PSI - Issue 75

D. Jbily et al. / Procedia Structural Integrity 75 (2025) 158–175 Author name / Structural Integrity Procedia (2025)

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stresses and reducing surface roughness, it also appears to accelerate the initiation of micropitting in the tooth dedendum areas during the early stages. However, shot peening delays the onset of macropitting, resulting in notable improvement in fatigue gear life. Observations and profile measurements on worn gear teeth show that the size of the micropitting ranges from approximately 10 to 100 μm, with a depth of 1 to 5 μm. These craters are similar for both REF and SP pinions after the testing. Microstructural analyses using EBSD revealed a distinct difference in grain structure between reference and shot peened gears. SP samples exhibited strong plastic deformation and misorientation near the surface before testing, indicating the presence of a hardened layer. After testing, a significant loss of crystallinity and indexing was observed at the extreme surface of SP teeth, suggesting the formation of an amorphous, wear prone layer. This transformation may reduce the resistance of SP surfaces to micropitting despite their improved fatigue resistance. The Tribological tests demonstrated that micropitting is reproducible on unpeened, hard-turned surfaces under reciprocating sliding conditions, while shot-peened specimens exhibited only polishing marks under identical test conditions. These results suggest that shot-peened surfaces may benefit from improved boundary lubrication behavior and reduced friction-induced damage. Nonetheless, the rolling-sliding nature of gear meshing appears more conducive to micropitting than pure sliding in the tribometer, highlighting the influence of real contact conditions. Current limitations, such as late-stage damage analysis and non-representative sampling in tribological testing, may hinder a comprehensive understanding of micropitting initiation and progression in both shot peened and non peened specimens. Future research should aim to address these gaps by investigating the early stages of micropitting and improving sample representativeness through the analysis of actual gear tooth surfaces. By overcoming these limitations, future studies will contribute to a deeper understanding of micropitting mechanisms and the influence of shot peening. Acknowledgements The research project was conducted and funded with the kind support of French Power Transmission committee. References ISO 10825 - 1:2022, Gears — Wear and damage to gear teeth — Part 1: Nomenclature and characteristics ISO 10825 - 1:2022, Gears — Wear and damage to gear teeth — Part 2: Supplementary information ISO/TS 6336 - 22:2018. Calculation of load capacity of spur and helical gears — Part 22: Calculation of micropitting load capacity. Liu, H., Liu, H., Zhu, C., Zhou, Y., 2019. A review on micropitting studies of steel gears. Coatings , 9(1), 42. https://doi.org/10.3390/coatings9010042 Oila, A., Bull, J., 2005. Assessment of the factors influencing micropitting in rolling/sliding contacts. Wear , 258, 1510 – 1524. Winkelmann, L., Omer, E. - S., Bell, M., 2008. The effect of superfinishing on gear micropitting, part II. American Gear Manufacturers Association Fall Technical Meeting . Carranza Fernandez, R., Tobie, T., 2025. Increase gear pitting capacity and reparability via improved gear surface technique. Gear Solutions Magazine . Lambert, R. D., Aylott, C. J., Shaw, B. A., 2018. Evaluation of bending fatigue strength in automotive gear steel subjected to shot peening techniques. Procedia Structural Integrity , 13, 1855 – 1860. Kobayashi, M., Hasegawa, K., 1990. Effect of shot peening on the pitting fatigue strength of carburized gears. Proceedings of the IV International Conference on Shot Peening , 465 – 476. Townsend, D. P., 1992. Improvement in surface fatigue life of hardened gears by high intensity shot peening. NASA Report , Lewis Research Center. Inoue, K., et al., 1989. The effect of shot peening on the bending strength of carburized gear teeth. JSME International Journal, Series III , 32(3). Güntner, C., Tobie, T., Stahl, K., 2017. Influence of the residual stress condition on the load carrying capacity of case hardened gears. AGMA Technical Paper 17FTM20 , American Gear Manufacturers Association. Geitner, M., Zornek, B., Hoja, S., Tobie, T., Stahl, K., 2023. Material influence on the micropitting and wear resistance of nitrided external and internal gears. HTM Journal of Heat Treatment and Materials , 78(6), 319 – 340. Höhn, B. - R., Oster, P., Emmert, S., 1996. Micropitting in case - carburized gears – FZG micro - pitting test. VDI Berichte , 1230, 331 – 344. Moorthy, V., Shaw, B., 2012. Contact fatigue performance of helical gears with surface coatings. Wear , 276 – 277, 130 – 140. Jbily, D., Lefebvre, F., Simonneau, A., 2024. The influence of shot peening on gear teeth micropitting and contact fatigue failure. Procedia Structural Integrity , 57, 199 – 216.