Deformation Control of Ion Nitriding of Double Inner Ring Gear of Large Mine Car
Our company has successfully undertaken the ion nitriding process on 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 requires 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, following grinding. To maintain the high-precision level, it is essential to strictly control deformation during nitriding.
In practice, distortion after nitriding has long been a major challenge in heat treatment processes, both domestically and internationally. If not controlled effectively, this deformation can reduce the gear's accuracy, leading to impact, vibration, and noise during operation. This, in turn, affects the reliability and lifespan of the entire machine. Through process innovation and experimental verification, we have largely resolved the issue of nitriding deformation for this ring gear.
**1. Material Process for the Large Double Ring Gear**
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 needs to be 0.3–0.5 mm, with a white bright layer less than 0.01 mm. Surface hardness should reach 53.5 HRC (tested using HR15N).
(2) Processing Procedure: The main steps include forging, normalization, quenching and tempering, vertical machining, tooth cutting, annealing, re-machining, grinding, stabilization aging, tooth grinding, nitriding, and final inspection.
Conventional heat treatment includes normalization at the forging factory, followed by quenching and tempering at (850±10)°C for 3–4 hours, oil cooling. Annealing is done at (530±10)°C for 3–4 hours, furnace cooled. Nitriding is performed at (510±10)°C for 10–12 hours, also furnace cooled.
Innovative process: Normalization is conducted at (880±10)°C for 3–4 hours, air-cooled. Quenching and tempering use water quenching followed by oil cooling. Annealing remains at (530±10)°C for 3–4 hours, but with a slower temperature rise (≤50°C/h). Stable aging is at (510±10)°C for 3–4 hours, furnace cooled, with a temperature change rate ≤25°C/h. Nitriding is optimized with improved furnace design and auxiliary heating devices, ensuring a temperature change rate ≤25°C/h.
**2. Test Results**
(1) Mechanical Properties: After cutting 50mm from the left end, the mechanical properties of the ring gear produced with water quenching and oil cooling meet the required standards, as shown in Table 2.
(2) Deformation After Annealing: After rough opening, significant surface stress is present, requiring stress-relief annealing. To control subsequent nitriding deformation, precise control of temperature changes is essential. Post-annealing deformation was measured at 0.05 mm ellipticity.
(3) Deformation After Aging: Coarse grinding may still generate machining stress, necessitating additional annealing. Using a regular electric furnace could cause slight oxidation, affecting nitriding quality. Instead, aging is done in the nitriding furnace, which helps eliminate residual stress, detect deformation, and verify tooling and loading methods. Data before and after aging is presented in Tables 3 and 4.
Before and after stable aging, the tooth profile tilt (fHα), tooth profile deviation (ffα), helix tilt deviation (fHβ), spiral shape deviation (ffβ), and cumulative pitch deviation (Fp) were all within acceptable limits, showing that the gear had minimal deformation, with an increase in diameter of 0.15 mm.
(4) Nitriding Deformation Detection: A 3D measuring device was used to assess deformation, with specific data provided in Table 5.
**3. Analysis of Factors Affecting Deformation**
(1) Tempering and Quenching: Changing from oil cooling to water quenching and oil cooling improves deformation control. This increases the hardened layer depth, ensures full hardening of tooth tops and roots, reduces structural differences, enhances core hardness, and improves resistance to high-temperature plastic deformation.
(2) Annealing and Stable Aging: Adding a stable aging process before tooth grinding helps reduce machining stress from coarse grinding. Stress relief annealing provides low thermal stress and slow heating, which are beneficial for stress release and lattice recovery, thus reducing deformation.
(3) During the initial stage of ion nitriding, temperature differences across parts can lead to thermal stress and deformation. Controlling the heating and cooling rates (≤25°C/h) and using auxiliary heating improves uniformity, minimizing deformation caused by temperature gradients.
**4. Conclusion**
(1) By optimizing 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, and ion nitriding distortion was effectively controlled.
(2) While most data is consistent, there is still some fluctuation in the total pitch deviation (Fp), which requires further investigation.
Author: Chen Fengyan, Zhang Changqing, Cao Fengjiao, Liu Jun, Dalian Huarui Heavy Industry Group Co., Ltd. Reducer Factory.
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