Machining method for transmission gear of medium-power turboprop engine

Abstract

The invention discloses a medium-power turboprop engine transmission gear machining method which comprises gear rough machining and subsequent gear finish machining, and deformation restraining treatment is connected between the gear finish machining and the gear rough machining. The deformation inhibition treatment comprises subsection carburization, isothermal quenching and subzero treatment; the subsection carburizing is divided into a strong carburizing stage, a diffusion stage and a soaking stage; the isothermal quenching adopts a nitrate bath; the temperature of the subzero treatment is-70 DEG C to-90 DEG C. The problem that in the prior art, after gear machining is completed, operation of a turboprop engine is easily affected after storage and transportation links are conducted and then the gear is applied to the turboprop engine is solved.

Description

Technical Field

[0001] This invention relates to the field of gear processing technology, and specifically to a method for processing transmission gears for a medium-power turboprop engine. Background Technology

[0002] In the transmission system of a medium-power turboprop engine, the core gear components are subjected to harsh conditions of high-frequency meshing impact (meshing frequency ≥ 500 Hz) and high alternating load (maximum contact stress ≥ 1100 MPa) for extended periods. Their machining quality directly determines the core performance indicators of the engine's transmission system, including a transmission efficiency of over 98%, operating noise controlled below 72 dB, and a fatigue life of no less than 6000 hours. As a crucial carrier of power transmission, the precision, roughness, and structural stability of the gear tooth surface are not only fundamental to ensuring transmission efficiency and operational stability but also directly related to the overall reliability and safety of the engine.

[0003] However, current mainstream gear machining processes are insufficient to meet the aforementioned high-performance requirements, exhibiting multiple key deficiencies in machining accuracy, surface quality, and structural stability. One particularly concerning issue is that after machining, gears undergoing storage and transportation for a period before being applied to turboprop engines may experience performance changes, potentially negatively impacting the engine's stable operation. The traditional hobbing and grinding processes suffer from significant accuracy bottlenecks, achieving only level 6 accuracy according to GB / T10095.1 standards. Post-machining tooth profile errors often exceed 0.008 mm, and tooth direction errors exceed 0.006 mm, resulting in gear meshing clearance fluctuations of ±0.05 mm, directly reducing transmission efficiency by 2%–3%. Furthermore, conventional grinding processes can only maintain a surface roughness of Ra 0.8–1.6 μm, with large microscopic peak-to-valley differences, easily generating wear debris during meshing, accelerating tooth surface fatigue and spalling, and shortening the actual service life of gears to below 4000 hours, far short of design goals.

[0004] More critically, the deficiencies in the existing process regarding heat treatment and process connections directly create hidden dangers for gear deformation during storage after machining. Environmental factors during storage and transportation may further amplify this problem, increasing the risk of impacting engine operation. The gears are made of high-strength alloy materials such as 20CrNi2MoA. After carburizing and quenching heat treatment, the radial deformation of the teeth exceeds 0.03mm. Although this can be corrected through repeated grinding, it not only significantly reduces machining efficiency but also disrupts the original precision balance of the tooth surface, creating potential for deformation during subsequent storage. Furthermore, the lack of effective deformation suppression processes between roughing, semi-finishing, and heat treatment leads to the accumulation of residual stress inside the gears, resulting in a final product tooth surface precision fluctuation exceeding ±0.005mm, exhibiting extremely poor consistency. During long-term storage, the internal stress of these gears with residual stress and precision defects gradually releases. Coupled with the influence of environmental factors such as temperature and humidity, the deformation further increases. After six months of storage, the tooth profile error of some gears can increase by 0.005-0.01 mm, completely exceeding the acceptable range. Furthermore, factors such as vibration and temperature and humidity changes during transportation may also lead to further degradation of gear precision. If these gears, whose precision has decreased after storage and transportation, are installed in the engine, it may lead to a decrease in meshing accuracy, a sharp increase in operating noise, and even premature fatigue failure, thus posing a threat to the operational safety of the turboprop engine. Summary of the Invention

[0005] The purpose of this invention is to solve the problem that after the gears are processed, they are stored and transported before being applied to the turboprop engine, which can easily affect the operation of the turboprop engine. This invention provides a method for processing transmission gears for medium-power turboprop engines.

[0006] To address the shortcomings of the aforementioned technical problems, the present invention employs the following technical solution: a method for machining transmission gears for a medium-power turboprop engine, comprising rough machining of the gear and subsequent finish machining of the gear, wherein a deformation suppression treatment is performed between the finish machining and the rough machining; the deformation suppression treatment includes segmented carburizing, isothermal quenching, and cryogenic treatment; the segmented carburizing is divided into a strong carburizing stage, a diffusion stage, and a homogenization stage; the isothermal quenching employs a nitrate salt bath; and the cryogenic treatment temperature is -70 to -90°C.

[0007] As a further optimization of the processing method for transmission gears of a medium-power turboprop engine of the present invention: the strong infiltration stage is in the temperature range of 910-930℃ and the carbon potential is between 0.9% and 1.2%C, and the temperature is held for 3.5-4.5 hours; the diffusion stage temperature is in the temperature range of 910-930℃ and the carbon potential is between 0.7% and 1.0%C, and the temperature is held for 2.5-3.5 hours; the soaking stage temperature is in the temperature range of 890-910℃ and the carbon potential is between 0.8% and 1.0%C, and the temperature is held for 0.5-1.5 hours.

[0008] As a further optimization of the processing method for transmission gears of a medium-power turboprop engine of the present invention: the isothermal quenching temperature is between 850 and 870°C, and the composition of the nitrate bath is 50% KNO3 and 50% NaNO2, and the holding time is 1.5 to 2.5 hours.

[0009] As a further optimization of the machining method for transmission gears of a medium-power turboprop engine of the present invention: the roughing process includes material pretreatment, roughing, semi-finishing, and stress control; the material pretreatment is a process of pretreating the material by isothermal forging; the roughing is a process of milling the surface of the pretreated material; the semi-finishing is a process of hobbing the milled material; and the stress control is a process of three-stage annealing of the rough gear formed by the hobbing material.

[0010] As a further optimization of the processing method for transmission gears of a medium-power turboprop engine of the present invention: the isothermal forging process is carried out between 1150 and 1180°C, the holding time is 2 to 2.5 hours, and the isothermal cooling rate is in the range of 5 to 8°C / min, so as to ensure uniform grain size of the forging.

[0011] As a further optimization of the machining method for transmission gears of a medium-power turboprop engine of the present invention: the rough machining process also includes machining of positioning reference holes on the material. The positioning reference holes are provided with two tooling clamping references that can provide tooling clamping references for subsequent machining, and the roughness of the hole wall of the positioning reference holes is less than 3.2μm.

[0012] As a further optimization of the method for machining transmission gears of a medium-power turboprop engine according to the present invention: the temperature of the first stage of stress control is 400-600℃, the heating rate is 90-110℃ / h, and the temperature is held for 0.5-1.5 hours; the second stage is to raise the temperature by 70-90℃ based on the temperature of the first stage, the heating rate is 45-55℃ / h, and the temperature is held for 2-3 hours; the third stage is to cool the gears in the furnace to 190-210℃ under the conditions of the second stage, and then use air for cooling.

[0013] As a further optimization of the processing method for transmission gears of a medium-power turboprop engine of the present invention: the finishing process includes precision grinding and ultra-precision machining. Precision grinding is the process of grinding the gears with a gear grinding machine, and ultra-precision machining includes precision honing and magnetorheological polishing processes in sequence.

[0014] As a further optimization of the processing method for transmission gears of a medium-power turboprop engine of the present invention: the grinding wheel of the gear grinding machine is a cubic boron nitride grinding wheel with a grit size of 120# and the grinding wheel bonding agent is resin.

[0015] As a further optimization of the processing method for transmission gears of a medium-power turboprop engine of the present invention: the grinding wheel of the precision honing machine is an ultra-fine alumina honing wheel with a particle size of 400# and a hardness of medium-soft; the polishing fluid used in magnetorheological polishing consists of 50% carbonyl iron powder with a particle size of 1-3μm, 30% silicone oil, and 20% Al2O3 with a particle size of 49-51nm.

[0016] Compared with the prior art, the present invention has the following beneficial effects: This invention utilizes segmented carburizing, isothermal quenching, and cryogenic treatment in the deformation-inhibiting heat treatment process to bridge the roughing and finishing stages, thereby significantly reducing stress concentration during roughing. Specifically, three-stage carburizing, isothermal salt bath treatment, and cryogenic cooling greatly reduce stress and gear deformation. This ensures that after storage and transportation, the gear's deformation parameters meet the requirements for turboprop engine operation, thus greatly minimizing the impact of the gear on turboprop engine operation. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the method steps of the present invention; Detailed Implementation

[0018] To better understand the present invention, the following embodiments further illustrate the content of the present invention, but the content of the present invention is not limited to the following embodiments.

[0019] A method for machining transmission gears for a medium-power turboprop engine includes the following steps: S1, Rough machining: High-strength carburized steel (20CrNi2MoA) is formed using isothermal forging. The forging temperature is 1150–1180℃, the holding time is 2–2.5 hours, and the isothermal cooling rate is 5–8℃ / min to ensure uniform grain size and a grain size of grade 8 or higher in the forging. After isothermal forging, the high-strength carburized steel undergoes dual stress relief through normalizing and high-temperature tempering. The normalizing temperature is 920–940℃, with a holding time of 3 hours. The high-temperature tempering temperature is 650–680℃, with a holding time of 4 hours. After tempering, the high-strength carburized steel is air-cooled to control its hardness to HB170–200 and achieve an internal stress relief rate of over 90%.

[0020] A DMU50 five-axis CNC milling machine was used to machine the gear blank from the high-strength carburized steel after machining. Specifically, the outer diameter was milled first, with the following standards: the outer diameter tolerance of the high-strength carburized steel was ±0.5mm, the flatness of the end face of the high-strength carburized steel was below 0.05mm, and a machining allowance of 2-2.5mm was left; at the same time, two positioning reference holes were machined on the high-strength carburized steel bar to serve as positioning references for subsequent machining processes, and the wall roughness of the positioning reference holes Ra≤3.2μm; After the positioning reference holes are machined, an ultrasonic flaw detector of model USM35 is used to inspect the high-strength carburized rod with two positioning reference holes to ensure that there are no cracks inside the high-strength carburized rod and that the defect equivalent of the high-strength carburized rod is between Φ1 and 3mm.

[0021] After ultrasonic testing, gears are machined using a GLEASON200G CNC high-speed gear hobbing machine. The cutting tool used is an ultra-fine grain high-speed steel hob, made of W6Mo5Cr4V2Al with a hardness of HRC 63-65. During the hobbing process, the hob speed is 120-150 r / min, the axial feed is 0.6-0.8 mm / r, and the radial feed is 0.3-0.4 mm / r. After hobbing the tooth profile, the tooth surface wear is 0.25-0.3 mm, the pitch error is controlled within ±0.02 mm, and the tooth direction error is ≤0.015 mm. Simultaneously, a high-pressure water gun with a pressure of 8-10 MPa is used to clean the tooth surface of metal chips to prevent residual impurities from causing scratches in subsequent machining.

[0022] The high-strength carburized rods, after cleaning and gear hobbing, undergo segmented heating and annealing to eliminate residual cutting stress in the gear teeth during the hobbing process. The specific segmented heating and annealing process is as follows: Phase 1: Raise the room temperature to 400-600℃ at a rate of 90-110℃ / hour, while maintaining the temperature for 0.5-1.5 hours. Second stage: Increase the temperature by 70-90°C from 400-600°C, at a rate of 45-55°C / h, and hold the temperature for 2-3 hours. The second stage: After being cooled in the furnace to 190-210℃, air cooling is used to ensure that the stress relief rate on the coarse gear formed by the high-strength carburized rod after gear hobbing is above 85%, and the radial deformation of the coarse gear teeth is between 0.004 and 0.006 mm. The coarse gears, after being subjected to segmented heating and annealing, are inspected using a HEXAGONGLOBAL coordinate measuring machine to check the position of the positioning reference hole, while ensuring that the reference deviation of the positioning reference hole is between 0.007 and 0.008 mm.

[0023] S2, Deformation Suppression: The coarse gear undergoes carburizing treatment, specifically using an IPSENVTTC120 vacuum carburizing furnace with a three-stage controlled carburizing process. This reduces deformation caused by uneven carbon concentration gradients in the gear teeth. The specific carburizing steps are as follows: Strong infiltration stage: within a temperature range of 910 to 930°C, and with a carbon potential between 0.9% and 1.2%C, while maintaining the temperature for 3.5 to 4.5 hours; The diffusion stage temperature is in the range of 910 to 930°C, and the carbon potential is between 0.7% and 1.0%C, while the temperature is maintained for 2.5 to 3.5 hours. The heat spread stage is in the temperature range of 890 to 910℃, with a carbon potential between 0.8% and 1.0%C, and is held at that temperature for 0.5 to 1.5 hours. During the carburizing process, an infrared carbon potential meter of model ADEVOSC is used for real-time monitoring, and the carbon potential fluctuation range is ≤ ±0.05%C.

[0024] S6. The carburized coarse gear is quenched with the following parameters: temperature is 850-870℃, hold for 1.5 hours, then transfer to an isothermal salt bath at 190-210℃. The salt bath composition is 50% KNO3 and 50% NaNO2, and hold for 2 hours. This will form a composite structure of bainite and martensite in the internal structure of the coarse gear, while reducing quenching stress. The quenched coarse gear undergoes a cryogenic treatment, with the following specific steps: The quenched coarse gear is transferred to a THORMAXTFX cryogenic chamber at -70 to -90℃ and held for 3 hours to convert the residual austenite in the coarse gear into martensite, reducing subsequent structural deformation; then, it undergoes low-temperature tempering at 210 to 230℃ for 4 hours, followed by air cooling to achieve a tooth surface hardness of HRC59 to 62, a core hardness of HRC32 to 35, and a tooth radial deformation ≤0.015mm.

[0025] S3, Finishing: The positioning reference hole was repaired using a CNC internal grinding machine of model STUDERS41. Specifically, the positioning reference hole was ground to the following standards: Φd+H7, hole wall roughness Ra≤0.8μm, and roundness error≤0.002mm. The repaired reference hole was positioned by rigidly connecting the mandrel to the machine tool fixture, with a positioning error ≤0.003mm.

[0026] The coarse gear surface was precision ground using a REISHAUERRZ150 CNC profile grinding machine. Cubic boron nitride grinding wheels with a grit size of 120# and resin as the bonding agent were used. The grinding wheel speed was 2000–2200 r / min, the radial feed was 0.01–0.02 mm / r, and the axial feed was 0.05–0.08 mm / r. The coarse gear grinding process involves the following three steps: Rough grinding: Remove 0.1-0.3 mm of excess material, and control the tooth profile error to ≤0.008 mm; Semi-finish grinding: removing 0.07-0.09mm of allowance, with a tooth profile error ≤0.006mm; Fine grinding: Remove 0.01-0.02mm of excess material. During the grinding process, the tooth profile and tooth direction errors are detected in real time by the built-in laser detection system of the grinding machine (model BLUMNOWOTESTLaserControl). The grinding parameters are dynamically adjusted. The final tooth profile error is ≤0.005mm, tooth direction error is ≤0.004mm, and cumulative tooth pitch error is ≤0.012mm. After fine grinding, the surface roughness of the coarse gear is initially controlled at Ra0.8-1.0μm, and the surface micromorphology is detected by a white light interferometer (model ZYGONewView9000).

[0027] The coarse gears after grinding were further processed using a KAPPNILESH X100 CNC precision honing machine. Ultra-fine alumina honing wheels with a grit size of 400# and a medium-soft hardness were used. The honing wheel speed was 800–1000 r / min, the workpiece speed was 100–120 r / min, and the radial pressure was 0.15–0.2 MPa. Homing fluid, composed of 30% extreme pressure lubricating oil and 70% deionized water, was sprayed during honing to cool the coarse gears and remove grinding debris. After honing, the surface roughness was reduced to Ra 0.4–0.6 μm, and the microscopic peak-valley height difference on the tooth surface was ≤0.3 μm.

[0028] After precision honing, the coarse gears are polished using a SINON MRF300 magnetorheological polishing system to remove microscopic defects such as wear marks and burrs from the tooth surface. The polishing slurry consists of 50% carbonyl iron powder with a particle size of 1–3 μm, 30% silicone oil, and 20% Al₂O₃ with a particle size of 49–51 nm. Specifically, the magnetic field strength is 0.8 T, the polishing head rotation speed is 500 r / min, the feed rate is 5 mm / s, and the polishing time is 3–5 min / tooth. After polishing, the final tooth surface roughness reaches Ra 0.2–0.4 μm. A Bruker Dimension Edge atomic force microscope is used to ensure that the surface micro-irregularities of the coarse gear are below 0.15 μm, thus transforming the coarse gear into a precision gear.

[0029] A GLEASON PFAFFINGER 3000 gear measuring center was used for full-parameter testing. The accuracy class was GB / T10095.1~20085, and the key parameters were as follows: tooth profile error ≤0.005mm, tooth direction error ≤0.004mm, cumulative tooth pitch error ≤0.012mm, and tooth surface roughness Ra≤0.4μm. Deformation testing maintained the radial runout of the coarse gear teeth ≤0.01mm and the end face runout ≤0.008mm.

[0030] S4. Inspection after processing: After passing the inspection, the gears undergo low-temperature aging treatment with the following parameters: temperature 119~121℃, held for 12 hours, and then cooled in the furnace to further eliminate residual stress from processing, ensuring that the residual stress is below 50MPa. This ensures the long-term stability of the precision gear tooth surface accuracy, meaning that the precision gear tooth surface accuracy will fluctuate within the range of 0.001~0.002mm after 6 months of storage.

[0031] S5. Practical Application Testing: The precision-machined gears were assembled into the transmission system of a 1000kW turboprop engine. After 8000 hours of continuous operation, the wear on the gear surface was 0.01mm, the transmission efficiency remained at 98.6%, the operating noise was 71dB, and there was no fatigue spalling.

[0032] The specific embodiments of the present invention have been described above. It should be understood that the present invention is not limited to the specific embodiments described above, and those skilled in the art can make various modifications or variations within the scope of the claims, which do not affect the essence of the present invention.

Claims

1. A method for machining transmission gears for a medium-power turboprop engine, characterized in that: This includes rough machining of gears and subsequent finish machining of gears, with deformation suppression treatment at the connection between the finish machining and rough machining of gears; The deformation suppression treatment includes segmented carburizing, isothermal quenching, and deep cryogenic treatment. The segmented carburizing process is divided into a strong carburizing stage, a diffusion stage, and a homogenization stage. The isothermal quenching is performed using a nitrate bath; The cryogenic treatment temperature is -70 to -90°C.

2. The method for machining transmission gears for a medium-power turboprop engine as described in claim 1, characterized in that: The strong infiltration stage is conducted at a temperature range of 910–930°C with a carbon potential between 0.9% and 1.2%C, and is held at this temperature for 3.5–4.5 hours; the diffusion stage is conducted at a temperature range of 910–930°C with a carbon potential between 0.7% and 1.0%C, and is held at this temperature for 2.5–3.5 hours; the homogenization stage is conducted at a temperature range of 890–910°C with a carbon potential between 0.8% and 1.0%C, and is held at this temperature for 0.5–1.5 hours.

3. The method for machining transmission gears for a medium-power turboprop engine as described in claim 1, characterized in that: The isothermal quenching temperature is between 850 and 870°C, and the salt bath composition is 50% KNO3 and 50% NaNO2, and the holding time is 1.5 to 2.5 hours.

4. The method for machining transmission gears for a medium-power turboprop engine as described in claim 1, characterized in that: The roughing process includes material pretreatment, roughing, semi-finishing, and stress control; material pretreatment is the process of pretreating isothermal forgings; roughing is the process of turning the surface of the pretreated material; semi-finishing is the process of hobbing the material after turning and rough grinding; stress control is the process of three-stage annealing of the rough gear formed by the hobbing material.

5. The method for machining transmission gears for a medium-power turboprop engine as described in claim 4, characterized in that: The isothermal forging process is carried out at 1150–1180°C, with a holding time of 2–2.5 hours and an isothermal cooling rate of 5–8°C / min to ensure uniform grain size in the forgings.

6. The method for machining transmission gears for a medium-power turboprop engine as described in claim 4, characterized in that: The roughing process also includes machining positioning reference holes on the material. The positioning reference holes have two references that can provide tooling clamping for subsequent machining, and the roughness of the hole wall of the positioning reference holes is less than 3.2 μm.

7. The method for machining transmission gears for a medium-power turboprop engine as described in claim 4, characterized in that: The first stage of stress control involves a temperature of 400–600°C, a heating rate of 90–110°C / h, and a holding time of 0.5–1.5 hours. The second stage involves raising the temperature by 70–90°C based on the first stage temperature, with a heating rate of 45–55°C / h, and a holding time of 2–3 hours. The third stage involves cooling the furnace to 190–210°C under the conditions of the second stage, followed by air cooling.

8. The method for machining transmission gears for a medium-power turboprop engine as described in claim 1, characterized in that: The finishing process includes precision grinding and ultra-precision machining. Precision grinding is the process of grinding gears with a gear grinding machine, and ultra-precision machining involves precision honing and magnetorheological polishing in sequence.

9. The method for machining transmission gears for a medium-power turboprop engine as described in claim 8, characterized in that: The grinding wheel of the gear grinding machine is a cubic boron nitride grinding wheel with a grit size of 120# and a resin binder.

10. The method for machining transmission gears for a medium-power turboprop engine as described in claim 8, characterized in that: The grinding wheel of the precision honing machine is an ultrafine alumina honing wheel with a particle size of 400# and a hardness of medium-soft. The polishing fluid used in magnetorheological polishing consists of 50% carbonyl iron powder with a particle size of 1-3μm, 30% silicone oil, and 20% Al2O3 with a particle size of 49-51nm.

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