Machining process for worm of linear steering system

Abstract

The invention relates to a machining process for a worm of a linear steering system and relates to the technical field of automobile part machining, the process selects special grade 42CrMoA alloy structural steel as a base material, and fatigue crack sources are eliminated by strictly controlling the content of non-metallic inclusions; the technological process comprises preheat treatment, multi-pass turning, cryogenic stabilizing treatment and self-adaptive accurate grinding. Wherein in the subzero treatment, residual austenite is promoted to be fully converted into martensite by utilizing an ultralow-temperature environment, the structure instability is eliminated, and beneficial pressure stress is introduced; a power monitoring system is adopted in the grinding process, the abrasion state of a grinding wheel is automatically judged according to real-time load changes of a spindle, finishing is triggered, and grinding burns are prevented. The problems of size distortion and fatigue fracture of the precision worm in long-term service are effectively solved, the surface integrity of batch production is guaranteed, the transmission precision of a linear steering system is improved, and the service life of the linear steering system is prolonged.

Description

Technical Field

[0001] This application relates to the field of automotive parts processing technology, and in particular to a processing technology for a worm gear used in a linear steering system. Background Technology

[0002] Linear steering systems are a key assembly in automotive chassis systems, with the worm gear as the core transmission component. During vehicle steering, it must withstand high-frequency reciprocating loads and significant torque. The manufacturing precision and mechanical properties of the worm gear directly determine the steering system's smoothness, quietness, and service life.

[0003] In the manufacture of such high-precision worm gears, existing technologies typically use alloy structural steels such as 42CrMoA as the base material. However, ordinary industrial-grade steel often contains a certain amount of non-metallic inclusions such as sulfides and oxides. These microscopic defects disrupt the continuity of the metal matrix and are prone to becoming initiation sources of fatigue cracks under alternating stress, thus limiting the fatigue limit of the component. Simultaneously, after traditional tempering and induction hardening processes, some metastable austenitic structure inevitably remains within the material. This structure is thermodynamically unstable and is prone to phase transformation into martensite under the heat of subsequent grinding or the alternating hot and cold environments of actual vehicle use. Because this phase transformation process is accompanied by volume expansion, it can cause slight dimensional distortion in the precision-machined worm gear, thereby compromising the meshing accuracy of the tooth surface and causing steering jamming or abnormal noise problems.

[0004] Furthermore, in precision manufacturing, grinding is the final process to ensure the surface quality of the worm gear. Current mass production typically employs a fixed-number dressing strategy, meaning that the grinding wheel is dressed once after processing a fixed number of workpieces, based on experience. This open-loop control method cannot detect the actual wear state of the grinding wheel. When the grinding wheel becomes clogged or dulled prematurely due to workpiece material fluctuations, if the set dressing quantity has not been reached, continuing processing will cause a sharp increase in grinding force and temperature, easily generating imperceptible grinding burns and residual tensile stress on the workpiece surface, damaging the surface integrity and fatigue resistance. Conversely, if over-dressing is performed while the grinding wheel is still sharp, it will result in wasted grinding tools and reduced production efficiency, making it difficult to achieve process optimization while ensuring quality. Summary of the Invention

[0005] The purpose of this application is to provide a machining process for worm gears used in linear steering systems, aiming to improve the problems in the prior art where worm gears suffer from poor fatigue resistance, low dimensional accuracy retention, and difficulty in ensuring surface integrity due to the presence of many non-metallic inclusions in the substrate, unstable microstructure caused by retained austenite, and insufficient control of thermal damage during the grinding process.

[0006] By adopting the above technical solution, the present invention provides a machining process for a worm gear in a linear steering system, which adopts the following technical solution: A machining process for a worm gear used in a linear steering system includes the following steps: S1. Raw material preparation: 42CrMoA alloy structural steel is selected as the base material and preheated. S2. Turning: The treated substrate is cut to a fixed length and then turned in multiple passes to produce a worm gear semi-finished product with a grinding allowance. S3. Cryogenic stabilization treatment: The machined worm gear semi-finished product is placed in a cryogenic chamber and cooled to -140℃~-130℃ at a predetermined rate. The temperature is maintained for 3.0~4.0 hours. Then, the temperature is raised to room temperature and low-temperature stress-relief tempering is performed immediately to obtain the stabilized worm gear semi-finished product. S4. Adaptive fine grinding: The stabilized worm gear semi-finished product prepared in step S3 is ground using a fully synthetic water-based cutting and grinding fluid. During the grinding process, the power monitoring system is turned on, and the dressing trigger threshold is set to 1.20 to 1.25 times the reference no-load power. When the real-time monitored power exceeds the dressing trigger threshold and continues for a predetermined time, or when the processing quantity reaches a preset value, the grinding wheel dressing is automatically triggered to obtain the finely ground worm gear finished product. S5. Inspection and warehousing: The finished worm gear after fine grinding is subjected to magnetic particle testing. After passing the test, it is demagnetized and put into storage.

[0007] By adopting the above technical solution, this invention introduces cryogenic stabilization treatment after turning and before fine grinding. Utilizing the thermodynamic driving force provided by the ultra-low temperature environment, it promotes the transformation of metastable residual austenite in the 42CrMoA steel matrix into stable martensite, significantly reducing subsequent volume expansion and internal stress caused by natural microstructure transformation, thus improving the dimensional stability and wear resistance of the workpiece. Combined with immediate low-temperature stress-relief tempering after cryogenic treatment, it eliminates the micro-stress generated by the phase transformation volume effect during cryogenic treatment, preventing the initiation of microcracks. Simultaneously, an adaptive power monitoring strategy is employed during the fine grinding stage. Utilizing the positive correlation between grinding power and grinding force, the sharpness of the grinding wheel is monitored in real time. When the power reaches 1.20 to 1.25 times the reference no-load power, it indicates that the abrasive wear or pore blockage of the grinding wheel has led to an abnormal increase in cutting resistance. At this point, automatic dressing is triggered, effectively avoiding grinding heat accumulation and workpiece surface burns caused by grinding wheel passivation, ensuring the surface integrity and machining consistency of the worm gear teeth.

[0008] Preferably, in step S3, the specific process of the cryogenic stabilization treatment is as follows: the predetermined rate is 1.0 to 2.0 °C / min, and after the isothermal period, the temperature is increased to room temperature at a rate of 1.0 to 1.5 °C / min; the heating temperature of the low-temperature stress-relief tempering is 160 °C to 180 °C, and the holding time is 2.0 to 2.5 hours.

[0009] By adopting the above technical solutions, the cooling and heating rates are strictly controlled, avoiding cracking caused by excessive thermal shock stress on the workpiece cross-section due to drastic temperature changes. The parameter window settings for tempering temperature and time maximize the release of residual processing stress while ensuring the high hardness (wear resistance) of the worm, achieving the best match between strength and toughness.

[0010] Preferably, in step S4, the fully synthetic aqueous cutting and polishing fluid comprises the following components by mass percentage: 15.0%–20.0% polyethylene glycol graft copolymer lubricant; 25.0%–30.0% sebacic acid-triethanolamine reactive rust inhibitor; 5.0%–8.0% borate ester extreme pressure anti-wear agent; 3.0%–5.0% fatty alcohol polyoxyethylene ether; 1.0%–2.0% polyacrylamide; 0.5%–1.0% hexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine; and the balance being deionized water.

[0011] By adopting the above technical solution, this fully synthetic water-based cutting and grinding fluid eliminates the traditional mineral oil component and is designed for heavy-duty worm gear grinding. The synergistic mechanism of each component is as follows: Polyethylene glycol graft copolymer lubricant: As a core lubricating component, it utilizes its unique amphiphilic structure to form a high-strength physical adsorption film on the metal surface, effectively reducing the coefficient of friction; Boronate ester extreme pressure anti-wear agent: provides chemical extreme pressure protection against instantaneous high temperatures in the grinding zone, preventing abrasive grains from sticking to the workpiece; sebacic acid-triethanolamine reactive rust inhibitor: provides inter-process rust prevention and pH buffering capacity; Polyacrylamide and fatty alcohol polyoxyethylene ether: respectively adjust the liquid adhesion and cleaning penetration to ensure timely removal of grinding debris and prevent grinding wheel clogging.

[0012] Preferably, the preparation method of the polyethylene glycol graft copolymer lubricant includes: heating and melting polyethylene glycol, raising the temperature to 85℃~90℃ under nitrogen protection; dissolving an initiator in 1-decene, and then slowly adding it dropwise to the molten polyethylene glycol; after the addition is complete, stirring the mixture at a constant temperature to remove unreacted monomers; wherein the weight parts of each reactant are: 13.0~19.5 parts of polyethylene glycol, 2.2~3.5 parts of 1-decene, and 0.04~0.07 parts of initiator. More preferably, the polyethylene glycol is PEG-1500; and the initiator is benzoyl peroxide.

[0013] By employing the above-mentioned technical solution, a lubricant is prepared using a melt free radical grafting method. Under the action of an initiator, active hydrogen on the hydrophilic polyethylene glycol (PEG) backbone is abstracted to form free radicals, initiating the graft polymerization of the hydrophobic 1-decene monomer. The comb-like copolymer prepared by this method possesses both excellent water solubility and lubricity: the PEG backbone ensures its solubility and dispersion stability in aqueous working fluids, while the grafted hydrophobic long alkyl side chains are oriented on the metal surface, forming a shear-resistant and high-load-bearing lubricating film through steric hindrance and long-chain entanglement, thus improving the insufficient lubricity of aqueous fluids.

[0014] Preferably, the preparation method of the sebacic acid-triethanolamine reactive rust inhibitor includes: adding sebacic acid and triethanolamine to deionized water, heating to 50℃~60℃ and stirring until the solution is clear and transparent; wherein, the weight parts of each reaction raw material are: 24.0~36.0 parts of deionized water, 8.5~11.5 parts of sebacic acid, and 12.0~19.5 parts of triethanolamine.

[0015] By employing the above-mentioned technical solution, a water-soluble carboxylic acid amine salt is synthesized through the neutralization reaction of a dicarboxylic acid (sebacic acid) and an organic alcohol amine (triethanolamine). In aqueous solution, the organic acid anions and organic amine cations ionized from this amine salt are adsorbed onto the anodic and cathodic regions of the metal surface, respectively, forming a dense monomolecular adsorption layer. This effectively blocks the contact channels between the corrosive medium (oxygen, water) and the metal substrate. Simultaneously, the excess triethanolamine provides an alkaline reserve, maintaining the system pH at approximately 9.0, further inhibiting metal corrosion.

[0016] Preferably, the preparation method of the borate ester extreme pressure anti-wear agent includes: adding boric acid in batches to diethanolamine, heating to dissolve, and then raising the temperature to 110°C to 120°C, reacting and dehydrating under vacuum until no fraction is distilled out; wherein, the weight parts of each reaction raw material are: 3.5 to 7.0 parts of diethanolamine and 1.4 to 2.8 parts of boric acid.

[0017] By employing the above-mentioned technical solution, nitrogen-containing borate esters are synthesized through a vacuum dehydration esterification reaction. In the high-temperature grinding zone (>500℃), the borate ester undergoes thermal decomposition, releasing active boron elements that undergo a tribochemical reaction with the iron matrix, generating in situ a hard chemical reaction film composed of iron boride (FeB / Fe2B) and boron nitride (BN). This reaction film exhibits extremely high heat resistance and shear strength, effectively preventing direct adhesive wear between the metal and abrasive grains. Furthermore, the nitrogen atoms in diethanolamine possess lone pairs of electrons, which can form stable intramolecular coordination bonds (N→B) with electron-deficient boron atoms. This unique coordination structure enhances the borate ester's resistance to water molecule attack, solving the technical problem of traditional borate esters' easy hydrolysis and failure in aqueous systems, thus ensuring the long-term extreme pressure anti-wear performance of the cutting fluid.

[0018] Preferably, in step S1, the pre-heat treatment includes: continuously induction heating the substrate at a temperature of 860℃~890℃ for 5~10 seconds, quenching it in PAG water-soluble quenching liquid; then induction tempering at a temperature of 540℃~580℃ for 30~60 seconds, and air cooling to room temperature.

[0019] By adopting the above technical solutions, rapid induction heating and PAG quenching refined the grains, resulting in a fine and uniform martensitic structure; high-temperature short-time tempering, while maintaining high strength, endowed the matrix with good plasticity and toughness, improved the machinability of the material, and provided a uniform microstructure basis for subsequent precision machining.

[0020] Preferably, in step S2, the multi-pass turning includes roughing and finishing. Finishing is performed in segments, with a spindle speed of 1800-2800 r / min, a feed rate of 0.10-0.20 mm / r, and a single-sided grinding allowance of 0.10-0.15 mm.

[0021] By adopting the above technical solutions, the segmented precision turning process reduces the radial deformation of worm gear parts with large length-to-diameter ratios during the machining process, and improves the geometric accuracy of the semi-finished products; the precise control of the grinding allowance (0.10~0.15mm) ensures that the tool marks and deteriorated layers left by turning can be completely removed, while avoiding the problems of excessive grinding force and severe heat accumulation caused by excessive allowance.

[0022] Preferably, in step S5, the magnetic particle inspection adopts a continuous wet method, with a circumferential magnetization current of 800-1200A and an energizing time of 1-3 seconds.

[0023] By adopting the above technical solution, and utilizing the high sensitivity of continuous wet magnetic particle testing, combined with high-current circumferential magnetization, it is possible to effectively detect minute grinding cracks and material hairline cracks on the worm gear tooth surface, ensuring zero defects in the products leaving the factory and guaranteeing the safety of the automotive steering system.

[0024] In summary, this application includes at least one of the following beneficial technical effects: 1. This application uses premium 42CrMoA alloy structural steel as the base material, which controls the content of non-metallic inclusions such as sulfides and oxides inside the material. The high purity of the matrix material ensures the continuity of the metal structure and effectively eliminates the micro-defect sources that lead to the initiation of fatigue cracks. Under the subsequent quenching and tempering treatment and cyclic loading during service, the base material avoids local stress concentration at the interface caused by the difference in thermal expansion coefficients between inclusions and the matrix. Combined with the subsequent surface strengthening process, the torsional fatigue strength of the worm gear under high-frequency reciprocating motion and extreme torque conditions in the linear steering system is improved, and the service life of the component is extended. 2. This application introduces a cryogenic stabilization process after semi-finishing, placing the workpiece in an ultra-low temperature environment to promote the full transformation of the unstable residual austenite after quenching into martensite, thus inducing and releasing the volume expansion caused by the phase transformation in advance. Combined with the subsequent low-temperature stress-relieving tempering, the workpiece achieves extremely high microstructure thermodynamic stability before entering the final fine grinding. This measure effectively solves the problem of dimensional distortion caused by the slow evolution of the internal structure of the worm gear under the cold and hot shock environment in subsequent processing and actual use. At the same time, it improves the ability to retain the residual compressive stress layer on the surface of the workpiece, ensuring the long-term geometric accuracy of the precision transmission components. 3. This application establishes an adaptive grinding dressing mechanism based on power monitoring. By collecting spindle load data in real time, it dynamically judges the wear and dulling degree of the grinding wheel. The system sets a specific power threshold as the dressing trigger condition, replacing the traditional fixed machining quantity dressing mode. It can keenly sense changes in the cutting state and perform dressing in time when the grinding wheel becomes clogged or the abrasive grains become dull. This ensures that the grinding process is always carried out under low grinding heat and low grinding force, fundamentally avoiding grinding burns, reduced hardness, and roughness deviations on the workpiece surface caused by the deterioration of grinding wheel performance, and ensuring the consistency of product surface quality in mass production. Detailed Implementation

[0025] Raw material specifications: In this embodiment of the invention, the 42CrMoA alloy structural steel selected in step S1 has the following chemical composition by mass percentage: carbon 0.38%-0.45%, silicon 0.17%-0.37%, manganese 0.50%-0.80%, chromium 0.90%-1.20%, molybdenum 0.15%-0.25%, with the balance being iron and unavoidable impurities; and its cleanliness meets the following requirements: non-metallic inclusions: A-class fine series ≤ 3.5 grade and coarse series ≤ 3.0 grade, B-class fine series ≤ 3.0 grade and coarse series ≤ 2.5 grade, C-class fine series ≤ 2.5 grade and coarse series ≤ 1.5 grade, D-class fine series ≤ 2.5 grade and coarse series ≤ 1.5 grade; grain size ≥ 5 grade.

[0026] Preparation Examples 1-3: Preparation Example 1: This preparation example provides a fully synthetic aqueous cutting and polishing fluid, the composition of which is as follows (by weight percentage): 17.5% polyethylene glycol graft copolymer lubricant, 27.5% sebacic acid-triethanolamine reactive rust inhibitor, 6.5% borate ester extreme pressure anti-wear agent, 4.0% fatty alcohol polyoxyethylene ether (AEO-9), 1.5% polyacrylamide (PAM), 0.8% hexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine, and the balance being deionized water. The preparation method of this cutting fluid includes the following steps: Preparation of polyethylene glycol graft copolymer lubricant: By weight, 16.5 kg of polyethylene glycol (PEG-1500) was added to a reaction vessel and heated to 65°C to melt, with nitrogen gas purging for protection; 2.75 kg of 1-decene was separately prepared, in which 0.055 kg of benzoyl peroxide (BPO) was dissolved; the reaction vessel was heated to 85°C-90°C, and the 1-decene solution containing the initiator was slowly added dropwise, with the addition time controlled at 1 hour. After the addition was completed, the reaction was stirred at a constant temperature for 3 hours, and unreacted monomers were removed by vacuum distillation to obtain the lubricant for later use; Preparation of sebacic acid-triethanolamine reactive rust inhibitor: Add 30.0 kg of deionized water, 10.0 kg of sebacic acid and 15.6 kg of triethanolamine to a reaction vessel, turn on the heating and stir, raise the temperature to 50℃-60℃, and react until the solution is clear and transparent to obtain the rust inhibitor solution for later use. Preparation of borate ester extreme pressure anti-wear agent: Add 5.3 kg of diethanolamine to a reaction flask, heat to 80 °C, add 2.1 kg of boric acid in batches, stir until the solid is completely dissolved, then raise the temperature to 110 °C-120 °C and dehydrate under vacuum. React for 2-3 hours until no distillate is distilled out to obtain extreme pressure anti-wear agent for later use. Compound preparation: Add an appropriate amount of deionized water to the mixing vessel, heat to 45°C, and then add 27.5 kg of the rust inhibitor solution prepared above, 17.5 kg of the lubricant prepared above, and 4.0 kg of fatty alcohol polyoxyethylene ether in sequence. Stir at high speed (900 r / min) for 30 minutes. Then add 6.5 kg of the extreme pressure anti-wear agent prepared above, 0.8 kg of hexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine, and 1.5 kg of polyacrylamide. Adjust the stirring speed to 300 r / min and continue stirring for 20 minutes. Finally, add deionized water to make the total weight 100 kg, and filter to obtain the final product.

[0027] Preparation Example 2: This preparation example provides a fully synthetic aqueous cutting and polishing fluid with the same components as before, but with adjusted proportions.

[0028] The preparation method of this cutting fluid includes the following steps: Preparation of polyethylene glycol graft copolymer lubricant: The operation process is the same as in preparation example 1, but the feed amount is adjusted to 14.1 kg of polyethylene glycol, 2.42 kg of 1-decene, and 0.044 kg of initiator to obtain the lubricant for later use; Preparation of sebacic acid-triethanolamine reactive rust inhibitor: The operation process is the same as in preparation example 1, but the amount of materials is adjusted to 25.0 kg of deionized water, 9.1 kg of sebacic acid, and 12.2 kg of triethanolamine to obtain the rust inhibitor solution for later use; Preparation of borate ester extreme pressure anti-wear agent: The operation process is the same as in preparation example 1, but the amount of feed is adjusted to 4.1 kg of diethanolamine and 1.6 kg of boric acid to obtain extreme pressure anti-wear agent for later use; Compound preparation: The operation process is the same as in Preparation Example 1, and the amount of each component added is as follows: 15.0 kg of the lubricant prepared above, 25.0 kg of the rust inhibitor solution prepared above, 5.0 kg of the extreme pressure anti-wear agent prepared above, 3.0 kg of fatty alcohol polyoxyethylene ether, 1.0 kg of polyacrylamide, 0.5 kg of hexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine, and deionized water is added to bring the total to 100 kg.

[0029] Preparation Example 3: This preparation example provides a fully synthetic aqueous cutting and polishing fluid with the same components as before, but with adjusted proportions.

[0030] The preparation method of this cutting fluid includes the following steps: Preparation of polyethylene glycol graft copolymer lubricant: The operation process is the same as in preparation example 1, but the feed amount is adjusted to 18.8 kg of polyethylene glycol, 3.2 kg of 1-decene, and 0.066 kg of initiator to obtain the lubricant for later use; Preparation of sebacic acid-triethanolamine reactive rust inhibitor: The operation process is the same as in preparation example 1, but the feed amount is adjusted to 35.0 kg of deionized water, 11.0 kg of sebacic acid, and 19.0 kg of triethanolamine to obtain the rust inhibitor solution for later use; Preparation of borate ester extreme pressure anti-wear agent: The operation process is the same as in preparation example 1, but the amount of feed is adjusted to 6.5 kg of diethanolamine and 2.53 kg of boric acid to obtain extreme pressure anti-wear agent for later use; Compound preparation: The operation process is the same as in Preparation Example 1, and the amount of each component added is as follows: 20.0 kg of the lubricant prepared above, 30.0 kg of the rust inhibitor solution prepared above, 8.0 kg of the extreme pressure anti-wear agent prepared above, 5.0 kg of fatty alcohol polyoxyethylene ether, 2.0 kg of polyacrylamide, 1.0 kg of hexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine, and deionized water is added to bring the total to 100 kg.

[0031] Examples 1-3: Example 1: This embodiment provides a machining process for a worm gear in a linear steering system. It uses 42CrMoA alloy structural steel (premium grade) from the aforementioned raw material portion as the base material and the fully synthetic water-based cutting and polishing fluid obtained in Preparation Example 1 (diluted to a 5% concentration). The process includes the following steps: S1. Raw Material Preparation (Preparatory Heat Treatment): φ18mm bar stock is continuously induction tempered at 875℃ for 8 seconds, followed by quenching in 25℃ PAG water-soluble quenching liquid. Induction tempering is then performed at 560℃ for 45 seconds, followed by air cooling to room temperature. The surface and core hardness are measured to be HRC32, with a hardness difference ΔHV of 15. S2. Turning: The treated substrate is cut to a fixed length using a band saw at a cutting speed of 52m / min. A 130mm cold-drawn deformation zone is removed from the head of the bar stock, resulting in a fixed-length sawing length of 120.2mm. Multiple turning passes are then performed to produce a worm gear semi-finished product with a grinding allowance. Rough turning uses a spindle speed of 2100r / min and a feed rate of 0.25mm / r. Finish turning uses a three-blade process at a speed of 2400r / min and a feed rate of 0.15mm / r, machining to a single-sided grinding allowance of 0.12mm. S3. Cryogenic Stabilization Treatment: The machined worm gear semi-finished product is placed in a cryogenic chamber and cooled to -135℃ at a rate of 1.5℃ / min, and kept at a constant temperature for 3.5 hours; then heated to room temperature at a rate of 1.2℃ / min; immediately subjected to low-temperature stress-relief tempering at 170℃ for 2.0 hours, and cooled with the furnace to obtain the stabilized worm gear semi-finished product. S4. Adaptive Fine Grinding: The stabilized worm gear semi-finished product prepared in step S3 is ground using a fully synthetic water-based cutting and grinding fluid. A 60# chromium corundum grinding wheel is used with a wheel linear speed of 45 m / s and a workpiece rotation speed of 135 r / min. During the grinding process, the power monitoring system is activated, and the dressing trigger threshold is set to 1.23 times the reference no-load power. When the real-time monitored power exceeds the dressing trigger threshold for 0.5 seconds, or when the number of processed parts reaches 60, the grinding wheel dressing is automatically triggered to obtain the finely ground worm gear finished product. The rough grinding feed is 0.7 mm / min, the fine grinding feed is 0.15 mm / min, and the finishing grinding takes 5 seconds. S5. Inspection and warehousing: The finished worm gear after fine grinding is subjected to magnetic particle testing. The continuous wet testing method is used, with a circumferential magnetization current of 1000A and energization for 2 seconds. After the test shows no cracks, the worm gear is demagnetized and put into storage.

[0032] Example 2: This embodiment provides a machining process for a worm gear in a linear steering system. It uses 42CrMoA alloy structural steel (premium grade) from the aforementioned raw material section as the base material and the fully synthetic water-based cutting and grinding fluid obtained in Preparation Example 2 (diluted to a 3% concentration for use, focusing on cooling and cleaning). This process selects boundary values ​​for process parameters (low temperature, long duration) and includes the following steps: S1. Raw material preparation (preparatory heat treatment): Induction heating temperature 860℃, heating time 10 seconds, quenching; tempering temperature 540℃, holding time 60 seconds, air cooling; the hardness was measured to be HRC34 (the hardness is slightly higher after low temperature tempering, which is in line with the pattern). S2. Turning: The treated substrate is cut to a fixed length at a cutting speed of 50 m / min, removing 120 mm of the head, and then sawn to a fixed length; then multiple turning passes are performed, with rough turning at a speed of 2000 r / min and a feed rate of 0.30 mm / r; finish turning at a speed of 1800 r / min and a feed rate of 0.20 mm / r, machining to a single-sided grinding allowance of 0.15 mm; S3. Cryogenic Stabilization Treatment: The machined worm gear semi-finished product is placed in a cryogenic chamber and cooled to -140℃ at a slow rate of 1.0℃ / min, and kept at a constant temperature for 4.0 hours; then the temperature is raised to room temperature at a rate of 1.0℃ / min; low-temperature stress-relief tempering is immediately carried out at a temperature of 160℃ for 2.5 hours, and then cooled with the furnace. S4, Adaptive fine grinding: grinding wheel linear speed 43m / s, workpiece rotation speed 120r / min; power monitoring system is turned on during grinding, and the dressing trigger threshold is set to 1.20 times the reference no-load power; the rest of the logic is the same as in Example 1; rough grinding feed 0.6mm / min, fine grinding feed 0.1mm / min, and finishing grinding 7 seconds; S5. Inspection and warehousing: Circumferential magnetization current 800A, energized for 3 seconds, demagnetized and warehousing after inspection for no cracks.

[0033] Example 3: This embodiment provides a machining process for a worm gear in a linear steering system. It uses 42CrMoA alloy structural steel (premium grade) from the aforementioned raw material section as the base material and the fully synthetic water-based cutting and polishing fluid obtained in Preparation Example 3 (diluted to a 10% concentration for lubrication). This process selects the other extreme boundary value of the process parameters (relatively high temperature and short time), and includes the following steps: S1. Raw material preparation (preparatory heat treatment): Induction heating temperature 890℃, heating time 5 seconds, quenching; tempering temperature 580℃, holding time 30 seconds, air cooling; hardness measured as HRC30. S2. Turning: The treated substrate is cut to a fixed length at a cutting speed of 55 m / min, removing 140 mm of the head, and then sawn to a fixed length; then multiple turning passes are performed, with rough turning at a speed of 2200 r / min and a feed rate of 0.15 mm / r; finish turning at a speed of 2800 r / min and a feed rate of 0.10 mm / r, machining to a single-sided grinding allowance of 0.10 mm; S3. Cryogenic Stabilization Treatment: The machined worm gear semi-finished product is placed in a cryogenic chamber and cooled to -130℃ at a rate of 2.0℃ / min, and kept at a constant temperature for 3.0 hours; then the temperature is raised to room temperature at a rate of 1.5℃ / min; low-temperature stress-relief tempering is immediately carried out at a temperature of 180℃ for 2.0 hours, and then cooled with the furnace. S4, Adaptive Fine Grinding: Grinding wheel linear speed 47m / s, workpiece rotation speed 150r / min; power monitoring system is turned on during grinding, and the dressing trigger threshold is set to 1.25 times the reference no-load power; the rest of the logic is the same as in Example 1; rough grinding feed 0.8mm / min, fine grinding feed 0.2mm / min, and finishing grinding 5 seconds; S5. Inspection and warehousing: Circumferential magnetization current 1200A, energized for 1 second, demagnetized and warehousing after inspection for no cracks.

[0034] Comparative Examples 1-3: Comparative Example 1: Compared with Example 1, the difference is that the raw material is replaced with the 42CrMoA alloy structural steel (ordinary grade, high inclusion content, whose cleanliness does not meet the requirements in the above "Raw Material Specifications"), and the rest are the same.

[0035] Comparative Example 2: Compared with Example 1, the difference is that the deep cryogenic stabilization treatment in step S4 is omitted. Instead, the workpiece after precision turning is directly subjected to stress-relieving tempering treatment at 170°C for 2.0 hours, and then directly proceeds to step S5. All other steps are the same.

[0036] Comparative Example 3: Compared with Example 1, the difference is that the power monitoring system is not turned on during the grinding process in step S5, the adaptive dressing function is canceled, and the traditional fixed-number dressing method is adopted instead, that is, the grinding wheel is fixedly dressed once after grinding 20 workpieces continuously, and the rest are the same.

[0037] Test Example 1-3: Test Example 1: The Influence of Substrate Cleanliness on the Fatigue Life of Worms Experimental description: This test aims to compare the fatigue resistance of the worm gears prepared in Example 1 and Comparative Example 1 under cyclic torsional loads. The main difference between the two lies in the metallurgical quality (premium grade vs. ordinary grade) of the raw material, 42CrMoA alloy structural steel.

[0038] The experimental steps are as follows: Sample selection: Ten worm gears were randomly selected from the final products of Example 1 and Comparative Example 1 as test samples, and labeled as Group A (Example 1) and Group B (Comparative Example 1), respectively.

[0039] Testing equipment: Electro-hydraulic servo torsional fatigue testing machine.

[0040] Installation and debugging: Install the worm sample horizontally in the testing machine fixture, ensuring that the coaxiality deviation is less than 0.02mm, so as to eliminate the interference of installation error on the test results.

[0041] Loading parameters: The loading waveform is set to a sine wave, and the stress ratio R = -1 (fully symmetrical cyclic loading). The loading torque is set to ±420 N·m (simulating high load conditions under extreme conditions), and the loading frequency is 15 Hz.

[0042] Termination criteria: When the specimen fractures or the stiffness decreases by more than 15% of the initial value, it is considered a failure, and the number of cycles at this time is recorded as the fatigue life; if the number of cycles reaches 2.0×10^6 and there is still no failure, the experiment is stopped and recorded as no failure.

[0043] Fracture observation: Perform a macroscopic visual inspection of the fracture location of the failed sample and record the location of the fracture initiation point (tooth root, shaft body or keyway).

[0044] Experimental data: Table 1. Record of Torsional Fatigue Life Test Data for Finished Worm Gear Sample number source Fatigue life (number of cycles, ×10^5) Failure status description A-01 Example 1 18.24 Tooth root fracture A-02 Example 1 >20.00 Not expired A-03 Example 1 19.56 Tooth root fracture A-04 Example 1 17.89 Tooth root fracture A-05 Example 1 >20.00 Not expired A-06 Example 1 18.92 Tooth root fracture A-07 Example 1 19.11 Tooth root fracture A-08 Example 1 16.55 Early crack propagation at the tooth root A-09 Example 1 >20.00 Not expired A-10 Example 1 19.03 Tooth root fracture B-01 Comparative Example 1 8.45 Shaft breakage at the junction B-02 Comparative Example 1 10.22 Tooth root fracture B-03 Comparative Example 1 6.78 Tooth surface peeling leads to fracture B-04 Comparative Example 1 9.15 Tooth root fracture B-05 Comparative Example 1 11.34 Tooth root fracture B-06 Comparative Example 1 7.92 Abnormal fracture at journal B-07 Comparative Example 1 5.66 Early tooth root fracture B-08 Comparative Example 1 9.88 Tooth root fracture B-09 Comparative Example 1 12.01 Tooth root fracture B-10 Comparative Example 1 8.13 Deep cracks on tooth surface Conclusions and Mechanism Analysis: Based on the test data and experimental process analysis in Table 1, the worm gear prepared in Example 1 is superior to that in Comparative Example 1 in terms of fatigue life and data stability.

[0045] Comparative Example 1 uses ordinary grade 42CrMoA, which has a high content of non-metallic inclusions (such as sulfides and oxides) in its matrix. These inclusions disrupt the continuity of the metal matrix. During the S1 heat treatment and S3 cryogenic treatment processes, due to the difference in thermal expansion coefficients between the inclusions and the matrix, a microscopic internal stress field is easily generated around the inclusions, leading to the initiation of microcracks. In the heavy grinding stages of S4 and S5, the hard inclusions on the surface are easily peeled off, leaving micropores on the workpiece surface or causing localized high-temperature accumulation in the grinding zone.

[0046] In subsequent fatigue tests, these inclusions and micropores located near or subsurface constituted stress concentration sources. Under alternating torsional stress, cracks rapidly initiated and propagated from these defects, resulting in generally low fatigue lives (mean approximately 9 × 10⁵ cycles) and extremely high data dispersion (5.66–12.01 × 10⁵ cycles) for the samples in Comparative Example 1. The atypical fracture locations (shaft body, journal) observed in samples B-01 and B-06 confirmed the impact of the randomness of internal defects in the raw materials on the overall structural strength.

[0047] Example 1 uses premium grade 42CrMoA, and the high cleanliness of the substrate ensures the isotropy and continuity of the material. This characteristic maximizes the effectiveness of subsequent processing steps. Effectiveness of cryogenic treatment: The cryogenic treatment in step S3 promotes the transformation of retained austenite into martensite, resulting in volume expansion and introducing beneficial residual compressive stress. Due to the purity of the substrate, this phase transformation stress is evenly distributed and does not form destructive tensile stress concentrations around inclusions as seen in Comparative Example 1.

[0048] Adaptive grinding coordination: High-purity substrates exhibit more consistent cutting performance during S4 and S5 grinding processes. Combined with a power monitoring system, it effectively avoids instantaneous grinding burns caused by material hard spots, ensuring surface integrity.

[0049] Ultimately, the worm gear of Example 1 exhibited a prolonged crack initiation period under high-cycle loading. The failure mode was primarily concentrated at the tooth root, where the design stress was highest, rather than at material defects. Furthermore, multiple samples remained intact after 2 million cycles, demonstrating the synergistic effect of the combined process of high-cleanliness substrate, cryogenic stabilization, and adaptive grinding in improving the fatigue resistance of key components in linear steering systems.

[0050] Test Example 2: Effect of Cryogenic Treatment on Dimensional Stability and Residual Stress Retention of Worm Gear. This test aims to compare the dimensional accuracy retention and surface residual compressive stress attenuation of worm gears prepared in Example 2 (including cryogenic stabilization treatment) and Comparative Example 2 (cryogenic treatment omitted, only conventional tempering) under thermal shock conditions.

[0051] The experimental steps are as follows: Sample selection: Eight samples were randomly selected from the finished products of Example 2 and Comparative Example 2, and labeled as Group C (Example 2) and Group D (Comparative Example 2), respectively.

[0052] Initial data acquisition: The total profile deviation (Fa) and radial runout (Fr) of all samples were measured using a gear measuring center and recorded as initial dimensional data.

[0053] Using an X-ray stress measuring instrument, sin 2 The ψ method involves selecting three test points at the pitch circle of the worm gear to measure the residual stress on the surface, and recording the average value as the initial stress value.

[0054] Thermal cycling test: All samples were placed in a high and low temperature test chamber for accelerated aging testing. A single cycle was set as follows: heating to 120℃ and holding for 2 hours, cooling to -40℃ within 30 minutes and holding for 2 hours, then returning to room temperature. A total of 50 cycles were performed. This process simulates the extreme temperature changes experienced by the steering system throughout its entire lifespan, inducing transformation of unstable internal structures and stress release within the material.

[0055] Final state data acquisition: After the cycle is completed, the sample is placed in a constant temperature room (20℃) for 24 hours, and the above dimensional parameters and residual stress are measured again.

[0056] Data processing: Calculate the radial runout change (ΔFr = |final value - initial value|) and the residual stress decay rate ((initial value - final value) / initial value × 100%).

[0057] Experimental data: Table 2. Record of dimensional stability and stress data of the finished worm gear after thermal cycling. Negative values ​​indicate compressive stress.

[0058] Conclusions and Mechanism Analysis: By comparing and analyzing the data in Table 2, Example 2 is superior to Comparative Example 2 in terms of dimensional stability and residual stress retention, which verifies the key role of the cryogenic stabilization treatment in step S3 in the manufacturing of precision worm gears.

[0059] Comparative Example 2 omitted cryogenic treatment and only underwent conventional tempering. After heat treatment and quenching, a certain amount of metastable austenite inevitably remains inside the material. This structure is thermodynamically unstable at room temperature. Under the thermal cycling of Test Example 2, or under temperature fluctuations in actual working conditions, the retained austenite will gradually decompose and transform into martensite or bainite. Since the transformation of austenite to martensite is accompanied by a volume expansion of about 1% to 3%, this non-uniform volume change directly leads to irreversible distortion of the workpiece dimensions. The radial runout of the samples in Group D in Table 2 is generally between 3 and 6 μm, and the dispersion is large, indicating that structural evolution and stress redistribution have occurred inside the material. At the same time, the phase transformation stress generated by the structural transformation is superimposed with the grinding residual stress, resulting in a large release of the beneficial compressive stress originally introduced by grinding (attenuation rate exceeding 20%), reducing the fatigue resistance of the workpiece surface.

[0060] Example 2 introduced a cryogenic stabilization treatment at -140°C. This process provides an extremely low temperature driving force, prompting the retained austenite to fully transform into martensite before finishing, thus completing most of the volume expansion process before machining. Subsequent low-temperature stress-relief tempering further stabilized the newly formed martensite structure and eliminated the microscopic internal stresses generated by the phase transformation.

[0061] Therefore, when the worm gear semi-finished product of Example 2 entered the grinding step S4, its matrix structure was already in a highly stable state. In the subsequent thermal cycling test, due to the lack of a material basis for phase transformation, the dimensional changes of the C group samples were minimal (average ΔFr value of approximately 0.2 μm), exhibiting excellent geometric stability. Furthermore, the stable matrix structure better maintains the residual compressive stress layer introduced by precision grinding, with its stress attenuation rate controlled within 6%. This dual stability of dimensional accuracy and surface stress state is a core factor ensuring that the linear steering system maintains high precision, low noise, and long service life during long-term operation.

[0062] Test Example 3: Surface Quality Consistency and Grinding Burn Monitoring Test in Batch Grinding Processes Experimental description: This test aims to compare the ability of Example 3 (adaptive dressing) and Comparative Example 3 (constant dressing) to control the surface quality of the worm under continuous production conditions, especially to evaluate the surface integrity fluctuations caused by grinding wheel wear.

[0063] The experimental steps are as follows: Experimental setup: Using the same CNC grinding machine, continuous grinding was performed according to the process logic of Example 3 and Comparative Example 3, respectively.

[0064] Sample size: 20 worm gears are processed continuously in each process group, and the workpieces are numbered according to the production sequence (No.1-No.20).

[0065] Process monitoring: Real-time recording of grinding machine spindle power data.

[0066] Example 3: Automatic adjustment is triggered based on the power monitoring system (the threshold is set to 1.25 times the reference no-load power).

[0067] Comparative Example 3: A fixed-number trimming strategy is adopted, with a trimming interval of 20 pieces (i.e., trimming is planned to be carried out after the 20th piece is processed).

[0068] Testing indicators: Surface roughness (Ra): Measured for each sample (No.1-No.20).

[0069] Surface hardness (HRC): Hardness test is performed on each sample at the pitch circle of the tooth surface. If the hardness is more than 2 units lower than the reference value (HRC30), it is considered to have a risk of burn.

[0070] Data logging: Records actual measured data and the timing of system-triggered adjustments.

[0071] Experimental data: Table 3 Surface roughness and hardness monitoring data during continuous grinding process Conclusions and Mechanism Analysis: Based on the test results of 20 full samples in Table 3, the impact of the two finishing strategies on product quality can be clearly observed: Wear evolution process: As the number of samples increased (No.1 to No.10), all indicators of both groups remained within a good range. However, starting from No.11, the abrasive grains of the grinding wheel showed obvious passivation, and the roughness of Comparative Example 3 rapidly deteriorated from 0.35 μm to 0.72 μm of No.20.

[0072] Disadvantages of constant adjustment: Comparative Example 3 was set to a fixed 20 pieces for dressing. During stages No. 13-No. 14, the grinding wheel had actually reached its wear critical point, and the sharp increase in cutting force led to heat accumulation. However, because the preset dressing quantity had not been reached, the system continued to force processing. This resulted in the hardness of sample No. 14 dropping to HRC 27.4, and the hardness of samples No. 15 to No. 20 all falling below HRC 27.0, with severely out-of-tolerance surface roughness. This indicates that severe grinding tempering (burning) occurred on the workpiece surface during periods No. 14 to No. 20, rendering the latter half of this batch entirely unusable.

[0073] Advantages of adaptive trimming: In Example 3, during the machining of No. 13, the power was monitored to be close to the threshold. During the machining of No. 14, the real-time power exceeded 1.25 times the idle power and remained so. The system immediately detected wheel passivation and automatically triggered dressing. Therefore, the surface roughness of No. 14 and subsequent samples No. 15-No. 20 rapidly recovered and stabilized at an excellent level of 0.21-0.24 μm, and the hardness also recovered to around HRC30, completely avoiding grinding burn.

[0074] In summary, during the small-batch continuous processing test of 20 pieces, Example 3 successfully intervened at the critical point of grinding wheel failure (No. 14) through power closed-loop feedback, while Comparative Example 3, lacking real-time sensing capabilities, resulted in batch quality accidents in the last 30% of its products. The process of this invention ensures the surface integrity and process stability of the worm gear manufacturing for linear steering systems.

Claims

1. A machining process for a worm gear used in a linear steering system, characterized in that, Includes the following steps: S1. Raw material preparation: 42CrMoA alloy structural steel is selected as the base material and preheated. S2. Turning: The treated substrate is cut to a fixed length and then turned in multiple passes to produce a worm gear semi-finished product with a grinding allowance. S3. Cryogenic stabilization treatment: The machined worm gear semi-finished product is placed in a cryogenic chamber and cooled to -140℃~-130℃ at a predetermined rate. The temperature is maintained for 3.0~4.0 hours. Then, the temperature is raised to room temperature and low-temperature stress-relief tempering is performed immediately to obtain the stabilized worm gear semi-finished product. S4. Adaptive fine grinding: The stabilized worm gear semi-finished product prepared in step S3 is ground using a fully synthetic water-based cutting and grinding fluid. During the grinding process, the power monitoring system is turned on, and the dressing trigger threshold is set to 1.20 to 1.25 times the reference no-load power. When the real-time monitored power exceeds the dressing trigger threshold and continues for a predetermined time, or when the processing quantity reaches a preset value, the grinding wheel dressing is automatically triggered to obtain the finely ground worm gear finished product. S5. Inspection and warehousing: The finished worm gear after fine grinding is subjected to magnetic particle testing. After passing the test, it is demagnetized and put into storage.

2. The machining process for a worm gear in a linear steering system according to claim 1, characterized in that, In step S3, the specific process of the cryogenic stabilization treatment is as follows: The predetermined rate is 1.0 to 2.0 °C / min, and after the constant temperature is completed, the temperature is increased to room temperature at a rate of 1.0 to 1.5 °C / min. The heating temperature for the low-temperature stress-relief tempering is 160℃~180℃, and the holding time is 2.0~2.5 hours.

3. The machining process for a worm gear in a linear steering system according to claim 1, characterized in that, In step S4, the fully synthetic aqueous cutting and polishing fluid comprises the following components by mass percentage: Polyethylene glycol graft copolymer lubricant 15.0%~20.0%; Sebacic acid-triethanolamine reactive rust inhibitor 25.0%~30.0%; Boronate ester extreme pressure anti-wear agent 5.0%–8.0%; Fatty alcohol polyoxyethylene ether 3.0%–5.0%; Polyacrylamide 1.0%–2.0%; Hexahydro-1,3,5-tris(2-hydroxyethyl)-s-triazine 0.5%–1.0%; The remainder is deionized water.

4. The machining process for a worm gear in a linear steering system according to claim 3, characterized in that, The preparation method of the polyethylene glycol graft copolymer lubricant includes: Polyethylene glycol is heated to melt and then heated to 85℃~90℃ under nitrogen protection; The initiator is dissolved in 1-decene and then slowly added dropwise to molten polyethylene glycol. After the addition is complete, the mixture is stirred at a constant temperature to react and unreacted monomers are removed to obtain the final product. The weight parts of each reactant are as follows: 13.0-19.5 parts of polyethylene glycol, 2.2-3.5 parts of 1-decene, and 0.04-0.07 parts of initiator.

5. The machining process for a worm gear in a linear steering system according to claim 4, characterized in that, The polyethylene glycol is PEG-1500; the initiator is benzoyl peroxide.

6. The machining process for a worm gear in a linear steering system according to claim 3, characterized in that, The preparation method of the sebacic acid-triethanolamine reactive rust inhibitor includes: Sebacic acid and triethanolamine were added to deionized water, heated to 50℃~60℃ and stirred until the solution was clear and transparent. The weight parts of each reaction raw material are as follows: 24.0-36.0 parts of deionized water, 8.5-11.5 parts of sebacic acid, and 12.0-19.5 parts of triethanolamine.

7. The machining process for a worm gear in a linear steering system according to claim 3, characterized in that, The preparation method of the borate ester extreme pressure anti-wear agent includes: Boric acid was added to diethanolamine in batches, heated to dissolve, and then heated to 110℃~120℃. The reaction was carried out under vacuum until no fraction was distilled off. The weight parts of each reactant are as follows: 3.5 to 7.0 parts of diethanolamine and 1.4 to 2.8 parts of boric acid.

8. The machining process for a worm gear in a linear steering system according to claim 1, characterized in that, In step S1, the pre-heat treatment includes: The substrate is continuously induction heated to a temperature of 860℃~890℃ for 5~10 seconds, and then quenched in PAG water-soluble quenching liquid. Then, induction tempering is performed at a temperature of 540℃~580℃, held for 30~60 seconds, and then air-cooled to room temperature.

9. The machining process for a worm gear in a linear steering system according to claim 1, characterized in that, In step S2, the multi-pass turning includes roughing and finishing. Finishing is carried out in segments, with a spindle speed of 1800-2800 r / min, a feed rate of 0.10-0.20 mm / r, and a grinding allowance of 0.10-0.15 mm left on one side.

10. The machining process for a worm gear in a linear steering system according to claim 1, characterized in that, In step S5, the magnetic particle inspection adopts the continuous wet method, with a circumferential magnetization current of 800-1200A and an energizing time of 1-3 seconds.

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