Our company is a professional manufacturer specializing in the production of automobile transmissions and front and rear axle gears. Among our product range, there are over ten types of internal spline gears, including the middle five-speed gear from the 255 transmission, two gears exported to Italy, and most recently, several varieties developed for Aike. During the manufacturing process, we often encounter issues with excessive deformation of the workpieces after carburizing and quenching. This deformation leads to problems such as the cross-bar distance exceeding the required tolerance during subsequent cutting, or the plug gauge failing to pass when measuring the gear. Additionally, when measuring the spline diameter using a large-diameter spindle, the taper mandrel may not fit properly, or the part might wear out quickly.
To address these challenges, we have enhanced our control over raw materials and optimized both hot and cold processing technologies. We also conducted a comprehensive analysis of the entire production process, focusing specifically on heat treatment. Through this effort, we implemented effective measures to reduce deformation during heat treatment, which has significantly improved the quality and consistency of our products.
**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 processing behavior of the gears. It directly affects the success of both cold and hot working processes.
**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 kept below 0.002%. The allowable deviation for chemical composition must follow the relevant national specifications, with aluminum and sulfur tolerances set at ±0.005% each.
**Steel Smelting Method and Delivery Condition**
The purity, hardenability, and original microstructure of the steel are critical factors. Vacuum degassing helps improve steel purity. Our steel is produced using electric furnaces or converters and undergoes vacuum degassing. It is delivered in a hot-rolled, slowly cooled state, with a hardness not exceeding 200–220 HBW.
**Low-Magnification Structure**
In the low-magnification structure of the steel, acid etching of the cross-section should reveal no visible shrinkage cavities, bubbles, cracks, inclusions, peeling, white spots, or residual dendrites. According to national standards, the levels of center looseness, general looseness, and ingot segregation should not exceed level 2.
**Non-Metallic Inclusions**
High-strength steels experience a rapid decrease in fatigue strength with increasing non-metallic inclusion content and size, especially oxides. These inclusions are classified 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.
**Grain Size**
Austenite grain size is a key structural parameter that influences quenching cracking and distortion tendencies. Fine and uniform grains help stabilize hardenability and reduce heat treatment deformation. Our steel has an austenite grain size ranging from grades 6 to 10. Coarser grains increase the risk of cracking and distortion, so we avoid mixed crystal structures to maintain strength and prevent brittle failure.
**Banded Structure**
Banding occurs due to dendritic segregation during solidification and rolling. Severe banding can lead to mixed crystals, uneven martensite formation, and inconsistent hardness. Our steel samples show a banded structure of ≤2 after normalization, ensuring better machinability and thermal stability.
**End Hardenability**
Hardenability is determined by the steel's chemical composition and ensures proper core hardness and a narrow hardened zone. The new GB/T 5216 standard reduces the hardenability bandwidth from 12 HRC to 8 HRC. For SAE8620H steel, our heat treatment parameters and end hardenability meet the requirements shown in Table 2, ensuring consistent performance and minimal deformation.
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