Causes of Thermal Treatment Distortion in Common Gears and Corresponding Mitigation Measures

(1) Cylindrical Gear Distortion
1) Cause of distortion: Carburized and quenched cylindrical gears suffer from poor fit due to quenching-induced deformation in splines and keyways. This deformation is caused by mass differences resulting from variations in shape.
2) Countermeasures: Design the part with a shape that is less prone to deformation—for example, by incorporating uniformly distributed hole patterns in the flange plates. Additionally, create lightening slots and holes, primarily to ensure uniform cooling. From a design perspective, if it’s not feasible to incorporate lightening slots or holes due to shape constraints, you can conduct experiments to obtain quantitative data on deformation. During machining, pre-allocate a margin for deformation and perform preliminary corrections accordingly.
After carburizing, the surface carburized layer is removed from all areas except the gear teeth, and then quenching is performed. This is also a method for preventing deformation and is particularly effective for large gears.
(2) Quenching Distortion of Helical Bevel Gears
1) Distortion hazards: Due to thermal treatment distortion, when meshing with standard gears, the contact points (contact patterns) shift significantly, making it unable to withstand operating loads and leading to premature gear failure.
2) Countermeasures. In general, this is often caused by differences in the hardenability of materials—specifically, differences in their chemical compositions. Therefore, it is essential to strictly control the uniformity of material composition, use materials with a narrow hardenability range, and select materials whose hardenability differences are not significant.
(3) Methods for Reducing Distortion in High-Frequency Hardened Gears
1) General method
① The material, gear size, and required hardening layer depth should be selected based on the gear specifications, while also taking into account its through-hardening capability. Typically, 40 and 45 carbon steels are commonly used.
② The forging of gear blanks should be thorough, ensuring that the blank’s fibers are non-directional, the grain structure is uniform, non-metallic inclusions are evenly distributed and have a specific shape.
③ Prior to high-frequency quenching, a preliminary treatment such as tempering or normalizing should be performed to meet the requirements of rapid high-frequency heating and uniform austenitization.
④ The appropriate hardening layer depth and shape should be selected. In particular, when the hardening layer depth exceeds a certain required value, it can become the primary cause of deformation. Generally, when the modulus is between 6 and 12 mm, the following hardening layer depths are recommended:
Hardened layer depth on the tooth surface = (0.2～0.3) × m
Depth of the root hardening layer = (0.16–0.28) × m
2) A deformation-prevention method unique to high-frequency quenching of gears
① The appropriate method can be selected based on the gear’s shape, size, material, and equipment condition. Different quenching methods result in varying tendencies and magnitudes of quenching-induced deformation.
② Methods for preventing quenching deformation from the perspectives of gear design and machining
a. The thickness of the spoke should be greater than one-third of the tooth width and positioned approximately symmetrically midway across the tooth width. Since, when the central part is heated, heat from the tooth surface diffuses toward the spokes, the entire tooth surface can be preheated to around 150℃.
b. The wall thickness of the hub should correspond to the small tooth profile to ensure sufficient strength.
c. Generally, the center of gear meshing should be located at the midpoint of the tooth width, and the teeth should be shaped as concave as possible, with a concavity depth of approximately 0.1 mm.
d. When the tooth tip expands and comes into rigid contact (interference) with the root of the meshing gear, the tooth tip must be repaired by methods such as grinding.
(4) Countermeasures for Distortion in Carburized Gear Quenching
1) Heat up slowly during preheating. This is especially important for large gears.
2) The pre-carburizing treatment is performed after rough machining, followed by tempering. For example, the workpiece is heated to 940℃, and the holding time is calculated at 30 minutes per inch; then it is oil-quenched. After oil quenching, the workpiece is subjected to spheroidizing treatment at 700℃, with a holding time of 60 minutes per inch. After this treatment, the as-cast microstructure achieves the desired refinement and homogenization, and there is no need to worry about the formation of ferritic banded or blocky structures.
3) Improve the carburizing and quenching process. To avoid rapid heating, preheat at 750℃ for 40 minutes near the phase transformation point; reduce the carburizing temperature to 900℃ and the quenching temperature to 810℃, followed by oil quenching for 30 minutes.
4) To prevent deformation of the spline hole during quenching, a tapered shaft sleeve can be inserted into the spline hole and then heated.