A heat treatment method for 34CrNi2Mo large forged steel parts after forging

Abstract

The application relates to the technical field of metal material heat treatment, and discloses a 34CrNi2Mo large forged steel part post-forging heat treatment method, which comprises the following steps of S1, preheating a heat treatment furnace to a certain temperature, loading workpieces into the furnace, segmentally heating the workpieces at different heating speeds, and respectively maintaining the workpieces for different time; S2, forcibly air cooling the workpieces to 550+ / -50 DEG C, and then air cooling the workpieces to 300+ / -50 DEG C; S3, again segmentally heating the workpieces at different heating speeds, and respectively maintaining the workpieces for different time; and S4, controlling the workpieces to be cooled to 400+ / -50 DEG C at a cooling speed of 50-90 DEG C / h, and then taking out the workpieces to be cooled to room temperature. The application makes the 34CrNi2Mo steel material after heat treatment have fine and uniform grains (5-8 levels), no mixed grains, and meanwhile, the product ultrasonic flaw detection quality is guaranteed; in addition, the product processing time is greatly shortened, the production efficiency is improved compared with a traditional process, and energy is saved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of heat treatment of large forged steel parts, in particular to a 34CrNi2Mo large forged steel part post-forging heat treatment method. BACKGROUND

[0002] 34CrNi2Mo steel has excellent hardenability due to high nickel and molybdenum content, and is often used in engine camshafts and connecting rods, and large forgings such as rotors, fin shafts, etc. However, for the manufacture of large forgings, they must be forged from large ingots, and during the manufacturing process, there will inevitably be non-uniformity in the organization and composition of the dendritic structure.

[0003] The reason for this phenomenon is that the high-temperature austenitizing of this steel is quite stable, and during air cooling, no pearlite phase change occurs. After normalizing, coarse austenite grains easily appear and there is a tendency for the organization to be inherited (i.e., the phenomenon of the original coarse grains being inherited and restored when the non-equilibrium original organization such as martensite and bainite is re-austenitized), resulting in coarse grains and abnormal, non-equilibrium organization in the 34CrNi2Mo large forged steel part after post-forging heat treatment. Due to the coarse-grained non-equilibrium organization of the forged steel, the waveform of the large forged steel appears as grassy waves during ultrasonic flaw detection, interfering with the defect signal, making it difficult to judge the internal quality of the forged steel, and even making it impossible to judge.

[0004] In order to solve the problem of coarse grains and non-uniformity caused by the organization inheritance of large forgings, the existing technology usually adopts multiple forging and post-forging heat treatment processes to improve it. However, this heat treatment process is complex and has a long process cycle, and in addition, large forgings heat slowly, with a large temperature difference between different parts. In actual production, it is difficult to balance the holding time of each part, resulting in poor refining effect. The conditions for refining grains by conventional normalizing are difficult to meet, and in areas with severe dendritic structures, there will still be severe coarse grains and mixed crystals, further increasing the number of normalizing / annealing times, time and energy consumption.

[0005] Therefore, in order to solve the problem of coarse grains and non-uniformity caused by the organization inheritance of 34CrNi2Mo large forgings in the prior art, while shortening the processing cycle and reducing energy consumption, it is necessary to provide a 34CrNi2Mo large forged steel part post-forging heat treatment method. SUMMARY

[0006] The present application aims to provide a 34CrNi2Mo large forged steel part post-forging heat treatment method to solve the problem of coarse grains and non-uniformity caused by the organization inheritance of 34CrNi2Mo large forgings in the prior art, while shortening the processing cycle and improving the production efficiency.

[0007] To achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0008] The application is used for providing a 34CrNi2Mo large forging steel piece post-forging heat treatment method, so that the grain size of the 34CrNi2Mo large forging steel piece after heat treatment is fine and uniform, the grain size meets the technical requirement of 5-8 levels, the ultrasonic flaw detection quality is guaranteed, the product processing time is greatly shortened, the production efficiency is improved compared with the traditional process, and the energy consumption is reduced; specifically, the 34CrNi2Mo large forging steel piece post-forging heat treatment method comprises the following steps:

[0009] S1, preheat the heat treatment furnace to a certain temperature, then load the workpiece into the furnace and segmentally heat the workpiece at different heating speeds, and respectively keep warm for different time;

[0010] S2, forcibly air cool the workpiece to 550±50℃, and then air cool to 300±50℃;

[0011] S3, segmentally heat the workpiece at different heating speeds again, and respectively keep warm for different time;

[0012] S4, control the workpiece to cool to 400±50℃ at a cooling speed of 50-90℃ / h, and then take out the workpiece to cool to room temperature.

[0013] The principle and advantages of the scheme are:

[0014] The principle of the scheme is that it is generally believed that the more needle-shaped austenite formed in the heating austenitizing process, the easier it is to restore the original coarse austenite grains due to the consistent spatial orientation of the needle-shaped austenite, and if more spherical austenite is formed, the original ordered spatial orientation can be disturbed, and the restoration of coarse austenite grains is prevented.

[0015] And research shows that when the non-equilibrium structure of 34CrNi2Mo steel is heated at a very slow speed (≤2-3℃ / min), needle-shaped austenite is easily generated and the organization inheritance is caused; when heated at a medium speed (100-300℃ / min), spherical austenite is easily generated, and no organization inheritance occurs; if the heating speed is extremely fast (>100-500℃ / s), the non-equilibrium structure of the steel will be directly restored to the original austenite grain morphology and orientation in the form of martensite reverse transformation.

[0016] Therefore, the present scheme has the advantages that, by using the preheating mode, the generation of acicular austenite is reduced first, then the generation of spheroidized austenite is facilitated by the segmented heating mode, so as to prevent the subsequent occurrence of the organizational inheritance phenomenon, and meanwhile, by controlling the temperature and the holding time in segments, the non-uniform grain structure after forging can be reordered and the grains can be refined. Then, by using the forced air cooling mode, the residual austenite generated by the normalizing can be fully decomposed to block the inheritance tendency. Then, the segmented temperature rising and holding treatment is performed to prevent the abnormal growth of the austenite grains, so as to completely block the organizational inheritance and achieve the effects of eliminating the mixed grains and refining the grains.

[0017] Preferably, as an improvement, in S1, the heat treatment furnace is preheated to 500±50℃. The heat treatment furnace is preheated first, so as to accelerate the heating speed of the large-scale forged piece and refine the grains.

[0018] Preferably, as an improvement, in S1, the workpiece is loaded on the trolley furnace in an amount less than the rated loading capacity of the heat treatment furnace, and the workpiece is isolated by using the pad iron on the trolley surface to make the distance between the workpieces ≥300mm. The larger interval distance can make the workpiece heating and cooling cycle more sufficient and the structure more uniform.

[0019] Preferably, as an improvement, in S1, the workpiece is first heated to a first temperature at a first heating speed and held for m minutes, and then heated to a second temperature at a second heating speed and held for n minutes. The workpiece is in a cold hard state above the first temperature, and in order to reduce the internal and external temperature difference and internal stress of the workpiece, it is necessary to hold at the first temperature for a period of time. Then, the workpiece is heated to the second temperature at the second heating speed and held, so as to fully austenitize the workpiece and be more conducive to the generation of spheroidized austenite.

[0020] Preferably, as an improvement, in S4, the workpiece is heated to a third temperature at a first heating speed and held for m minutes, and then heated to a fourth temperature at a second heating speed and held for n minutes. The workpiece is austenitized twice, the first time is heated to the third temperature and held, to eliminate the stress of the workpiece, and the second time is heated to the fourth temperature to austenitize, to ensure that the workpiece is fully austenitized and is more conducive to the generation of spheroidized austenite.

[0021] Preferably, as an improvement, the first heating speed is 80-120℃ / h; and the second heating speed is 300-500℃ / h. When the two-phase region (austenite and ferrite coexist) is rapidly heated, it is easy to generate spheroidized austenite and prevent the subsequent occurrence of the organizational inheritance phenomenon.

[0022] Preferably, as an improvement, the first temperature and the third temperature are the same, both being 600-650℃; the second temperature is 880-920℃; and the fourth temperature is 840-880℃. The grains are further refined by holding at a slightly lower temperature.

[0023] Preferably, as an improvement, the holding time is related to the workpiece thickness. Let the workpiece thickness be δ mm, then the holding time m = (0.5~0.8)×δ min; the holding time n = (1~1.5)×δ min. Strict control of temperature and holding time is crucial to avoid excessively long times or excessively high temperatures, thus preventing abnormal growth of austenite grains.

[0024] Preferably, as an improvement, in S2, an axial convection fan is used to force-cool the workpiece. The greater degree of supercooling allows the residual austenite produced during normalizing to decompose fully, effectively blocking the tendency for its inheritance.

[0025] Preferably, as an improvement, in S4, when removing the workpiece, the workpiece is completely driven out of the furnace along with the trolley, allowing the workpiece to be fully exposed to air and cooled to room temperature. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of the process of the present invention;

[0027] Figure 2 This is a schematic diagram of the process temperature curve of the present invention. Detailed Implementation

[0028] The following detailed description illustrates the specific implementation method:

[0029] Example 1

[0030] The basics are as follows: Figure 1 The method described is a post-forging heat treatment method for large 34CrNi2Mo forgings. For large 34CrNi2Mo forgings (≥φ400mm), the process involves preheating, austenitizing, rapid cooling, followed by preheating again, lowering the temperature for further austenitizing, and then controlled cooling. This results in a fine and uniform grain size in the heat-treated 34CrNi2Mo forgings, meeting the technical requirement of grade 5-8. This ensures the quality of ultrasonic testing and eliminates the need for additional heat treatment, reducing the heat treatment production cycle and costs, and improving production efficiency. See attached diagram for details. Figure 2 As shown, it includes the following steps:

[0031] Step 1: Preheat the heat treatment furnace to 500±50℃ and wait for the material. Load the workpiece onto the trolley furnace with a load capacity less than the rated capacity of the heat treatment furnace. At the same time, use shims to isolate the workpiece on the trolley surface, so that the distance between the workpieces is ≥300mm, to ensure that the heating and cooling cycle of the workpiece is more complete and the structure is more uniform. Send the workpiece and the trolley together into the heat treatment furnace.

[0032] Step 2: Turn on the heating system of the heat treatment furnace and simultaneously turn on the furnace fan to heat the workpiece to 600℃ at a rate of 80℃ / h and hold it at that temperature for m minutes. Controlling the heating rate of the workpiece helps to form spherical austenite and reduce the content of acicular austenite. In this embodiment, the holding time is related to the thickness of the workpiece. For a large 34CrNi2Mo forged steel workpiece, assuming an effective thickness of 500mm, the holding time m is 250 minutes to reduce the internal and external temperature difference and internal stress within the workpiece, ensuring full austenitization.

[0033] Step 3: After the initial heat treatment, heat the workpiece to 880℃ at a rate of 300℃ / h and hold for n minutes. The holding time n is set to 500 minutes based on the effective thickness of the workpiece. Rapid heating of the two-phase region (i.e., the region where austenite and ferrite coexist) facilitates the formation of spherical austenite, preventing subsequent microstructure inheritance. Simultaneously, strict control of temperature and holding time ensures that the higher temperature causes the uneven grain structure after forging to rearrange, achieving a grain refinement effect.

[0034] Step 4: After the process holding time is reached, stop heating, fully open the furnace door, and completely drive the workpiece out of the furnace along with the trolley, exposing it to the air. Use an axial convection fan to force-cool the workpiece to a temperature of 550±50℃, then turn off the axial convection fan and allow the workpiece to air-cool to 300±50℃. This significant undercooling refines the grains and inhibits the inheritance of genetic tendencies.

[0035] Step 5: Push the workpiece back into the heat treatment furnace, turn on the furnace heating system, and simultaneously turn on the furnace fan to raise the workpiece to 600°C at a heating rate of 80°C / h, and hold it at that temperature for 250 minutes.

[0036] Step 6: After the heat treatment is completed, turn on the heating system of the heat treatment furnace and simultaneously turn on the furnace fan to raise the workpiece to 840℃ at a heating rate of 300℃ / h, and hold it at that temperature for 500 minutes. Hold it at a slightly lower temperature to further refine the grains, while strictly controlling the temperature and holding time to prevent abnormal growth of austenite grains.

[0037] Step 7: After reaching the required holding time, stop heating and cool the workpiece to 400±50℃ at a controlled cooling rate of 50℃ / h. Slow cooling completely blocks the microstructure, thereby eliminating mixed crystals and refining the grains. After reaching the desired temperature, fully open the furnace door and remove the workpiece along with the trolley from the furnace, exposing it to air to cool to room temperature. The test results after heat treatment are detailed in Table 1.

[0038] In the existing heat treatment process, in order to ensure the performance of the workpiece after heat treatment, the focus is placed on the heating process and the quenching process, and emphasis is placed on one-time processing treatment to ensure that the workpiece fully reacts, and it is also considered that the workpiece has an irreversible phenomenon in the reaction process, so it is considered that the heat treatment reaction can only be completed once, and the reaction effect is ensured as much as possible to meet the performance requirements. If the performance of the workpiece after heat treatment does not meet the requirements, the heat treatment process must be performed again to improve or enhance the performance of the workpiece.

[0039] In the embodiment, the inherent thinking that austenitizing and quenching in heat treatment can only be performed once is broken, and the austenitizing of the workpiece in heat treatment is divided into four stages, and austenitizing is performed again after cooling. The heating rate, temperature and holding time of each austenitizing stage are strictly controlled, so as to fundamentally control the generation and transformation conditions of globular austenite, reduce acicular austenite, make the grain size small and uniform, meet the technical requirements, avoid repeated heat treatment, greatly reduce the production cycle and improve the production efficiency.

[0040] Example Two

[0041] In the embodiment, unlike the first embodiment,

[0042] In step 2, the temperature is raised to 650°C at a rate of 120°C / h, and the holding time is 400 minutes.

[0043] In step 3, the temperature is raised to 920°C at a rate of 500°C / h, and the holding time is 750 minutes.

[0044] In step 5, the temperature is raised to 650°C at a rate of 120°C / h, and the holding time is 400 minutes.

[0045] In step 6, the temperature is raised to 880°C at a rate of 500°C / h, and the holding time is 750 minutes.

[0046] In step 7, the temperature is controlled at a cooling rate of 90°C / h.

[0047] The detection results after heat treatment are shown in Table 1.

[0048] Example Three

[0049] In the embodiment, unlike the first embodiment,

[0050] In step 2, the temperature is raised to 620°C at a rate of 100°C / h, and the holding time is 325 minutes.

[0051] In step 3, the temperature is raised to 900°C at a rate of 450°C / h, and the holding time is 625 minutes.

[0052] In step 5, the temperature is raised to 620℃ at a rate of 100℃ / h, and the holding time is 325 minutes.

[0053] In step 6, the temperature is raised to 860℃ at a rate of 450℃ / h, and the holding time is 625 minutes.

[0054] In step 7, the temperature is controlled at a cooling rate of 75℃ / h.

[0055] The detection results after heat treatment are shown in Table 1.

[0056] Comparative Example 1

[0057] The difference between the present comparative example and the present application is that the existing technology of multiple normalizing (more than 3 times) after forging heat treatment process is used to heat treat the 34CrNi2Mo large forgings to improve the problem of coarse and uneven grain size of the 34CrNi2Mo large forgings. The detection results after heat treatment are shown in Table 1.

[0058] Table 1

[0059] Index Grain size Ultrasonic flaw detection Total processing time Energy consumption cost Requirement index ≥ 5 levels Pass / / Example one 5 levels Pass About 52 hours About 8000 yuan Example two 6 levels Pass About 72 hours About 12000 yuan Example three 5 levels Pass About 61 hours About 10000 yuan Comparative example one 5 levels Pass About 87 hours About 17000 yuan

[0060] From the results of the above examples and comparative examples, it can be seen that by sequentially performing the step 1 to step 7 of the present application on the 34CrNi2Mo large forgings, the grain size of the 34CrNi2Mo large forgings after heat treatment is fine and uniform, the grain size meets the technical requirement of 5-8 grade, and the ultrasonic flaw detection quality is guaranteed. At the same time, there is no need for additional repeated heat treatment, and the production cycle is reduced by at least 15 hours, which greatly reduces the heat treatment production time, saves production energy, reduces production cost by more than 30%, and improves production efficiency.

[0061] The above is only an embodiment of the present application, and the well-known specific technical solutions and / or common knowledge of characteristics in the scheme are not described in detail. It should be noted that for those skilled in the art, without departing from the technical solutions of the present application, a number of modifications and improvements can be made, which should also be considered as the protection scope of the present application, and these will not affect the effect and practicality of the present application. The protection scope of the present application should be subject to the content of its claims, and the specific implementation mode and the like in the specification can be used to explain the content of the claims.

Claims

1. A heat treatment method for 34CrNi2Mo heavy forging steel after forging, characterized in that, The method comprises the following steps: S1, preheat the heat treatment furnace to 500±50℃, load the workpiece into the furnace and segmentally heat the workpiece at different heating rates and respectively keep warm for different time; first heat the workpiece to the first temperature at the first heating rate and keep warm for m minutes, then heat to the second temperature at the second heating rate and keep warm for n minutes; the first temperature is 600~650℃; the second temperature is 880~920℃; S2, use the axial convection fan to forcibly air cool the workpiece to 550±50℃, then air cool to 300±50℃; S3, again segmentally heat the workpiece at different heating rates and respectively keep warm for different time; heat the workpiece to the third temperature at the first heating rate and keep warm for m minutes, then heat to the fourth temperature at the second heating rate and keep warm for n minutes; the first heating rate is 80~120℃ / h; the second heating rate is 300~500℃ / h; the third temperature is 600~650℃; the fourth temperature is 840~880℃; S4, control the temperature of the workpiece to cool to 400±50℃ at the cooling rate of 50~90℃ / h, then take out the workpiece to cool to room temperature.

2. The heat treatment method of a 34CrNi2Mo heavy steel forging after forging according to claim 1, characterized in that: In S1, load the workpiece on the trolley furnace at less than the rated loading capacity of the heat treatment furnace; at the same time, use the pad iron on the trolley surface to isolate the workpieces, so that the distance between the workpieces is ≥300mm.

3. The heat treatment method of a 34CrNi2Mo heavy steel forging after forging according to claim 1, characterized in that: The keeping warm time is related to the thickness of the workpiece, assuming that the thickness of the workpiece is δmm, then the keeping warm time m=(0.5~0.8)×δmin; the keeping warm time n=(1~1.5)×δmin.

4. The heat treatment method of a 34CrNi2Mo heavy steel forging after forging according to claim 1, characterized in that: In S4, when taking out the workpiece, completely open the workpiece and the trolley from the furnace, so that the workpiece is completely exposed to the air to cool to room temperature.

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