Bending fatigue behavior of case-hardened Pyrowear 53 steel gears with increased retained austenite content

Date

2025-9-01

Author

Çınar, İrem

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The bending fatigue behavior of Pyrowear 53 (AMS6308) alloy gears with retained austenite (RA) content was examined. RA levels were adjusted by modifying the carbon potential during carburizing and application of sub-zero heat treatments, resulting in RA contents of 17.4%, 47.4%, and 47.9% in the test groups, compared to 15.4% in the reference group. The effect of surface carbon altering on the RA content was concluded to be minimal, compared to the sub-zero treatment. Bending fatigue performance was evaluated using the Single Tooth Bending Fatigue (STBF) test method. The test results showed a significant improvement in bending fatigue strength, with an increase of over 25% in endurance limit when RA content elevated from 17.4% to 47.9%. This improvement was mainly linked to the crack closure effect caused by martensitic transformation of the RA phase. Despite a faster crack propagation rate was observed in gears with higher RA content, their overall fatigue performance in STBF tests was concluded to be superior. This was attributed to delayed crack initiation and the dampening of small sub-surface cracks, as observed through Scanning Electron Microscopy (SEM). Microstructural evaluations and microhardness measurements were repeated after the tests, indicating RA to martensite transformation occurred at high load. The lack of this transformation at lower loads with high RAsamples were observed to lead to great decreases in the hardness, which can potentially result in early wear and surface fatigue failure of the gear. While increased RA phase content was concluded to contribute positively to bending fatigue performance of case-carburized Pyrowear 53 alloy steels, risks were highlighted as for high performance transmission applications as dimensional instability, lower surface hardness, surface fatigue failure potential and instable nature of the RA phase.

Subject Keywords

Pyrowear 53 Alloy Steel, Single Tooth Bending Fatigue Test, Retained Austenite, Gears

URI

https://hdl.handle.net/11511/116065

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Graduate School of Natural and Applied Sciences, Thesis