The cone ring support plate, a sub-box component of a multi-speed transmission in heavy-duty trucks, plays a critical role in gear shifting despite experiencing relatively low loads during operation. Its internal teeth must withstand friction and maintain wear resistance, which makes precision manufacturing essential. Due to the tight tolerances required for mating with other components, deformation control during heat treatment is crucial.
The cone ring support plate is initially forged from 40Cr steel. The production process includes forging, preliminary tempering, machining, internal tooth strengthening, finishing, and final heat treatment. To meet the hardness requirement of at least 50HRC on the internal teeth, two heat treatment methods—ion nitriding and induction hardening—are evaluated. This paper compares their effectiveness and economic feasibility to determine the optimal strengthening process.
**Ion Nitriding**
Ion nitriding, a surface modification technique, offers advantages such as high-quality layer formation, precise control over the process, minimal workpiece deformation, and a shorter production cycle. It significantly enhances surface hardness, improving wear resistance and fatigue strength.
The ion nitriding process involves cleaning the workpiece, loading it into the furnace, evacuating air, creating an arc, heating, holding at temperature, and then cooling. For the cone ring support plate with a 10mm wall thickness, careful control of current and voltage during temperature rise is necessary to prevent deformation. The heating and holding steps are repeated until reaching (500±20)°C, with a hold time under 4 hours to avoid coarse nitride structures that reduce hardness. Proper furnace loading ensures even heating, and special fixtures are used to prevent uneven temperature distribution.
After ion nitriding, the internal teeth achieve a surface hardness of 550–600 HV. The nitrided layer consists of a compound white layer (2–5 μm) and a diffusion layer (0.15–0.25 mm). The roundness deformation of the inner teeth is within 0.05–0.07 mm, meeting the design specification of less than 0.08 mm. The white layer provides excellent wear resistance and stability, making ion nitriding a viable option for internal tooth strengthening.
**Induction Heat Treatment**
Induction heat treatment is a localized method known for its fast heating, high efficiency, and minimal deformation. It allows for precise control over the heating process, resulting in high-quality heat treatment. It also offers energy savings, pollution-free operation, and the potential for automation.
For the cone ring support plate, a custom internal tooth quenching sensor was designed, featuring a "Π"-shaped magnet to enhance heating efficiency. A specialized fixture was developed to secure the workpiece during quenching, reducing deformation. The optimized process parameters included a current of 155A, voltage of 250V, and a frequency of 23kHz, with a heating time of 9 seconds and cooling time of 8 seconds. After quenching, low-temperature tempering was performed at 180°C for 2 hours to stabilize the structure.
Deformation issues were addressed by increasing the PVA quenching medium concentration from 0.3% to 0.5%, reducing spray pressure from 0.15MPa to 0.10MPa, and optimizing the groove structure for symmetry. These adjustments improved the process and met the design requirements. The hardened layer depth reached about 2mm at the tooth root, with a fine tempered martensite structure. The inner ring gear deformation remained below 0.07 mm, satisfying technical specifications.
**Economic Comparison**
Both ion nitriding and induction hardening are environmentally friendly and energy-efficient. However, cost is a key factor. Ion nitriding costs approximately 26 yuan per piece, while induction hardening costs only 4.2 yuan per piece. With an annual output of 30,000 units, the cost difference amounts to 654,000 yuan. Given the similar performance and lower cost, induction hardening is preferred when both technical and deformation requirements are satisfied.
In conclusion, while ion nitriding offers superior surface properties, induction hardening presents a more economical solution for strengthening the internal teeth of the cone ring support plate. This makes it the preferred choice for mass production in the automotive industry.
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