Optimizing Heat Treatment of Spiral Bevel Gears: Reducing Distortion through Thermodynamic Modeling of Carburizing and Quenching Process

Significance 

Spiral bevel gears are integral components in many high-performance mechanical applications that requires high precision and durability. These gears which are known for their complex geometry and superior load-bearing capabilities usually undergo heat treatment processes such as carburizing and quenching to improve their mechanical properties which can induce significant distortions and compromise the dimensional accuracy and performance of the gears. This is a major challenge in manufacturing because it results in additional corrective procedures that are both time-consuming and costly. The primary challenge with heat-treated spiral bevel gears lies in the non-uniform temperature distribution and phase transformations that occur during the quenching process. The complex geometry of these gears makes the problem even worse and can lead to uneven cooling rates and non-simultaneous phase transformations. These irregularities contribute to dimensional changes and shape distortions, which in turn affect the gear’s functionality and reliability. Previous studies have predominantly focused on simpler geometries, which left a knowledge gap regarding the specific behaviors of spiral bevel gears during heat treatment. Recognizing this gap, the researchers at Northwestern Polytechnical University, led by Professor Yingqiang Xu, undertook a detailed investigation into the cooling characteristics and phase transformations of spiral bevel gears made from medium-alloyed Cr–Ni steel. The study, was conducted by PhD candidates Junpeng Li, Youwei Liu, and Hui He, aimed to deepen the understanding of the factors influencing heat-treated distortions in these gears. The authors successfully developed a new thermodynamic-based model for the martensitic start temperature (Ms) and proposed innovative methods to characterize temperature and martensite distribution non-uniformities, which addressed the complexities inherent in the heat treatment of spiral bevel gears. The research work is now published in the Journal of Materials Science.

The researchers selected spiral bevel gears made of 18Cr2Ni4WA steel due to its high fatigue strength and toughness, making it ideal for aeronautical applications. The gears underwent a specific heat treatment process involving carburizing, diffusion, pre-cooling, and quenching. Initially, the gears were heated to 925°C for carburizing in a carbon-rich atmosphere to enhance surface hardness while maintaining a tough core. The gears were then quenched in oil at varying temperatures (70°C, 80°C, and 100°C) to investigate the effects of quench rate on temperature distribution and phase transformation. The authors initially focused on understanding the cooling characteristics of different parts of the spiral bevel gear during quenching. They used finite element modeling (FEM) to simulate the quenching process and monitor temperature changes in real-time. The findings revealed significant non-uniformity in cooling rates across different parts of the gear. The heel and toe regions exhibited faster cooling rates due to larger heat transfer areas and earlier rupture of the insulating film blanket. In contrast, the bottom of the gear cooled more slowly because of mass concentration and smaller heat transfer areas. These variations in cooling rates were found to contribute significantly to the overall distortion of the gear. Afterward, the researchers conducted experiments to further analyze the temperature distribution along two paths: the tooth length and tooth width directions. By making virtual cuts and tracking temperature distribution over time, they found that temperature variation was more pronounced along the tooth length direction. Moreover, they found that the heel and toe exhibited significant temperature differences, with the heel cooling faster due to its larger heat transfer area. The researchers also observed that increasing the quenching medium temperature reduced the non-uniformity in temperature distribution, which lead to a more uniform phase transformation across the gear tooth profile.

The researchers also studied the evolution of carbon concentration during the carburizing and diffusion stages. Using FEM, they monitored carbon diffusion from the surface to the core. The experiments showed a rapid initial increase in carbon concentration at the surface, followed by a slower increase in the subsurface regions. During the diffusion stage, the carbon atoms deposited on the surface diffused towards the core, creating a gradient that improved the performance of the gear. The pre-cooling stage slightly reduced surface carbon concentration due to differential diffusion rates. These findings underscored the importance of controlling carbon diffusion to achieve desired surface and core properties. Additionally, the researchers developed a new thermodynamic-based model for the start temperature of Ms, incorporating the effects of austenite grain size and carbon content gradient. The experiments showed that martensitic transformation started in the core and progressed outward to the surface. The volume fraction of martensite was highest in the core and decreased towards the surface. Lower quenching temperatures led to higher martensite fractions due to lower Ms temperatures. The researchers also found that the distribution of martensite across the tooth profile was influenced by local cooling characteristics and carbon concentration profiles.

The team compared different quenching medium temperatures, to investigate their impact on the uniformity of temperature distribution and martensitic transformation and found that higher quenching medium temperatures improved the uniformity of temperature distribution, reducing non-uniform martensitic transformation. This uniformity was important in minimizing distortions.  The authors also showed that choosing a higher quenching medium temperature is beneficial for controlling temperature uniformity and preventing quenching cracks. They also examined the volume fraction of martensite in different parts of the gear tooth and found that the martensite distribution in the carburized layer varied significantly, influenced by carbon concentration and local cooling conditions. The peak and bottom of the gear showed different martensite profiles, with the peak having a more gradual gradient which highlight that the local uneven carburization and subsequent quenching is significantly impacted the martensite distribution and, consequently, the distortion of the gear.

In conclusion, Professor Yingqiang Xu and his team developed a new thermodynamic-based model for the start temperature of martensitic transformation and proposed innovative methods to characterize temperature and martensite distribution non-uniformities which are significant advancement in the field.  The study has important practical implications, for example, with the control of the quenching medium temperature, manufacturers can achieve more uniform temperature and phase transformations, which reduces distortions and leads to improved dimensional accuracy and performance of spiral bevel gears, which are important in aerospace and other high-performance applications. Moreover, the resulted reduced distortions during heat treatment will minimize the need for expensive and time-consuming post-processing steps such as hard grinding and surface modifications. This will significantly cut down production costs and also shortens manufacturing cycles, and by this enhances the overall operational efficiency.

Optimizing Heat Treatment of Spiral Bevel Gears: Reducing Distortion through Thermodynamic Modeling of Carburizing and Quenching Process - Advances in Engineering

About the author

Junpeng Li is currently pursing Ph.D. degree in Northwestern Polytechnical University. His research interest include surface strengthening of steel materials, mechanical response of steel materials during heat treatment, calculation of residual stress and distortion control of engineering steel parts such as spiral bevel gears.

About the author

Yingqiang Xu received the Ph.D. degree from Northwestern Polytechnical University, Xi’an, Shaanxi, China. He is now Professor in Northwestern Polytechnical University, Excutive Director of Shaanxi Tribology Society, Director of Shaanxi Automotive Engineering Society and Senior Member of Chinese Mechanical Engineering Association.

His research interests include modern design theories and methods, mechanical system vibration, noise control technology, advanced research in high-tech automotive technology, and advanced technology for new vehicles.

About the author

Youwei Liu is currently pursing Ph.D. degree in Northwestern Polytechnical University. His research interest is surface strenghthening of materials.

About the author

Hui He received the M.S. degree from Northwestern Polytechnical University.

Reference

Li, J., Xu, Y., Liu, Y. et al. Investigation of non-uniformity of temperature distribution and phase transformation in spiral bevel gears during carburizing and quenching. J Mater Sci 59, 609–630 (2024). https://doi.org/10.1007/s10853-023-09182-z

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