Our company is a professional manufacturer specializing in the production of automobile transmissions and front and rear axle gears. We offer over ten types of internal spline gears, including the middle five-speed gear for the 255 transmission, two gears exported to Italy, and several other models recently introduced to the market. In addition, we have developed more than ten kinds of gears that are exported by Aike. However, during the manufacturing process, especially after carburizing and quenching, we often encounter issues with excessive workpiece deformation. This leads to problems such as the cross-bar distance exceeding the required range when measured during subsequent cutting processes, or the inability of the integrated plug gauge to pass. Similarly, when measuring the spline diameter using a large-diameter spindle and taper mandrel, the results may not meet specifications, or the part may experience wear issues.
To address these challenges, we have enhanced our control over raw materials and optimized both hot and cold processing technologies. Additionally, we conducted a thorough analysis of the entire production process, particularly focusing on the heat treatment stage. As a result, we implemented measures to reduce heat treatment deformation, which have yielded significant improvements.
**The Influence and Control of Raw Materials on Deformation**
The metallurgical quality of gear steel plays a crucial role in determining the mechanical properties, fatigue performance, and overall processability of the gear. Several key factors influence this quality:
1. **Chemical Composition**
The carbon content and alloying elements in the steel must comply with national standards for automotive gear steel. The oxygen content should be less than 0.002%. The allowable deviation for aluminum is ±0.005%, and for sulfur, it is also ±0.005%.
2. **Steel Smelting Method and Delivery Condition**
Our steel is produced using electric or converter furnaces with vacuum degassing, ensuring high purity. It is delivered in a hot-rolled, slow-cooled state, with a hardness not exceeding 200–220 HBW.
3. **Low-Magnification Structure**
The low-magnification structure must be free from visible defects such as shrinkage cavities, cracks, or inclusions. According to national standards, the levels of center looseness, general looseness, and ingot segregation should not exceed level 2.
4. **Non-Metallic Inclusions**
Non-metallic inclusions significantly affect the fatigue strength of the steel. They are categorized into sulfides (Class A), aluminas (Class B), silicates (Class C), spheroidal oxides (Class D), and single-particle spheroids (Ds). Our steel meets the inclusion grade requirements outlined in Table 1.
5. **Grain Size**
The austenite grain size has a major impact on quenching distortion and cracking. Finer grains improve hardenability and reduce deformation. Our steel has a grain size between grades 6 and 10. Coarser grains can lead to mixed crystals, reducing strength and increasing brittleness.
6. **Banded Structure**
Banded structures, formed during solidification and rolling, can cause uneven hardening and poor machinability. Our steel samples show a banded structure rating of ≤2 after normalization.
7. **End Hardenability**
End hardenability is determined by the chemical composition and affects the core hardness and hardened layer width. The revised GB/T 5216 standard now limits the hardenability bandwidth to 8HRC. For SAE8620H steel, our heat treatment parameters and end hardenability meet the requirements specified in Table 2.
By carefully managing these aspects, we ensure the quality and consistency of our products, ultimately improving the efficiency and reliability of the gear manufacturing process.
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