Deformation Control of Ion Nitriding of Double Inner Ring Gear of Large Mine Car
Our company has taken on the challenge of ion nitriding the inner ring gear surface of a large mine car reducer. The ring gear, as shown in the photo, is a special double-connected, thin-walled structure that demands high precision. The left end has a module of 11.7, while the right end has a module of 8.7. The material used is 40CrNiMo. Nitriding is the final step in the gear manufacturing process, performed after grinding the finished product. Ensuring the high precision of the gear requires strict control over nitriding deformation. In actual production, distortion after nitriding has long been a persistent issue in heat treatment, both domestically and internationally. If deformation cannot be effectively controlled, it can reduce the gear's accuracy, leading to impact, vibration, and noise during operation, which lowers the system’s reliability and shortens the machine's lifespan. Through process innovation and experimental verification, we have largely resolved the problem of nitriding deformation in the ring gear.
**1. Large Double Ring Gear Material Process**
The chemical composition of the ring gear is detailed in Table 1.
(1) Technical Requirements: Tempering hardness should be between 280–310 HBW. The nitriding layer depth should be 0.3–0.5 mm, with a white bright layer less than 0.01 mm. Surface hardness must reach 53.5 HRC (measured using HR15N).
(2) Processing Process: The main steps for the ring gear include forging → normalization → quenching and tempering → vertical machining → tooth cutting → annealing → vertical machining → tooth cutting → vertical grinding → stable aging → tooth grinding → nitriding → final inspection.
Conventional heat treatment: Normalizing at the forging factory; quenching and tempering at (850±10)°C for 3–4 hours, oil-cooled; annealing at (530±10)°C for 3–4 hours, furnace-cooled.
Innovative process: Normalizing at (880±10)°C for 3–4 hours, air-cooled; quenching and tempering at (850±10)°C for 3–4 hours, water-quenched and oil-cooled; annealing at (530±10)°C for 3–4 hours, furnace-cooled with a heating rate ≤50°C/h; stable aging at (510±10)°C for 3–4 hours, furnace-cooled with a heating rate ≤25°C/h; nitriding at (510±10)°C for 10–12 hours, furnace-cooled, with temperature rise/fall rate ≤25°C/h and improved furnace design with auxiliary heating devices.
**2. Test Results**
(1) Mechanical Properties: After cutting 50mm from the left end, the mechanical properties of the ring gear produced using the water quenching and oil cooling process met the requirements, as shown in Table 2.
(2) Deformation After Annealing: After rough opening, significant surface stress was present, requiring stress-relief annealing. To control deformation from subsequent nitriding, strict control over temperature changes and holding time was necessary. After annealing, the ellipticity was measured at 0.05 mm.
(3) Deformation After Aging: Coarse grinding introduced residual machining stress, requiring additional annealing. Using a regular electric furnace caused slight oxidation on the surface, affecting nitriding quality. Therefore, aging was done in the nitriding furnace. This helped further eliminate processing stress, detect deformation, and test the nitriding tooling and loading method. Deformation before and after aging is shown in Tables 3 and 4.
Before and after stable aging, deviations in tooth profile tilt (fHα), tooth profile deviation (ffα), helix tilt deviation (fHβ), spiral shape deviation (ffβ), and cumulative pitch deviation (Fp) were all within the 7th level or better, with Fp reaching up to the 9th level. This indicates some deformation in the diameter direction, with the ring gear diameter increasing by 0.15 mm.
(4) Detection of Nitriding Deformation: A three-coordinate measuring device was used to detect deformation, with specific data shown in Table 5.
**3. Analysis of Factors Affecting Deformation**
(1) Tempering and Quenching: Changing from oil cooling to water quenching and oil cooling improved deformation control. This increased the hardened layer depth, ensuring full hardening of the tooth top and root, reducing structural deformation. It also improved core hardness and high-temperature plastic deformation resistance, and enhanced tempering temperature to relieve internal stresses.
(2) Annealing and Stable Aging: Adding stable aging before tooth grinding reduced machining stress from coarse grinding. Stress relief annealing provided two advantages: low heating temperature and slow heating rate, both of which aided in stress release and lattice recovery, thus minimizing deformation.
(3) During the initial stage of ion nitriding, large temperature differences across different parts generated thermal stress, potentially causing deformation. Controlling the temperature rise and fall rate during nitriding to ≤25°C/h, along with improved heating uniformity via auxiliary devices, helped prevent deformation due to thermal stress.
**4. Conclusion**
(1) By adjusting the cold working process, improving preliminary heat treatment, eliminating stress, and applying an innovative nitriding process, the base strength of the ring gear was enhanced, allowing effective control of ion nitriding distortion.
(2) According to the data analysis, there is still some fluctuation in the total pitch deviation (Fp), which requires further study.
Author: Chen Fengyan, Zhang Changqing, Cao Fengjiao, Liu Jun, Dalian Huarui Heavy Industry Group Co., Ltd. Reducer Factory.
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