to 700°C, the waiting holding time h1 ranges from 10 to 25h; the low holding temperature T2 ranges from 200 to 250°C, and the low holding time h2 ranges from 10 to 25h. In addition, after the waiting is completed, the forging is cooled to T2 at a cooling rate of 5 to 25°C / h.

[0053] It should be noted that the material temperature T1 and the low temperature holding temperature T2 are determined according to the material characteristics of the shaft head forgings. Exceeding the corresponding temperature range will lead to insufficient organizational transformation; the cooling rate and holding time h1, h2 ranges are determined according to the cross-sectional dimensions of the shaft head forgings. Too slow cooling speed or too long holding time will increase production costs, while too fast cooling rate and too short holding time will increase the non-uniformity of the temperature field and increase the asynchronicity of organizational transformation. The present invention improves the uniformity and synchronization of the temperature field of the forgings by controlling the above parameters within a reasonable range, and ensures that the forgings complete a full organizational transformation.

[0054] like Figure 2 As shown, in step S12, when heating is performed in stages, in the first stage, the shaft head forgings are heated from a low holding temperature to a stage temperature T3, and the stage holding time is h3; in the second stage, the shaft head forgings are heated to a first austenitizing temperature T4, and the austenitizing holding time is h4.

[0055] It should be noted that the AC1 temperature of the shaft head forging (i.e., the starting temperature of the austenitization transformation of the structure during the heating process) is between T3 and T4. The present invention heats the shaft head forging in stages so that the shaft head forging is fully insulated at the T3 temperature before the austenite transformation occurs, so that the temperature of the surface and the core of the forging tends to be consistent. Then, after heating and entering the austenitization transformation temperature range, the temperature difference between the surface and the core of the shaft head forging can be reduced, and the synchronization of the structure transformation is improved. Moreover, after being fully insulated at the austenitization temperature T4, the forging is completely austenitized, so that various alloy elements are fully dissolved.

[0056] Among them, for the shaft head forgings in the above-mentioned chemical composition range, in the first stage, the forgings are heated to the stage temperature T3 at a heating rate of 20 to 50°C / h, the T3 temperature range is 630 to 670°C, and the stage insulation time h3 ranges from 10 to 30h; in the second stage, the forgings are heated to the first austenitizing temperature T4 at a heating rate of 70 to 100°C / h, the T4 temperature range is 870 to 930°C, and the austenitizing insulation time h4 ranges from 20 to 40h.

[0057] It should be noted that the heating rate and holding time range in each stage of step S12 are determined according to the cross-sectional dimensions of the shaft head forgings. A heating rate that is too slow and a holding time that is too long will increase production costs, while a heating rate that is too fast and a holding time that is too short will lead to increased temperature field non-uniformity, increased asynchrony in structural transformation, and insufficient austenitization process.

[0058] In some embodiments, in step S13, Figure 2 As shown, when the forging is subjected to isothermal transformation treatment, the forging is cooled to the isothermal transformation temperature T5 and kept warm, and the isothermal transformation time is h5; after the isothermal transformation is completed, the forging can be taken out of the furnace and air-cooled.

[0059] Specifically, the isothermal transformation temperature T5 is 620-670° C., and the isothermal transformation time h5 is 100-300 h.

[0060] It should be noted that the isothermal transformation process is the most critical link in the preparatory heat treatment process of the present invention. After being fully austenitized, the shaft head forgings are cooled to the isothermal transformation temperature T5. The T5 temperature range is the pearlite transformation temperature range of the shaft head forgings. The purpose of the embodiment of the present invention to keep the forgings at T5 temperature for a long time is to make the large outer diameter shaft head forgings fully undergo pearlite transformation from the surface to the center, obtain a fine and uniform microstructure grain state, and prepare the organization for subsequent performance heat treatment.

[0061] Among them, the temperature range of the isothermal transformation temperature T5 is determined according to the inherent material characteristics (TTT transformation curve) of the shaft head forging steel, and the isothermal transformation time is determined by the size specifications of the shaft head forgings. If the temperature range is exceeded or the holding time is too short, the pearlite transformation on the surface and core of the shaft head forgings cannot be fully completed. If the holding time is too long, the production cycle cost will increase.

[0062] There are forging pits, heat treatment scale, etc. on the surface of the forging that has undergone preliminary heat treatment. In order to remove the above-mentioned pits, scale, etc., the embodiment of the present invention performs mechanical processing in step S2, such as rough processing, to smooth the surfaces of the forging so that it meets the conditions for performance heat treatment.

[0063] In some embodiments, after mechanical processing in step S2, the roughness of the forging surface reaches Ra3.2. If the shaft head forgings are not rough-machined before the performance heat treatment, the black skin on the surface of the shaft head forgings will affect the heat conduction during the heating process; in addition, some small pits, surface cracks and other defects on the surface of the shaft head forgings may cause cracking during the quenching process. In order to avoid cracking of the forgings and poor heat conduction, the present invention processes the surface of the shaft head forgings to be smooth before the performance heat treatment.

[0064] In some embodiments, in step S3, the forging is subjected to a performance heat treatment, including:

[0065] Step S31, loading the forging into a furnace and performing low temperature insulation treatment;

[0066] Step S32, performing a second stage heating treatment on the forging after the low temperature insulation treatment, wherein in the first stage, the forging is heated to the stage temperature and kept warm, and in the second stage, the forging is further heated to the second austenitizing temperature and kept warm;

[0067] Step S33, taking the forging out of the furnace and quenching;

[0068] Step S34, tempering the quenched forging.

[0069] In the embodiment of the present invention, when performing performance heat treatment, the forging is subjected to low temperature heating, stage heating, furnace quenching and tempering treatment in sequence, thereby improving the mechanical properties of the forging and achieving the refinement and homogenization of the forging structure grains. Among them, the re-austenization of the forging is achieved by stage heating, which can ensure the uniformity and synchronization of the structure transformation of each part of the forging.

[0070] In some embodiments, in step S31, after the forging 2 is loaded into the heat treatment furnace 1, the central axis of the forging 2 remains horizontal. Figure 3As shown, a bottom shim 3 is arranged at the bottom of the heat treatment furnace 1, and the forging 2 is placed on the bottom shim 3. At the same time, shims 4 of different heights are arranged at different positions of the forging 2, such as below the shaft diameter and the flange, so that the forging 2 remains in a horizontal state.

[0071] In the embodiment of the present invention, the diameters of the shaft body and the shaft diameter of the large outer diameter shaft head forgings are very different. The shaft body has a large diameter and a large weight, while the shaft diameter is small and the axial length is long. This shape feature is easy to bend and deform during the heat treatment heating process. Therefore, the embodiment of the present invention adopts Figure 3 In the furnace loading method shown, the shaft body is directly in contact with the bottom pad iron to bear the weight of the forging, and the small shaft diameter and flange parts are supported by pad irons of different heights, which can make the central axis of the shaft head forgings in a horizontal state, greatly reducing the bending deformation of the shaft head forgings during the heat treatment process, especially in the high temperature stage of heat treatment. If the forgings are severely deformed, pressure straightening and stress relief treatment are required, which increases the manufacturing cost and cycle. For some special shaft head forgings, straightening is not allowed after performance heat treatment.

[0072] In some embodiments, when the forging is subjected to low temperature insulation in step S31, Figure 4 As shown, the forging is placed in a heat treatment furnace with a low temperature holding temperature T6 for holding, and the low temperature holding time h6. Specifically, the low temperature holding temperature T6 ranges from 200 to 400°C, and the low temperature holding time h6 ranges from 10 to 25 hours.

[0073] like Figure 4 As shown, in step S32, when heating in stages, in the first stage, the shaft head forgings are first heated from the low temperature insulation temperature T6 to the stage temperature T7, and the stage insulation time is h7; in the second stage, the shaft head forgings are heated to the second austenitizing temperature T8, and the austenitizing insulation time is h8.

[0074] It should be noted that the AC1 temperature of the shaft head forgings (i.e., the starting temperature at which the austenitization transformation of the structure begins during the heating process) is between T7 and T8. The purpose of the staged heating in step S32 of the present invention is the same as that in step S12, both of which are to reduce the temperature difference between the surface and the core of the forging during the austenitization process and improve the synchronization of the structure transformation.

[0075] In the embodiment of the present invention, the heating speed and holding time range of each stage in step S32 are determined according to the cross-sectional size of the shaft head forging. If the heating speed is too slow and the holding time is too long, the production cycle and cost will be increased. If the heating speed is too fast and the holding time is too short, the temperature field non-uniformity will increase, the asynchronism of the organizational transformation will increase, and the austenitization will be insufficient. The austenitization temperature range T8 is determined according to the material composition of the shaft head forging, which is generally 30 to 50°C higher than its AC3 temperature (the final temperature at which the organizational structure is completely transformed into austenite). If the austenitization temperature is too high, the grains are easy to grow, and if the austenitization temperature is too low, the austenitization process is insufficient.

[0076] In some embodiments, for shaft head forgings within the above-mentioned chemical composition range, in the first stage, the forgings are heated to the stage temperature T7 at a heating rate of 20 to 50°C / h, the T7 temperature range is 630 to 670°C, and the stage insulation time h7 ranges from 10 to 30 hours; in the second stage, the forgings are heated to the second austenitizing temperature T8 at a heating rate of 70 to 100°C / h, the T8 temperature range is 840 to 880°C, and the austenitizing insulation time h8 ranges from 20 to 40 hours.

[0077] It should be noted that the second austenitizing temperature T8 in step S32 is slightly lower than the first austenitizing temperature T4 in step S12, which can ensure that the alloy elements in the forging are fully dissolved through austenitizing during the preliminary heat treatment process, and the chemical composition is more uniform. Even if the microstructure grains are large, the grains can be refined through subsequent performance heat treatment; and in the performance heat treatment process, in order to take into account the final mechanical properties of the forging and make the grain size of the forging finer, the austenitizing temperature range is lowered to ensure the refinement of the microstructure grains of the final forging.

[0078] like Figure 4 As shown, after the second austenitizing temperature is maintained, the forging is taken out of the furnace and quenched.

[0079] In some embodiments, in step S34, Figure 4 As shown, the tempering treatment of the forging includes: heating the forging to a low temperature T9 and keeping it warm for a low temperature holding time of h9; after the holding is completed, the forging is further heated to a tempering temperature T10 and kept warm for a holding time of h10.

[0080] The embodiment of the present invention divides the tempering heating process into two stages. In the first stage, the temperature is kept at a low temperature of T9 to reduce the temperature difference between the surface and the core of the forging during the subsequent heating process. When the temperature just reaches T10, the temperature difference between the surface and the core of the shaft head forging with a large outer diameter will be very large, and the temperature of the core will be lower than the surface. In the second stage, a long period of tempering and heat preservation is performed. After heat transfer, the core of the forging reaches the same temperature as the surface, and finally the surface and the core of the forging obtain a very uniform microstructure grain state.

[0081] In some embodiments, during tempering, the low temperature T9 is 100-300°C, and the tempering temperature T10 is 580-620°C. Specifically, in the first stage, the forging is kept at a low temperature of 100-300°C, and the low temperature holding time h9 is 10-25h; in the second stage, the forging is heated to a tempering temperature of 580-620°C at a rate of 20-50°C / h, and the tempering time h10 is 30-60h, and then the forging is cooled to below 200°C at a rate of 5-25°C / h and then taken out of the furnace.

[0082] The embodiment of the present invention controls the tempering holding temperature T10 in the range of 580-620°C, which can remove the stress generated in the previous quenching process in this temperature range and make the forging obtain a uniform microstructure and grain state. If the tempering temperature T10 exceeds the range of 580-620°C, the final required microstructure and grain state and mechanical property level cannot be obtained.

[0083] In addition, the heating and cooling rates and the holding time in each stage of step S34 are determined according to the cross-sectional dimensions of the shaft head forgings. If the heating or cooling rate is too fast, the temperature difference between the surface and the core of the shaft head forgings will increase during the tempering process, thereby generating large internal stresses. If the heating or cooling rate is too slow, the production cycle and cost will be increased. If the holding time is too short, the tempering will be insufficient and the organizational state will not be uniform. If the time is too long, the cycle cost will be increased.

[0084] In some embodiments, when the forging is taken out of the furnace for quenching, the forging is immersed in a water tank for quenching. Figure 5 As shown, after the insulation is completed, the power is turned off and the furnace is opened, and the forging 2 is lifted horizontally by the quenching hanger 5 and immersed in the water tank 6 for quenching. In this process, the lifting time of the forging from the furnace to the water is controlled to be no more than 5 minutes to ensure that the temperature of the forging when entering the water is not lower than 840°C.

[0085] During the period from the opening of the heat treatment furnace door to the immersion of the forgings in water for quenching, the heat treatment furnace has been powered off, the forgings are lifted out and exposed to the atmosphere, and the surface temperature has been decreasing, so the forgings need to be lifted and quenched in water as soon as possible. The use of a dedicated quenching sling in the present invention facilitates the lifting of shaft head forgings by lifting equipment, which can greatly shorten the time from the opening of the furnace to the immersion of the shaft head forgings in water, so that the temperature of the shaft head forgings before immersion in water is maintained above 840°C.

[0086] In some embodiments, Figure 5As shown, the water tank 6 can be cylindrical, and an overflow tank 7 is formed in the water tank 6, and the overflow tank 7 is arranged along the circumferential direction of the water tank. During quenching, the diving position of the forging 2 in the water tank 6 is controlled so that the central axis of the forging 2 is located at the height center of the water tank 6, and the cooling water circulation is controlled to achieve quenching. Specifically, the circulating cooling water is controlled to flow from the bottom to the top of the water tank 6 and flow into the overflow tank 7; the water in the overflow tank 7 flows out into the water storage device and is cooled by the cooling equipment, and the cooled water enters the bottom of the water tank 6 to achieve cooling water circulation. Among them, the water circulation volume of the water tank is 7000~9000T / hour, and the water temperature is ≤18℃.

[0087] The embodiments of the present invention use circulating cooling water. Compared with non-circulating static water cooling, on the one hand, it can quickly take away the heat on the surface of the shaft head forgings, increase the heat transfer coefficient, and improve the cooling rate; on the other hand, it ensures that the cooling water temperature will not increase significantly during the entire quenching process, thereby improving the cooling effect.

[0088] In some embodiments, when the forgings are taken out of the furnace for quenching, the forgings are immersed in a water tank for quenching, and the temperature of the end core of the forging is cooled to below 50°C before the water is removed to ensure that the forgings achieve a good quenching effect and improve the mechanical properties of the forgings.

[0089] To ensure that the temperature of the core of the end face of the forging is cooled to below 50°C, after the forging is quenched and taken out of water, the re-temperature of the core of the end face of the forging is measured after a predetermined time; when the re-temperature is greater than 50°C, the forging is continued to be cooled according to the re-temperature; when the re-temperature does not exceed 50°C, the forging can be charged into the furnace for tempering as soon as possible.

[0090] The temperature of the shaft head forgings will rise after leaving the water, especially the temperature rise phenomenon of the core of the shaft body section of the shaft head forgings is the most obvious. The present invention measures the temperature return of the center of the shaft body end face after waiting for half an hour to determine whether the core of the forging has achieved the expected cooling effect. If it exceeds 50°C, it indicates that the cooling time is insufficient and the cooling should be continued for a certain period of time according to the measured temperature return data. Specifically, if the temperature return is greater than 50°C, the cooling should be continued for 10 minutes for every 1°C higher the temperature return is than 50°C.

[0091] In some embodiments, step S4, the surface to be tested of the forging is processed smooth to meet the ultrasonic testing requirements. Specifically, the roughness of the surface to be tested is at least Ra3.2. If the surface of the forging is too rough, it will affect the injection of ultrasonic waves and the reception of reflected echoes, which will have an adverse effect on the detection accuracy.

[0092] After the surface to be inspected of the forging is processed, the forging can be subjected to external cylindrical ultrasonic flaw detection.

[0093] The heat treatment method of the present invention is further described below with reference to specific examples.

[0094] Example 1

[0095] The steel grade of the shaft head forgings is 25Cr2Ni2MoV, which belongs to the Ni-Cr-Mo-V series steel with low carbon content. Its specific chemical composition is shown in Table 2.

[0096] Table 2 Chemical composition of the shaft head forgings in Example 1

[0097] C Si Mn Cr Ni Mo V 0.22wt.% 0.05wt.% 0.23wt.% 2.20wt.% 2.25wt.% 0.70wt.% 0.06wt.%

[0098] The specific dimensions of the forging are: the outer diameter D1 of the shaft section is φ3000mm, the thickness H of the shaft section is 800mm, the shaft diameter D2=1000mm, the flange diameter D3=1500mm, and the total length L of the forging is 5000mm.

[0099] Heat treatment of the above forgings:

[0100] S1. Isothermal transformation preparatory heat treatment

[0101] S11: Load the furnace and wait for the material to cool down. Figure 6 As shown, the forged large outer diameter shaft head forgings are loaded into a heat treatment furnace for waiting at a temperature of 680±10°C and a holding time of 15 hours. Thereafter, the temperature is cooled to a low holding temperature of 230±10°C at a cooling rate of 10°C / h for 15 hours.

[0102] S12: Heating in stages. Figure 6 As shown, in the first stage, the forgings are heated from 230±10℃ to the stage temperature of 650±10℃ at a speed of 30℃ / h, and the stage insulation time is 20h; in the second stage, the shaft head forgings are heated to the austenitizing temperature of 900±10℃ at a speed of 80℃ / h, and the insulation time is 30 hours.

[0103] S13: Isothermal transformation. Figure 6 As shown, the forging is first cooled to the isothermal transformation temperature of 660±10℃ at a rate of 10℃ / h, kept at this temperature for 200 hours to fully complete the isothermal transformation, and then taken out of the furnace and air-cooled to room temperature.

[0104] S2. Machining

[0105] After the preliminary heat treatment is completed, the forgings are rough-machined to remove the forging pits, oxide scale, etc. on the surface of the forgings to make them bright, in preparation for the performance heat treatment.

[0106] S3, Performance Heat Treatment

[0107] S31: Load the furnace and keep it warm at low temperature. According to the size of the forging, the outer diameter of the shaft body is φ3000mm, the shaft body thickness is 800mm, and the total length is 5000mm. A bell-type heat treatment electric furnace with a size of φ9m×5m is selected to perform the performance heat treatment process. First, place a 1000mm high pad horizontally on the heat treatment furnace table, and then place the forging horizontally on the pad. The shaft diameter and flange parts are supported and stabilized with pads of different heights to prevent deformation. The axis of the forging is parallel to the plane of the furnace table. Figure 3 As shown. At this time, the height of the central axis of the forging is about 2500mm, and the forging is in the center of the effective heating area of ​​the heat treatment furnace. Figure 7 As shown, the low temperature insulation temperature is 250±10℃, and the low temperature insulation time is 15h.

[0108] S32: Heating in stages. Figure 7 As shown, in the first stage, the shaft head forgings are heated from 250±10℃ to the stage temperature of 650±10℃ at a speed of 30℃ / h, and the stage holding time is 20h; in the second stage, the shaft head forgings are heated to the austenitizing temperature of 860±10℃ at a speed of 80℃ / h, and the austenitizing holding time is 30h, and then they are taken out of the furnace for quenching.

[0109] S33: Quenching after leaving the furnace. Figure 5 As shown, the shaft head forgings are hoisted horizontally by quenching hoists and immersed in a cylindrical water tank for quenching. The hoisting time of the shaft head forgings from the furnace to the water is ≤5min, and the surface temperature of the forgings when entering the water is not less than 840℃. The diving position of the shaft head forgings is controlled so that the horizontal axis position is located at the height center of the water tank. The water circulation volume of the water tank is 8000T / hour, and the water temperature is ≤18℃. After 12 hours of water circulation cooling, the water is discharged, and the temperature of the center of the end face is measured after half an hour of placement. If the return temperature is ≤50℃, it can be loaded into the furnace for tempering as soon as possible. If the return temperature of the end face center is greater than 50℃, it is cooled for 10 minutes more for every 1℃ higher than 50℃.

[0110] S34: Tempering. Figure 7 As shown, in the first stage, the forgings are kept at a low temperature of 150±10℃ for 15h; in the second stage, the shaft head forgings are heated to the tempering temperature of 610±10℃ at a rate of 30℃ / h, the tempering holding time is 50h, and then the forgings are cooled to 200℃ at a rate of 10℃ / h and taken out of the furnace.

[0111] S4. Processing ultrasonic flaw detection surface

[0112] Before ultrasonic testing, the outer cylindrical surface of the forging to be tested needs to be processed to light and processed to a roughness of Ra3.2 to meet the surface condition requirements of ultrasonic flaw detection.

[0113] The ultrasonic flaw detection capability of the forging of Example 1 was evaluated.

[0114] Use a 4MHz frequency ultrasonic instrument probe for testing, evenly apply engine oil as a coupling agent on the outer surface of the shaft body, and scan defects from the outer diameter of the shaft body of the forging to the center. Because the diameter of the shaft body D1 = Φ3000mm, the sound path of ultrasonic testing emitted from the outer circle of the shaft body to the center is the longest, the scattering and absorption of ultrasonic waves is the most serious, and the detection difficulty is the greatest. Therefore, the central limit sensitivity of UT testing at the outer circle of the shaft body can represent the ultrasonic flaw detection capability of the entire shaft head forging.

[0115] It should be noted that usually the outer circle of the shaft body section, the outer circle of the shaft diameter and the outer circle of the flange of the shaft head forging need to be tested by UT, because D1>D2, D1>D3, and the diameters of D1, D2 and D3 are very different. Therefore, if Figure 8 As shown, when emitting ultrasonic waves from the outer circle to the center, using an ultrasonic probe of the same frequency, it is much more difficult to detect defects of the same equivalent size in the center of the shaft section at the outer circle of the D1 section than at the outer circles of D2 and D3.

[0116] In actual production, large-sized shaft sections are often reworked and reheated because the center limit sensitivity of the outer circle UT detection does not meet the requirements. Therefore, this embodiment only examines the center limit sensitivity that can be achieved by ultrasonic scanning from the outer circle of the shaft section of the forging to the center to evaluate the ultrasonic flaw detection capability of the shaft head forging, that is, the minimum equivalent size of the defect at the center of the shaft that can be detected, such as Φ2, Φ1.6, Φ1.0, etc., in mm. The smaller the equivalent size, the higher the center limit sensitivity of UT detection and the stronger the detection capability.

[0117] like Fig. 9 As shown, the minimum equivalent size of defects that can be detected in the center of the shaft section is Φ1.4mm, that is, the UT detection of this forging from the outer circle of the shaft section is Φ1.4mm. It can be seen that for the forging with an extra-large outer diameter in Example 1, after isothermal transformation preparatory heat treatment and performance heat treatment, the ultrasonic detection center limit sensitivity of the forging shaft section is already very high, which fully meets the requirements of the product-related standards for the ultrasonic detection center limit sensitivity.

[0118] Comparative Example 1

[0119] The chemical composition and size of the forgings of Comparative Example 1 are similar to those of Example 1. The difference is that the preliminary heat treatment process of Comparative Example 1 does not use a staged heating method, does not contain an isothermal transformation process, and the performance heat treatment process does not use a staged heating method, as follows.

[0120] The steel grade of the shaft head forging is 25Cr2Ni2MoV, which belongs to the Ni-Cr-Mo-V series steel with low carbon content. The specific chemical composition (wt.%) is shown in Table 3.

[0121] Table 3 Chemical composition of forgings in comparative example 1

[0122] C Si Mn Cr Ni Mo V 0.22 0.06 0.20 2.23 2.26 0.68 0.05

[0123] The specific dimensions are: the shaft body section outer diameter D1 is φ2950mm, the shaft body section thickness H is 780mm, the shaft diameter D2=950mm, the flange diameter D3=1450mm, and the total length of the forging L=4950mm.

[0124] The above forgings are heat treated, and the specific process is as follows:

[0125] 1. Preparatory heat treatment

[0126] (1) Load the furnace and wait for the material to cool down. Fig.10 As shown, the forged large outer diameter shaft head forgings are loaded into a heat treatment furnace for waiting at a temperature of 680±10°C and a holding time of 15 hours. Thereafter, the temperature is cooled to a low holding temperature of 230±10°C at a cooling rate of 10°C / h for 15 hours.

[0127] (2) Heating and normalizing. Fig.10 As shown, the forging is heated from 230±10℃ to the austenitizing temperature of 900±10℃ at a rate of 30℃ / h, kept at this temperature for 30 hours, and then taken out of the furnace for air cooling and normalizing.

[0128] It should be noted that before the forging is heated to the austenite transformation temperature range, no intermediate stage insulation is set, so there is a large temperature difference between its surface and core. In this state, after the heating temperature enters the austenite transformation temperature (AC1), the surface temperature is high and the austenite structure transformation occurs first. At this time, the core temperature has not reached the austenite transformation temperature (AC1) and no structure transformation occurs, that is, the structure transformation lags. Therefore, the austenite transformation of the forging as a whole is very asynchronous, which can easily cause uneven grain size. Even if the austenitizing temperature is kept at 900±10℃ for a long time, the previously formed unevenness is difficult to eliminate, resulting in uneven grain state of the forging before performance heat treatment.

[0129] (3) Tempering. Fig.10 As shown, the forging is heated from room temperature to the tempering temperature of 650±10℃ at a rate of 30℃ / h, kept at 650±10℃ for 50 hours, and then cooled to 200℃ at a rate of 10℃ / h and taken out of the furnace.

[0130] It should be noted that after the forging is cooled from the higher austenitizing temperature of 900±10℃ through normalizing and air cooling, the surface hardness is high, which is not conducive to cutting. This comparative example reduces the hardness of the forging by tempering to facilitate subsequent rough machining. In contrast, the forging of Example 1 is cooled from the isothermal transformation temperature of 660±10℃ to room temperature, and the hardness is relatively low, which is suitable for mechanical rough machining and does not require tempering.

[0131] 2. Machining

[0132] After the preliminary heat treatment is completed, the forgings are rough-machined to remove the forging pits, oxide scale, etc. on the surface of the forgings to make them bright, in preparation for the performance heat treatment.

[0133] 3. Performance heat treatment

[0134] (1) Load the furnace and keep it warm at low temperature. According to the size of the forging, the outer diameter of the shaft body is φ2950mm, the shaft body thickness is 780mm, and the total length is 4950mm. A bell-type heat treatment electric furnace with a size of φ9m*5m is selected to perform performance heat treatment. Figure 3 As shown in the figure, first place a 1000mm high pad horizontally on the heat treatment furnace, then place the forging horizontally on the pad, and use pads of different heights to support the shaft diameter and flange to prevent deformation. The axis of the forging is parallel to the plane of the furnace. At this time, the height of the central axis of the forging is about 2500mm, and the forging is in the center of the effective heating area of ​​the heat treatment electric furnace. Fig.11 As shown, the low temperature insulation temperature is 250±10℃, and the low temperature insulation time is 15h.

[0135] (2) Heating. Fig.11 As shown, the forging is heated from 250±10℃ to the austenitizing temperature of 860±10℃ at a speed of 30℃ / h, kept at this temperature for 30h, and then taken out of the furnace for quenching.

[0136] It should be noted that in the heating process of the forging in Example 1, there is no intermediate stage of heat preservation, so there is a large temperature difference between the surface and the core before the heating enters the austenite transformation temperature range. In this state, after the heating temperature enters the austenite transformation start temperature (AC1), the temperature of the surface of the forging is high, and the austenite structure transformation occurs first. At this time, the temperature of the core has not reached the austenite transformation temperature (AC1), and the structure transformation has not occurred, that is, the structure transformation is delayed. Therefore, the austenite transformation of the forging as a whole in Example 1 is very asynchronous, which can easily cause uneven grain size. Even if the austenitizing temperature of 860±10℃ is kept for a long time, the previously formed unevenness is difficult to eliminate, resulting in uneven grain state of the structure of the forging in Example 1 after the performance heat treatment is completed, which has an adverse effect on the center limit sensitivity of ultrasonic flaw detection.

[0137] (3) Quenching after exiting the furnace. Use a quenching hoist to lift the shaft head forgings horizontally and immerse them in a cylindrical water tank for quenching. The lifting time of the shaft head forgings from exiting the furnace to entering the water is ≤5min. The temperature of the forgings when entering the water shall not be lower than 840℃. The diving position of the shaft head forgings is controlled so that the horizontal axis position is located at the height center of the water tank. The water circulation volume of the water tank is 8000T / hour and the water temperature is ≤18℃. After 12 hours of water circulation cooling, the water is discharged. After half an hour of standing, the temperature of the center of the end face is measured. If the return temperature is ≤50℃, it can be loaded into the furnace for tempering as soon as possible. If the return temperature of the center of the end face is greater than 50℃, it is calculated as 10 minutes more cooling for every 1℃ higher than 50℃.

[0138] (4) Tempering. Fig.11 As shown, the forging is first heated from room temperature to a tempering temperature of 610±10°C at a rate of 30°C / h, and the tempering holding time is 50h. Then, the forging is cooled to 200°C at a rate of 10°C / h and taken out of the furnace.

[0139] 4. Processing of ultrasonic flaw detection surface

[0140] Before ultrasonic testing, the outer cylindrical surface of the forging to be tested is processed to light, and the roughness is at least Ra3.2, which meets the surface condition requirements of ultrasonic flaw detection.

[0141] The forgings of Comparative Example 1 were evaluated for their ultrasonic flaw detection capabilities.

[0142] Use a 4MHz frequency ultrasonic instrument probe for testing, evenly apply engine oil as a coupling agent on the outer surface of the shaft body, and scan defects from the outer diameter of the shaft body of the forging to the center. Because the shaft body diameter D1 = 2950mm, the sound path of ultrasonic testing from the outer circle of the shaft body to the center is the longest, the scattering and absorption of ultrasonic waves is the most serious, and the detection difficulty is the greatest. Therefore, the central limit sensitivity of UT testing at the outer circle of the shaft body can represent the ultrasonic flaw detection capability of the entire forging.

[0143] like Fig.12 As shown, the minimum equivalent size of defects that can be detected in the center of the shaft body section of the shaft head type forging in comparative example 1 is Φ2.5mm, that is, the radial scanning center limit sensitivity of UT detection of this forging from the outer circle direction of the shaft body is Φ2.5mm. The ultrasonic flaw detection capability of this shaft head type forging cannot meet the requirements of the ultrasonic detection center limit sensitivity, and rework and re-heat treatment are required, which greatly increases the production cycle and cost.

[0144] In addition, the UT test diagram of the comparative example 1 ( Figure 8 ) and the UT test diagram of Comparative Example 1 ( Fig.12), the outer diameter and axial thickness of the shaft section of the shaft head forgings of Example 1 are slightly smaller than those of Example 1. Assuming that the same preparatory heat treatment process and performance heat treatment process are implemented for both, theoretically, the outer circle is scanned from the outer circle to the center using an ultrasonic probe of the same frequency, the ultrasonic detection sound path of Example 1 is shorter, and the center limit sensitivity should be better. However, the measured UT center limit sensitivity of Example 1 is much worse than that of Example 1, indicating that the preparatory heat treatment method and the performance heat treatment method of the present invention effectively improve the center limit sensitivity of ultrasonic flaw detection of shaft head forgings with large outer diameter.

[0145] The above description is only a 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 conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A heat treatment method for improving the ultrasonic flaw detection capability of large outer diameter shaft head forgings, characterized in that: include: Step S1, performing isothermal transformation preparatory heat treatment on the forging, in which the forging is subjected to isothermal transformation treatment after austenitization, so that pearlite transformation fully occurs in the forging from the surface to the center; Step S2, machining the forging to make the surface of the forging smooth; Step S3, performing a performance heat treatment on the forging to refine the microstructure grains of the forging.

2. The heat treatment method according to claim 1, characterized in that In the step S1, the forging is subjected to isothermal transformation preliminary heat treatment, including: Step S11, loading the forging into a furnace and performing a material cooling treatment; Step S12, performing a first stage heating treatment on the forging after the material cooling treatment, wherein in the first stage, the forging is heated to a stage temperature and kept warm, and in the second stage, the forging is further heated to a first austenitizing temperature and kept warm, so as to austenitize the forging; Step S13, performing isothermal transformation treatment on the forging.

3. The heat treatment method according to claim 2, characterized in that: In the step S13, the forging is subjected to the isothermal transformation treatment, including: The forging is cooled to an isothermal transformation temperature and kept warm, wherein the isothermal transformation temperature is 620-670° C. and the isothermal transformation time is 100-300 hours.

4. The heat treatment method according to claim 2, characterized in that: In the step S12, the stage temperature is 630-670°C, and the first austenitizing temperature is 870-930°C.

5. The heat treatment method according to claim 1, characterized in that: In the step S3, the forging is subjected to a performance heat treatment, including: Step S31, loading the forging into a furnace and performing low temperature insulation treatment; Step S32, performing a second stage heating treatment on the forging after the low temperature insulation treatment, wherein in the first stage, the forging is heated to the stage temperature and kept warm, and in the second stage, the forging is further heated to the second austenitizing temperature and kept warm; Step S33, taking the forging out of the furnace and quenching it; Step S34, tempering the forged part after quenching.

6. The heat treatment method according to claim 5, characterized in that: In the step S32, during the second staged heat treatment, the stage temperature is 630-670°C, and the second austenitizing temperature is 840-880°C.

7. The heat treatment method according to claim 5, characterized in that: In the step S34, performing the tempering treatment on the forging includes: heating the forging to a cryogenic temperature and maintaining the temperature; After the heat preservation is completed, the forging is further heated to the tempering temperature and then kept warm.

8. The heat treatment method according to claim 7, characterized in that: In the tempering treatment, the low temperature is 100-300°C, and the tempering temperature is 580-620°C.

9. The heat treatment method according to claim 5, characterized in that: When the forging is taken out of the furnace for quenching, the forging is immersed in a water tank for quenching, and the temperature of the end face center of the forging is cooled to below 50° C. and then the water is removed; After the forging is quenched and taken out of water, the temperature of the center of the end face of the forging is measured after being left for a predetermined time; When the re-warming temperature is greater than 50° C., the forging is further cooled according to the re-warming temperature.

10. The heat treatment method according to any one of claims 1 to 9, characterized in that: The method further comprises: step S4, machining the surface of the forging to be inspected to be smooth.

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