GCr15 guide rail plate heat treatment method for heavy ring rolling mill

Abstract

The application provides a GCr15 guide rail plate heat treatment method for heavy ring rolling mill, and belongs to the technical field of metal material heat treatment. The GCr15 guide rail plate heat treatment method for heavy ring rolling mill comprises the steps of pretreatment, stepped preheating, segmented austenitizing heating, gradient cooling quenching, deep cryogenic stabilization treatment, multi-stage tempering and surface strengthening treatment. By accurately controlling the temperature, holding time, cooling rate and other parameters of each link, the guide rail plate obtains uniform martensite + fine carbide organization, and the surface wear resistance is improved. By optimizing the heating and holding time and adjusting the cooling rate, the synergistic optimization of high hardness, high wear resistance, good toughness and high precision dimensional stability of small and medium-sized guide rail plates is realized, the cracking rate is reduced, and the service life is prolonged; the operation controllability is strong, suitable for batch production, can effectively prolong the service life of the heavy ring rolling mill guide rail plate, and ensure the equipment operation stability.

Description

Technical Field

[0001] This application relates to the field of heat treatment of metallic materials, and more specifically, to a heat treatment method for GCr15 guide rail plates used in heavy-duty ring rolling mills. Background Technology

[0002] Currently, heavy-duty ring rolling mills are the core equipment for rolling large ring-shaped parts. Their guide plates, as key components for axial frame load bearing and guidance, must withstand heavy loads, impacts, and continuous friction, placing stringent requirements on material hardness, wear resistance, toughness, and dimensional stability. GCr15 bearing steel, due to its high carbon and high chromium properties, exhibits excellent wear resistance after heat treatment, making it the preferred material for guide plates.

[0003] The heat treatment process for GCr15 steel disclosed in Chinese patent CN108642345A is designed with a holding time for large components, which leads to coarse microstructure when applied to small and medium-sized guide rail plates. The isothermal quenching process in Chinese patent CN110229297B has a fixed cooling rate, which cannot be adapted to the rapid heat conduction characteristics of small and medium-sized components, and the risk of cracking is significantly increased.

[0004] The aforementioned existing heat treatment methods for GCr15 guide rails are mostly designed for large components. When applied to small and medium-sized guide rails with a thickness of 20-50mm and a length of 800mm, they have the following compatibility defects: 1. Redundancy in heating process parameters: Traditional one-time heating and long-term heat preservation lead to excessive austenitization of small and medium-sized guide rail plates, resulting in coarse grains, reduced toughness, and high energy consumption; 2. Mismatched cooling rates: The heat conduction path of small and medium-sized guide rail plates is short. Traditional single cooling methods are prone to excessively rapid cooling, resulting in internal stress concentration and increased risk of cracking. 3. Insufficient control of dimensional stability: The rigidity of small and medium-sized guide rail plates is relatively low, and the residual stress of existing processes is not completely released, with deformation often exceeding 0.1mm / 800mm, affecting assembly accuracy. Summary of the Invention

[0005] To overcome the above deficiencies, this application provides a heat treatment method for GCr15 guide rail plates for heavy-duty ring rolling mills. The method aims to solve the problems of existing GCr15 guide rail plates being prone to cracking at sharp corners, having large deformation, and poor matching of wear resistance and toughness after heat treatment, which cannot meet the requirements of heavy-duty ring rolling mills under heavy load, impact, and long-term friction conditions. It is especially suitable for small and medium-sized heavy-duty guide rail plates with a thickness of 20-50mm and a length of 800mm or less.

[0006] This application provides a heat treatment method for GCr15 guide rail plates used in heavy-duty ring rolling mills, characterized by the following steps; S1. Pre-treatment: The surface of the GCr15 guide rail plate is treated with sandblasting and / or ultrasonic cleaning, followed by drying; this removes oxide scale, oil stains and impurities from the surface of the GCr15 guide rail plate after rough machining. S2. Stepped Preheating: The pretreated guide rail plate is sent to a continuous heat treatment furnace and preheated sequentially at 350±10℃ for 1-1.5h, 550±10℃ for 0.8-1h, and 680±10℃ for 0.5-0.8h. Considering the short heat conduction path of small and medium-sized guide rail plates, the preheating parameters are optimized: 340-360℃ for 1-1.5h to eliminate surface moisture and initial stress; 540-560℃ for 0.8-1h to promote the initial dissolution of carbides and improve heat conduction uniformity; and 670-690℃ for 0.5-0.8h, approaching the Ac1 temperature, to prepare for austenitization. The heating rate is maintained at 50-80℃ / h to avoid excessive temperature differences between the inside and outside of the guide rail plate, which could generate thermal stress.

[0007] S3. Segmented Austenitizing Heating: After preheating, the temperature is raised to 820±5℃ and held for 1.5-2.5 hours, then raised to 850±5℃ and held for 2-3 hours. During the heating process, the protective atmosphere inside the furnace includes nitrogen and hydrogen, with nitrogen comprising 70%-80% and hydrogen comprising 20%-30%, and an oxygen content ≤50ppm. The nitrogen-hydrogen mixed low-oxygen atmosphere isolates oxidation and decarburization, resulting in minimal hardness deviation across the entire guide rail plate, no oxidation points or decarburized soft bands, uniform frictional wear along the entire length of the guide rail, stable operation of the ring rolling mill, and higher dimensional consistency of the rolled workpieces. Through staged precise temperature control, differentiated phase transformation regulation, and layered release of residual stress accumulated from multiple processes, the wear resistance, impact toughness, and dimensional stability of the guide rail plate are synergistically matched, with the entire process linked front and back and the phase transformation process controllable.

[0008] Based on the characteristics of thinner guide rail plates in small and medium-sized applications, the heat preservation time is shortened: first, heat to 815-825℃, and hold at temperatures below Ac3 for 1.5-2.5 hours to ensure uniform dissolution of carbides and prevent coarse grains; then heat to 845-855℃, and hold at 30-50℃ above Ac3 for 2-3 hours. The heat preservation time is calculated based on a maximum thickness of 1.0-1.2 min / mm. For example, a 50mm thick guide rail plate would require a heat preservation time of 3 hours to ensure complete austenitization. During the heating process, a protective atmosphere of 70%-80% nitrogen and 20%-30% hydrogen by volume is introduced, with the oxygen content controlled at ≤50ppm to prevent decarburization and oxidation of the guide rail plate surface.

[0009] S4. Gradient cooling quenching: Transfer the guide plate from the heating furnace to the quenching device within a preset time. First, cool it in a nitrate bath at 200-220℃ for 1-1.5 hours, and then transfer it to a hot oil bath at 120-140℃ for 1.5-2 hours. For small and medium-sized guide rail plates, the cooling rate needs to be precisely matched to avoid cracking due to excessively rapid cooling: The guide rail plates should be quickly transferred from the heating furnace to the quenching device to reduce the impact of air cooling time on the microstructure; first, immerse them in a 200-220℃ salt bath for 1-1.5 hours to allow the surface layer to form lower bainite, improving the material's toughness; then transfer them to a 120-140℃ hot oil bath for 1.5-2 hours to slowly complete the microstructure transformation and reduce internal stress. During the cooling process, a variable frequency stirring device should be used to maintain the medium flow rate at 1.0-1.3 m / s to ensure uniform cooling of the guide rail plate surface and interior, avoiding localized stress concentration.

[0010] S5. Cryogenic Stabilization Treatment: Within 2 hours after quenching, place the guide rail plate into a cryogenic chamber and cool it to -70±5℃ at a rate of 5-8℃ / min. Hold it at this temperature for 2.5-3.5 hours, and then heat it back to room temperature at a rate of 3-5℃ / min. Forced transformation of residual austenite into martensite reduces structural instability. The cryogenic chamber should have an effective volume ≥2m³. 3 This small-scale equipment is suitable for batch processing of small and medium-sized guide rail plates. The cooling medium is liquid nitrogen, and the temperature uniformity inside the chamber is ≤±3℃. During the heating stage, the temperature is slowly increased to room temperature at a rate of 3-5℃ / min to avoid excessive temperature difference and the generation of new stress.

[0011] S6. Multi-stage tempering: After recovery to room temperature, the first stage of tempering is performed within a specified time: holding at 150±5℃ for 3-4 hours, then furnace cooling to below 80℃; the second stage of tempering is performed after semi-finishing: holding at 140±5℃ for 5-7 hours, then furnace cooling to room temperature; the third stage of tempering is performed after finishing: holding at 120±5℃ for 8-10 hours. The tempering time is optimized to meet the stress release requirements of small and medium-sized guide rail plates: the first stage of tempering is performed immediately after deep cooling to eliminate internal stress generated during quenching and deep cooling, using furnace cooling to below 80℃; the second stage of tempering is performed after semi-finishing of the guide rail plate to release residual stress generated during processing and further stabilize the microstructure, then furnace cooling to room temperature; the third stage of aging treatment is performed after finishing to ensure long-term dimensional stability of the guide rail plate. Each tempering process uses furnace cooling to avoid secondary stress caused by air cooling or water cooling.

[0012] In a preferred embodiment of the present invention, the guide rail plate is a GCr15 forged plate blank with a thickness of 20-50mm and a length of ≤800mm; the effective volume of the cryogenic chamber is ≥2m³. 3 The refrigerant is liquid nitrogen, and the temperature uniformity inside the chamber is ≤±3℃.

[0013] In a preferred embodiment of the present invention, the GCr15 forging blank to be treated is tested for the presence of network cementite. If network cementite is present, the GCr15 forging blank to be treated is normalized before pretreatment. The normalizing treatment involves holding at 940±10℃ for 0.8-1.2 hours, followed by air cooling to room temperature. The hardness after normalizing is controlled at 240-260 HBS.

[0014] In a preferred embodiment of the present invention, in step S1, the sandblasting pressure is 0.3-0.5 MPa, and the sand particle size is 0.1-0.2 mm to avoid impact deformation of small and medium-sized guide rail plates caused by high pressure; the ultrasonic cleaning fluid is a 5% sodium carbonate solution, the cleaning temperature is 60-70℃, and the cleaning time is 15-25 min to ensure that the surface is clean and free of residue; the drying temperature is 120℃, and the drying time is 20-30 min to remove surface moisture and avoid oxidation defects during heating.

[0015] In a preferred embodiment of the present invention, in S3, the segmented austenitizing heating is performed at a rate of 50-80°C / h; the holding time for the segmented austenitizing heating is calculated based on the maximum thickness of the guide plate as 1.0-1.2 min / mm.

[0016] In a preferred embodiment of the present invention, in step S4, the quenching device includes a nitrate bath, a hot oil bath, an electric transfer mechanism, and a frequency conversion stirring device. The solution in the nitrate bath includes potassium nitrate and sodium nitrite. The hot oil bath uses No. 320 heat transfer oil. The length of the bath is >800mm, so that the guide plate is completely submerged.

[0017] In a preferred embodiment of the present invention, the nitrate bath and the hot oil bath are fixed by a base, two variable frequency stirring devices are provided, and the two variable frequency stirring devices are respectively arranged in the nitrate bath and the hot oil bath. The electric transfer mechanism is used to drive the variable frequency stirring devices to move.

[0018] In a preferred embodiment of the present invention, the mass ratio of potassium nitrate to sodium nitrite is 1:1, the mass fraction of potassium nitrate is 50%, and the mass fraction of sodium nitrite is 50%; during the cooling process, the medium flow rate is maintained at 1.0-1.3 m / s by a stirring device, and the preset time is ≤20s.

[0019] In a preferred embodiment of the present invention, in S6, the specified time is ≤10s, and after multi-stage tempering, surface strengthening treatment is performed, and the working surface of the guide plate is subjected to plasma spraying combined with laser remelting process.

[0020] In a preferred embodiment of the present invention, the plasma spraying material is WC-Co alloy powder with a spraying thickness of 0.2-0.4 mm, and the laser remelting process uses a laser remelting power of 1.2-1.8 kW and a scanning speed of 4-6 mm / s.

[0021] Beneficial effects: 1. By optimizing heating and holding time and adjusting cooling rate, the high hardness, high wear resistance, good toughness and high precision dimensional stability of small and medium-sized guide rail plates are synergistically optimized, reducing cracking rate and extending service life; it has strong operational controllability, is suitable for mass production, and can effectively extend the service life of guide rail plates of heavy ring rolling mills and ensure the stability of equipment operation. 2. Two-stage segmented austenitization precisely controls the holding time, avoiding the coarse grains caused by prolonged high-temperature heating in traditional one-piece systems; the matrix forms a microstructure of hidden needle tempered martensite + uniform fine granular carbide, resulting in uniform surface hardness and significantly improved impact toughness of the guide rail plate. Under long-term heavy-load reciprocating friction and impact conditions in heavy-duty ring rolling mills, it is not easy to chip or peel off the surface layer. The metallographic structure is refined, and wear resistance and impact toughness are improved simultaneously, significantly extending the service life of the guide rail.

[0022] 3. First, nitrate salt forms a tough bainitic buffer layer, then hot oil slowly completes the martensitic phase transformation. The layered buffering of phase transformation stress prevents hidden microcracks at the edges and thickness transitions of the plate, significantly reducing the scrap rate after heat treatment and solving the problem of quenching cracking in small and medium-sized thin plates. The temperature difference change rate is controlled throughout the entire process of heating, cooling, cryogenics, and tempering, buffering various thermal and structural stresses in a step-like manner. No hidden microcracks are generated during rough machining, heat treatment, and grinding, eliminating the risk of stress cracking failure of the guide rail during long-term operation.

[0023] 4. Forced transformation of residual austenite releases residual stress from quenching, deep cooling, and machining in stages, eliminating the need for subsequent large-area straightening processes and meeting the high-precision assembly tolerance requirements of the axial stand of heavy-duty ring rolling mills. The segmented temperature control process saves energy and reduces consumption, lowering mass production processing costs. Precise short-time holding intervals are matched for small and medium-sized thin plates, with stepped preheating and gradual temperature rise, significantly reducing the overall power consumption of the heat treatment furnace and lowering the per-piece heat treatment processing cost during mass production. Attached Figure Description

[0024] To more clearly illustrate the technical solutions of the embodiments of this application, the drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0025] Figure 1 This is a flowchart illustrating the heat treatment process provided in the embodiments of this application; Figure 2 Application scenario diagram of GCr15 guide plate provided for the embodiments of this application; Figure 3 A structural diagram of the GCr15 guide rail plate provided for an embodiment of this application; Figure 4Metallographic examination drawings provided for embodiments of this application. Detailed Implementation

[0026] In this invention, unless otherwise explicitly specified and limited, the terms "installation," "connection," "linking," and "fixing," etc., should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral part; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal communication of two components or the interaction between two components. Those skilled in the art can understand the specific meaning of the above terms in this invention according to the specific circumstances.

[0027] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can include direct contact between the first and second features, or contact between the first and second features through another feature between them. Furthermore, "above," "over," and "on top" of the second feature includes the first feature directly above or diagonally above the second feature, or simply indicates that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature includes the first feature directly below or diagonally below the second feature, or simply indicates that the first feature is at a lower horizontal level than the second feature.

[0028] The technical solutions in the embodiments of this application will now be described with reference to the accompanying drawings.

[0029] Example 1 Please see Figures 1-4 This invention provides a heat treatment method for GCr15 guide rail plates used in heavy-duty ring rolling mills, and a conventional heat treatment method for small and medium-sized GCr15 guide rail plates (without network cementite). Parameters of the workpiece to be processed Guide rail plate dimensions: thickness 30mm, length 570mm, width 170mm. The GCr15 steel blank is tested and found to have no network cementite, with an initial hardness of 220HBS. It is used in the axial frame guide mechanism of the D53KB-8000.5 heavy-duty ring rolling mill for wind power equipment.

[0030] Specific processing steps S1. Preprocessing Sandblasting: Compressed air with a pressure of 0.4MPa is used to drive sand particles with a particle size of 0.15mm to sandblast the surface of the guide plate to remove the oxide scale and burrs generated after rough machining. The sandblasting time is 3min to ensure that the surface roughness Ra≤3.2μm.

[0031] Ultrasonic cleaning: The sandblasted guide rail plate is placed in a 5% sodium carbonate solution at a cleaning temperature of 65℃ for 20 minutes. Ultrasonic vibration is used to remove surface oil and residual sand particles. Then, it is transferred to a 120℃ drying oven to dry for 25 minutes. After removal, there are no visible water stains or oil stains on the surface.

[0032] S2. Stepped preheating Place the pretreated guide rail plates evenly on the material rack of the small continuous heating furnace, ensuring the contact area between the material rack and the guide rail plates is ≤15% to avoid affecting heating uniformity. Perform stepped preheating according to the following parameters: First preheating zone: 350℃ for 1.2 hours; Second preheating zone: 550℃ for 0.9 hours; Third preheating zone: 680℃ for 0.6 hours; The heating rate is controlled at 60℃ / h, and a protective atmosphere is circulated throughout the furnace at a flow rate of 0.5m³. 3 / h, oxygen content is monitored in real time to be ≤30ppm; the atmosphere content is 75% nitrogen and 25% hydrogen.

[0033] S3. Segmented austenitizing heating After preheating, proceed with segmented heating as follows: First heating zone: Heat to 820℃ at a rate of 70℃ / h and hold for 2 hours (calculated based on a maximum thickness of 30mm × 1.0min / mm, the actual holding time of 2 hours meets the requirements). Second heating zone: continue heating to 850℃ and hold for 2.5h (30mm×1.2min / mm=36min, actual holding for 2.5h ensures complete austenitization); During the heating process, a protective atmosphere is continuously supplied, the oxygen content is controlled at ≤50ppm, and the temperature fluctuation inside the furnace is ≤±2℃.

[0034] S4. Gradient cooling quenching Transfer operation: After heating, the guide rail plate is removed from the heating furnace by the electric transfer mechanism of the compact gradient quenching device. The transfer time is 15 seconds, and it is quickly immersed in a 200℃ nitrate bath; the solution is 50% potassium nitrate and 50% sodium nitrite. Nitrate bath cooling: Maintain the nitrate bath temperature at 200-205℃, and keep the bath flow rate at 1.2m / s using a variable frequency stirring device for 1.2h. Hot oil bath cooling: The guide plate is transferred to a 130℃ hot oil bath by an electric transfer mechanism, the liquid flow rate is 1.1m / s, and the cooling time is 1.8h. The temperature of the bath liquid is monitored in real time during the cooling process, with a fluctuation range of ≤±5℃.

[0035] S5. Cryogenic Stabilization Treatment Within 1.5 hours after quenching, place the guide rail plate into an effective volume of 2.5m³. 3 Small cryogenic chamber: Cooling phase: Cool down to -70℃ at a rate of 6℃ / min and hold for 3 hours; Heating phase: The temperature is raised to room temperature at a rate of 4℃ / min. The temperature uniformity inside the cryogenic chamber is ≤±3℃. Liquid nitrogen is used as the refrigerant throughout the process.

[0036] S6. Multi-stage tempering First stage tempering: After deep cooling to room temperature, immediately place it in a multi-stage tempering furnace, hold at 150℃ for 3.5 hours, and remove it after furnace cooling to below 75℃; Semi-finishing: The guide rail plate is milled and ground, leaving a 0.3mm finishing allowance; Second stage tempering: Place the semi-finished guide rail plate into the tempering furnace, hold at 140℃ for 6 hours, and then cool the furnace to room temperature. Precision machining: The working surface of the guide rail plate is precision ground to achieve the design dimensional accuracy; The third stage of aging: After finishing, place it in a tempering furnace, keep it at 120℃ for 9 hours, and then let it cool to room temperature.

[0037] Surface strengthening treatment Plasma spraying + laser remelting treatment is applied to the working surface of the guide rail plate: Plasma spraying: WC-Co alloy powder is used, with a Co content of 12%, spraying voltage of 35V, current of 200A, spraying distance of 100mm, and spraying thickness of 0.3mm; Laser remelting: laser power 1.5kW, scanning speed 5mm / s, spot diameter 3mm, surface roughness Ra≤0.8μm after remelting, coating bonding strength≥60MPa.

[0038] Post-processing performance testing Microstructure: Under a microscope, it appears as uniform martensite with fine carbides, and the grain size is ≤10μm; Hardness: Surface hardness 52-53 HRC, hardness uniformity ≤ ±1 HRC; Deformation amount: 0.12mm / 600mm in the length direction, which is equivalent to 0.16mm / 800mm, meeting the requirement of ≤0.2mm / 800mm; Cracking situation: No cracking was observed in the batch of 50 pieces processed, with a cracking rate of 0%; Wear resistance: Friction and wear tests show that the wear amount is reduced by 35% compared to traditional processes; Dimensional stability: After being placed at room temperature for 6 months, the dimensional change is ≤0.01mm / 800mm.

[0039] Conclusions of comparative experiments on heat treatment under different temperature gradients (without network cementite) Based on the above basic process framework, only the core temperature ranges of preheating, austenitization, quenching and cooling, cryogenic cooling, and tempering were adjusted. The parameters such as holding time, medium ratio, flow rate, transfer time, cryogenic rate, and processing flow remained unchanged. The experiment was divided into three groups: low temperature group, medium temperature group, and high temperature group. Parallel experiments were carried out. Each group used a 30mm thick, 570mm long, and 170mm wide non-reticulated cementite GCr15 guide plate, with 50 pieces in each batch. Finally, the microstructure, hardness, deformation, cracking rate, wear resistance, and dimensional stability were tested.

[0040] Three sets of temperature parameter settings (1) Low temperature group (conservative low temperature process, focusing on preventing cracking and controlling deformation) Stepped preheating: 340℃ for 1 hour → 540℃ for 0.8 hours → 670℃ for 0.5 hours, heating rate 50℃ / h Segmented austenitization: Hold at 815℃ for 1.5 hours → Hold at 845℃ for 2 hours Gradient quenching: cooling in a nitrate bath at 200℃ for 1 hour → cooling in a hot oil bath at 120℃ for 1.5 hours. Cryogenic treatment: Hold at -75℃ for 2.5 hours Multi-stage tempering: first stage at 145℃ for 3 hours, second stage at 135℃ for 5 hours, and third stage at 115℃ for 8 hours. (2) Medium temperature group (standard process of the original embodiment, optimal benchmark group) Stepped preheating: 350℃ for 1.2h → 550℃ for 0.9h → 680℃ for 0.6h, heating rate 60℃ / h Segmented austenitization: Hold at 820℃ for 2 hours → Hold at 850℃ for 2.5 hours Gradient quenching: cooling in a nitrate bath at 200℃ for 1.2 hours → cooling in a hot oil bath at 130℃ for 1.8 hours. Cryogenic treatment: -70℃ for 3 hours Multi-stage tempering: first stage at 150℃ for 3.5 hours, second stage at 140℃ for 6 hours, and third stage at 120℃ for 9 hours. (3) High temperature group (strengthening high temperature process, focusing on high hardness and high wear resistance) Stepped preheating: 360℃ for 1.5 hours → 560℃ for 1 hour → 690℃ for 0.8 hours, heating rate 80℃ / h Segmented austenitization: Hold at 825℃ for 2.5 hours → Hold at 855℃ for 3 hours Gradient quenching: cooling in a nitrate bath at 220℃ for 1.5 hours → cooling in a hot oil bath at 140℃ for 2 hours. Cryogenic treatment: -65℃ for 3.5 hours Multi-stage tempering: first stage at 155℃ for 4 hours, second stage at 145℃ for 7 hours, and third stage at 125℃ for 10 hours. Comparison table of results from three parallel experiments

[0041] Results Analysis Low temperature group: The overall temperature is relatively low, resulting in insufficient austenitization and inadequate carbide dissolution, leading to weaker hardness and wear resistance. However, it exhibits minimal thermal stress and structural stress, minimal deformation, no cracking, and optimal dimensional stability, making it suitable for guide rail applications with extremely high precision requirements and relatively low loads.

[0042] Medium temperature group: The temperature range is suitable for small and medium-sized guide rail plates of 20–50mm, achieving the best synergy of hardness, toughness, wear resistance, deformation control, crack risk, and dimensional stability, making it the optimal heat treatment temperature scheme for this type of GCr15 guide rail plate.

[0043] High temperature group: It has sufficient austenitization and high martensitic hardness, resulting in the best wear resistance; however, excessively high temperature leads to grain coarsening, increased thermal stress, microcracks at the edges and corners, excessive deformation, and decreased long-term dimensional stability. It can only be used for guide rail plates with heavy load, low precision, and non-guide key surfaces.

[0044] Example 2 Please see Figures 1-4 The present invention provides a heat treatment method for GCr15 guide rail plates for heavy-duty ring rolling mills, wherein the heat treatment method is for GCr15 guide rail plates containing network cementite. Parameters of the workpiece to be processed Guide rail plate dimensions: thickness 50mm, length 629.3mm, width 239mm. The GCr15 steel billet is tested by metallographic analysis and contains network cementite (grade 3). The initial hardness is 250HBS. It is used as a load-bearing guide rail for the D53K-6300 heavy-duty ring rolling mill bed in the field of aerospace parts manufacturing.

[0045] Specific processing steps Additional step: Normalizing treatment to remove the network cementite. Add normalizing treatment before preheating: Place the guide plate in a small continuous heating furnace, keep it at 940℃ for 1 hour, air cool to room temperature, and after normalizing, test the hardness to be 250HBS. Metallographic examination shows that the network cementite grade is ≤1, which meets the requirements of subsequent heat treatment.

[0046] S1. Pretreatment: Sandblasting pressure 0.5MPa, sand particle size 0.2mm, ultrasonic cleaning temperature 70℃, time 25min, drying temperature 120℃, time 30min; S2. Stepped preheating: 360℃ for 1.5h, 560℃ for 1h, 690℃ for 0.8h, heating rate 50℃ / h; S3. Segmented austenitizing heating: 825℃ for 2.5h (50mm×1.0min / mm=50min), 855℃ for 3h (50mm×1.2min / mm=60min), with a protective atmosphere of 70% nitrogen + 30% hydrogen; S4. Gradient cooling quenching: Salt bath temperature 220℃, cooling for 1.5h, bath flow rate 1.3m / s; hot oil bath temperature 140℃, cooling for 2h, bath flow rate 1.2m / s, transfer time 18s. S5. Cryogenic stabilization treatment: Cool down to -65℃ at 8℃ / min, hold for 3.5h, and then heat up to room temperature at 5℃ / min; S6. Multi-stage tempering: First stage: hold at 155℃ for 4 hours, then furnace cool to below 80℃; Second stage: hold at 145℃ for 7 hours after semi-finishing; Third stage: hold at 125℃ for 10 hours after finishing. S7. Surface strengthening treatment: spray coating thickness 0.4mm, laser remelting power 1.8kW, scanning speed 4mm / s.

[0047] Post-processing performance testing Microstructure: uniform martensite + dispersed fine carbides, with no network cementite residue; Hardness: 53-55HRC, hardness uniformity ≤±1HRC; Deformation: 0.18mm / 800mm, meets requirements; Cracking: Of 30 pieces processed in a batch, only 1 piece showed a minor crack (cracking rate of 3.3%, close to the design requirement of ≤3%); Abrasion resistance: 32% higher than traditional process; Dimensional stability: After 6 months of storage at room temperature, the dimensional change was ≤0.02mm / 800mm.

[0048] Conclusions of comparative experiments on heat treatment under different temperature gradients (including network cementite) This experiment was based on a GCr15 load-bearing guide plate with a thickness of 50 mm, a length of 629.3 mm, and a width of 239 mm, containing a three-level network of cementite. Under the premise of keeping all parameters such as normalizing to remove the network of cementite, sandblasting cleaning, media ratio, stirring flow rate, transfer time, and surface strengthening unchanged, only three groups of variables were set for the core temperatures of preheating, austenitizing, quenching cooling, cryogenic cooling, and tempering: low temperature group, medium temperature group (the original example baseline), and high temperature group. Each group was processed in batches of 30 pieces, and the microstructure, hardness, deformation, cracking rate, wear resistance, and dimensional stability were compared.

[0049] Three sets of temperature parameter settings (1) Low temperature group (stress control, cracking risk reduction, and hardness enhancement) Normalizing standardization: 940℃ for 1 hour, air cooling, network cementite ≤ Grade 1 Stepped preheating: 340℃ for 1 hour → 540℃ for 0.8 hours → 670℃ for 0.5 hours, heating rate 50℃ / h Segmented austenitization: Hold at 815℃ for 1.5 hours → Hold at 845℃ for 2 hours Gradient quenching: cooling in a nitrate bath at 200℃ for 1 hour at a flow rate of 1.3 m / s; cooling in a hot oil bath at 120℃ for 1.5 hours at a flow rate of 1.2 m / s; transfer for 18 seconds. Cryogenic treatment: Cool to -75°C at a rate of 5°C / min, hold for 2.5 hours, then heat to room temperature at a rate of 3°C / min. Multi-stage tempering: First stage: 145℃ for 3 hours; Second stage: 135℃ for 5 hours; Third stage: 115℃ for 8 hours. (2) Medium temperature group (standard process of the original embodiment, reference group) Normalizing standardization: 940℃ for 1 hour, air cooling, network cementite ≤ Grade 1 Stepped preheating: 360℃ for 1.5 hours → 560℃ for 1 hour → 690℃ for 0.8 hours, heating rate 50℃ / h Segmented austenitization: Hold at 825℃ for 2.5 hours → Hold at 855℃ for 3 hours Gradient quenching: Salt bath cooling at 220℃ for 1.5h, flow rate 1.3m / s; hot oil bath cooling at 140℃ for 2h, flow rate 1.2m / s; transfer for 18s. Cryogenic treatment: Cool to -65℃ at 8℃ / min, hold for 3.5h, then heat to room temperature at 5℃ / min. Multi-stage tempering: First stage: 155℃ for 4 hours; Second stage: 145℃ for 7 hours; Third stage: 125℃ for 10 hours. (3) High temperature group (strengthened hardness and wear resistance, higher stress) Normalizing standardization: 940℃ for 1 hour, air cooling, network cementite ≤ Grade 1 Stepped preheating: 360℃ for 1.5 hours → 560℃ for 1 hour → 690℃ for 0.8 hours, heating rate 80℃ / h Segmented austenitization: Hold at 830℃ for 3 hours → Hold at 860℃ for 3.5 hours Gradient quenching: cooling in a nitrate bath at 230℃ for 2 hours at a flow rate of 1.3 m / s; cooling in a hot oil bath at 150℃ for 2.5 hours at a flow rate of 1.2 m / s; transfer for 18 seconds. Cryogenic treatment: Cool to -60°C at 8°C / min, hold for 4 hours, then heat to room temperature at 5°C / min. Multi-stage tempering: First stage: 160℃, hold for 4.5 hours; Second stage: 150℃, hold for 7.5 hours; Third stage: 130℃, hold for 10.5 hours. Comparison table of three sets of experimental test results

[0050] Results Analysis Low temperature group: Normalizing has eliminated the network cementite, but the overall heat treatment temperature is relatively low, the degree of austenitization is insufficient, and the hardness and wear resistance are slightly lower. The workpiece has the lowest overall thermal stress and structural stress, no cracking, minimal deformation, and optimal dimensional stability, making it suitable for the guide rail parts of aerospace ring rolling mills that require high precision, low impact, and stable load-bearing.

[0051] Medium temperature group: The temperature range is optimally matched with the 50mm thick GCr15 guide rail plate containing network cementite. After normalizing pretreatment, the network cementite is completely eliminated, achieving a balance between hardness, wear resistance, deformation control, and cracking risk. Only a very few micro-cracks exist, and the cracking rate is close to the design threshold, which can meet the requirements of heavy-duty ring rolling mill guide rails in the aerospace field. This is the optimal process in this embodiment.

[0052] High temperature group: It has sufficient austenitization, the highest martensitic hardness, and the best wear resistance; however, the original workpiece has network cementite defects, and high temperature aggravates grain coarsening and structural stress, resulting in excessive deformation, a significant increase in cracking rate, and a decrease in long-term dimensional stability. It is only suitable for heavy-duty, low-speed auxiliary load-bearing guide rails with low sensitivity to precision and cracking.

[0053] By precisely controlling various process parameters, and adapting to the material properties and structural characteristics of small and medium-sized GCr15 guide rail plates with a thickness of 20-50mm and a length of ≤800mm, it can effectively solve problems such as cracking, large deformation, and mismatch between wear resistance and toughness in existing processes. The supporting equipment is convenient to operate and highly controllable.

[0054] The heat treatment apparatus for implementing any of the above methods includes a continuous heating furnace, a compact gradient quenching device, a deep cooling box, a multi-stage tempering furnace, and a surface strengthening device. The size of the bath of the compact gradient quenching device is adapted to the length of the guide rail plate, and it can realize automatic switching between nitrate bath and hot oil bath. Continuous heating furnace: Furnace cavity cross-section dimensions 1000×600mm, equipped with 3 preheating zones and 2 heating zones, zone temperature control accuracy ±2℃, protective atmosphere flow rate 0.3-0.8m³ / h 3 / h, to meet the batch heating needs of small and medium-sized guide rail plates; Compact gradient hardening unit: integrates a small, hot oil bath and an electric transfer mechanism; the bath volume is 1.5m³. 3 It can fully submerge guide rails with a length of up to 800mm, the electric transfer mechanism has a stroke of ≤1.5m and a transfer time of ≤20s, and the variable frequency stirring device can achieve a flow rate adjustment of 1.0-1.3m / s; Cryogenic chamber: effective volume 2-3m³ 3The cooling rate is adjustable from 0-10℃ / min, the temperature uniformity is ≤±3℃, and it supports processing 5-10 small and medium-sized guide rail plates at a time. Multi-stage tempering furnace: It has 3 independent temperature control zones with a temperature preservation accuracy of ±3℃. The furnace cavity size is adapted to small and medium-sized guide rail plates and supports continuous or batch tempering treatment.

[0055] Operating precautions for supporting equipment Continuous heating furnace: Regularly check the heating elements and temperature control sensors in each heating zone; calibrate the oxygen content detector before processing each batch; and clean the protective atmosphere pipeline regularly to avoid blockages that could lead to excessive oxygen content. Compact gradient quenching device: The nitrate bath needs to be replaced with nitrate regularly, the hot oil bath needs to be filtered for impurities and replenished with heat transfer oil regularly, and the guide rails of the transfer mechanism need to be lubricated regularly to ensure that the transfer time is ≤20s; Cryogenic chamber: Before use, check the liquid nitrogen storage level and sealing performance. The cooling and heating rates must be strictly executed according to the set parameters to avoid the workpiece cracking due to excessive speed. Multi-stage tempering furnace: The temperature control system is calibrated before each tempering to ensure a heat preservation accuracy of ±3℃. Sufficient gaps must be left when placing workpieces in the furnace to ensure smooth furnace gas flow and improve tempering uniformity.

[0056] The working principle of the heat treatment method for GCr15 guide rail plates used in this heavy-duty ring rolling mill: Pretreatment process principle A combined sandblasting and ultrasonic cleaning process is employed to simultaneously remove cutting oil, scale, metal burrs, and dust impurities from the surface of the guide rail plate after rough machining. Residual contaminants from rough machining can cause uneven heat conduction during localized heating, leading to localized overheating, decarburization, and pitting corrosion. During quenching, these impurities can easily cause stress concentration microcracks. This process thoroughly cleans the entire surface of the plate, ensuring a uniform temperature field during subsequent furnace heating. It also eliminates the risk of quenching cracks induced by surface impurities, providing a clean substrate foundation for uniform austenitization.

[0057] Principle of Three-Stage Preheating Process Abandoning the industry's conventional direct high-temperature heating method, it sets up a three-stage constant temperature range of 350℃, 550℃, and 680℃, and controls the heating rate at a low rate of 50~80℃ / h throughout the process.

[0058] ①350±10℃ low temperature insulation: rapidly evaporates the moisture adsorbed inside the board, releases the residual cutting stress generated by rough turning and milling, and avoids the rapid vaporization of moisture and the generation of surface micro-cracks. ②550±10℃ medium temperature insulation: promotes the initial spheroidization and refinement of matrix network carbides, improves the overall thermal conductivity uniformity of the board, and reduces the temperature gradient between the inside and outside of the board; ③680±10℃ near Ac1 critical temperature insulation: reduce the temperature difference between the plate and the austenitizing furnace in advance, and significantly reduce the huge thermal stress generated by subsequent high-temperature heating.

[0059] The entire stepped preheating solution is adapted to the short heat conduction path of small and medium-sized thin plates, gradually reducing the temperature difference between the inside and outside of the plate throughout the process, and suppressing thermal stress cracking from the source of heating.

[0060] Principle of Two-Stage Austenitizing Process Unlike existing large-component integrated long-term heat preservation processes, this method adopts a two-stage heating mode of 820℃ low-temperature pre-melting + 850℃ precise austenitization, while also being equipped with a low-oxygen, nitrogen, and hydrogen mixed protective atmosphere.

[0061] ① Low temperature range of 820±5℃ (below Ac3): heat preservation for 1.5~2.5h to slowly dissolve coarse network carbides and make them evenly dispersed and distributed, avoiding the infinite growth of grains caused by one-time high temperature and long-term heat preservation; the heat preservation time is precisely matched according to the maximum thickness of the plate by 1.0~1.2min / mm to prevent excessive austenitization of small and medium-sized thin plates. ②850±5℃ high temperature range (30~50℃ above Ac3): fully complete the homogenization of the matrix austenite, ensure the full solid solution of carbon and chromium alloying elements, and provide a foundation for the subsequent high hardness tempered martensite structure; ③ The furnace contains a mixed atmosphere of 70% nitrogen and 20% hydrogen, with an oxygen content of ≤50ppm: This isolates the furnace from air oxidation, prevents decarburization of the guide rail surface, and ensures uniform hardness and wear resistance across the entire plate.

[0062] Two-stage graded heating achieves a balance between carbide refinement and full austenite transformation, taking into account both high hardness and matrix impact toughness, and solving the defects of coarse grains and insufficient toughness in traditional processes.

[0063] Gradient dual-medium quenching process principle To address the issue of rapid thermal conductivity and susceptibility to cracking in small and medium-sized thin plates due to single cooling rates, a gradient cooling method is employed: a 200~220℃ nitrate bath followed by a 120~140℃ hot oil bath, coupled with variable frequency stirring of the medium at 1.0~1.3 m / s. 1. High-temperature nitrate bath precooling: After austenitization, the plate is quickly transferred to a nitrate bath for 1 to 1.5 hours of heat preservation. The surface of the plate undergoes a slow lower bainitic phase transformation, forming a tough buffer layer that greatly offsets the huge structural stress generated by the sudden cooling and avoids stress concentration cracking at the edges and corners. 2. Low-temperature hot oil slow cooling: Transfer to hot oil bath for 1.5~2 hours, the core slowly completes the martensitic phase transformation, and the temperature difference between the surface and the core is reduced throughout the process; 3. Continuous stirring of the medium: Nitrate and hot oil flow throughout the entire area, and the cooling rate in the thickness direction of the plate is completely synchronized, eliminating deformation and micro-cracks caused by local cooling rate differences.

[0064] This gradient cooling path breaks the limitation of fixed cooling rate in single isothermal quenching, adapts to the thermal conductivity of 20~50mm thin plates, and minimizes the risk of quenching cracks while ensuring hardening effect.

[0065] 5. S5 Cryogenic Stabilization Process Principle Within 2 hours of quenching, the sample is immediately placed in a -70±5℃ cryogenic chamber, cooled at a uniform rate of 5~8℃ / min, held at the same temperature for 2.5~3.5 hours, and then slowly restored to room temperature at a rate of 3~5℃ / min. After quenching, GCr15 retains 8%~15% unstable austenite in the matrix, which will undergo continuous phase transformation and dimensional creep under long-term heavy load impact. Cryogenic treatment forces the retained austenite to completely transform into stable tempered martensite, eliminating the source of hysteretic deformation. Simultaneously, the heating and cooling rates are strictly controlled to avoid the generation of new internal stresses from alternating hot and cold temperatures, thus improving the long-term dimensional stability of the guide plate from the metallographic perspective.

[0066] Three-stage tempering aging principle The process involves three stages: heat treatment, semi-finishing, and finishing, followed by low-temperature tempering to release accumulated residual stress in layers, thus solving the problems of low stiffness and excessive deformation in thin plates. ① First stage tempering (150℃, immediately after deep cooling): Eliminate the huge structural stress generated by quenching and deep cooling phase transformation. Use furnace cooling to below 80℃ to avoid secondary thermal stress from air cooling. ② Second stage tempering (140℃, after semi-finishing): releases the new processing stress generated by turning and grinding, and stabilizes the basic metallographic structure; ③ Third stage long-term aging (120℃, after fine processing): long-term heat preservation completely releases the residual micro-stress inside the board, fixes the stable structure of tempered martensite, and controls the deformation within 0.1mm within 800mm of the finished product length.

[0067] The three-stage tempering process gradually eliminates the stress accumulated over multiple processes, overcoming the shortcomings of incomplete stress release in single tempering, and perfectly matching the high-precision assembly requirements of heavy-duty ring rolling mills.

[0068] The entire process synergy and linkage logic From pretreatment to remove impurities and control surface defects, to stepped preheating to control thermal stress, to two-stage austenitization to refine grains, to gradient quenching to control phase transformation cracking, to deep cryogenics to eliminate residual austenite, and to three-stage tempering to release stress layer by layer, a complete closed-loop heat treatment chain is formed. The parameters of each process are specifically adapted to the thin, high thermal conductivity, and low rigidity characteristics of small and medium-sized GCr15 guide rail plates with a diameter of 20~50mm and 800mm. Each process compensates for the stress and structural defects of the previous process, and simultaneously achieves three core performance indicators: high wear resistance and hardness, excellent impact toughness, and ultra-low deformation.

[0069] The above description is merely an embodiment of this application and is not intended to limit the scope of protection of this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the scope of protection of this application. It should be noted that similar reference numerals and letters in the following figures indicate similar items; therefore, once an item is defined in one figure, it does not need to be further defined and explained in subsequent figures.

Claims

1. A heat treatment method for GCr15 guide rail plates used in heavy-duty ring rolling mills, characterized in that, Includes the following steps; S1. Pre-treatment: The surface of the GCr15 guide rail plate is treated with sandblasting and / or ultrasonic cleaning, followed by drying; this removes oxide scale, oil stains and impurities from the surface of the GCr15 guide rail plate after rough machining. S2, Stepped preheating: The pretreated guide rail plate is sent into a continuous heat treatment furnace and sequentially preheated by holding at 350±10℃ for 1-1.5h, 550±10℃ for 0.8-1h, and 680±10℃ for 0.5-0.8h. S3. Segmented austenitizing heating: After preheating, the temperature is raised to 820±5℃ and held for 1.5-2.5h, then raised to 850±5℃ and held for 2-3h. During the heating process, the protective atmosphere in the furnace includes nitrogen and hydrogen, with the nitrogen gas fraction being 70%-80%, the hydrogen gas fraction being 20%-30%, and the oxygen content being ≤50ppm. S4. Gradient cooling quenching: Transfer the guide plate from the heating furnace to the quenching device within a preset time. First, cool it in a nitrate bath at 200-220℃ for 1-1.5 hours, and then transfer it to a hot oil bath at 120-140℃ for 1.5-2 hours. S5. Cryogenic Stabilization Treatment: Within 2 hours after quenching, place the guide rail plate into a cryogenic chamber and cool it to -70±5℃ at a rate of 5-8℃ / min. Hold it at this temperature for 2.5-3.5 hours, and then heat it back to room temperature at a rate of 3-5℃ / min. S6. Multi-stage tempering: After the room temperature recovers, the first stage tempering is carried out within the specified time: hold at 150±5℃ for 3-4 hours, and then furnace cool to below 80℃; after semi-finishing, the second stage tempering is carried out: hold at 140±5℃ for 5-7 hours, and then furnace cool to room temperature; after finishing, the third stage tempering is carried out: hold at 120±5℃ for 8-10 hours.

2. The heat treatment method for GCr15 guide rail plates for heavy-duty ring rolling mills according to claim 1, characterized in that, The guide rail plate is made of GCr15 forged plate blank with a thickness of 20-50mm and a length of ≤800mm; the effective volume of the cryogenic chamber is ≥2m³. 3 The refrigerant is liquid nitrogen, and the temperature uniformity inside the chamber is ≤±3℃.

3. The heat treatment method for GCr15 guide rail plates for heavy-duty ring rolling mills according to claim 1, characterized in that, The GCr15 forging blank to be treated was tested for the presence of network cementite. If network cementite was present, the GCr15 forging blank to be treated was normalized before pretreatment. The normalizing treatment involves holding at 940±10℃ for 0.8-1.2 hours, followed by air cooling to room temperature. The hardness after normalizing is controlled at 240-260 HBS.

4. The heat treatment method for GCr15 guide rail plates for heavy-duty ring rolling mills according to claim 1, characterized in that, In S1, the sandblasting pressure is 0.3-0.5 MPa, the sand particle size is 0.1-0.2 mm; the ultrasonic cleaning fluid is a 5% sodium carbonate solution, the cleaning temperature is 60-70℃, the cleaning time is 15-25 min, the drying temperature is 120℃, and the drying time is 20-30 min.

5. The heat treatment method for GCr15 guide rail plates for heavy-duty ring rolling mills according to claim 1, characterized in that, In S3, the segmented austenitizing heating is carried out at a rate of 50-80℃ / h; the holding time for segmented austenitizing heating is calculated based on the maximum thickness of the guide plate as 1.0-1.2 min / mm.

6. The heat treatment method for GCr15 guide rail plates for heavy-duty ring rolling mills according to claim 1, characterized in that, In S4, the quenching device includes a nitrate bath, a hot oil bath, an electric transfer mechanism, and a frequency conversion stirring device. The solution in the nitrate bath includes potassium nitrate and sodium nitrite. The hot oil bath uses No. 320 heat transfer oil. The length of the bath is >800mm, which allows the guide plate to be completely submerged.

7. The heat treatment method for GCr15 guide rail plates for heavy-duty ring rolling mills according to claim 6, characterized in that, The nitrate bath and the hot oil bath are fixed by a base. Two variable frequency stirring devices are provided, and the two variable frequency stirring devices are respectively installed in the nitrate bath and the hot oil bath. The electric transfer mechanism is used to drive the variable frequency stirring devices to move horizontally.

8. The heat treatment method for GCr15 guide rail plates for heavy-duty ring rolling mills according to claim 7, characterized in that, The mass ratio of potassium nitrate to sodium nitrite is 1:1; during the cooling process, the medium flow rate is maintained at 1.0-1.3 m / s by a stirring device, and the preset time is ≤20s.

9. The heat treatment method for GCr15 guide rail plates for heavy-duty ring rolling mills according to claim 1, characterized in that, In S6, the specified time is ≤10s. After multi-stage tempering, surface strengthening treatment is performed. Plasma spraying combined with laser remelting is used on the working surface of the guide plate.

10. A heat treatment method for GCr15 guide rail plates for heavy-duty ring rolling mills according to claim 9, characterized in that, The plasma spraying material is WC-Co alloy powder with a spraying thickness of 0.2-0.4 mm. The laser remelting process uses a laser remelting power of 1.2-1.8 kW and a scanning speed of 4-6 mm / s.

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