Investigation of Subsurface Fatigue Degradation Processes in Rolling–Sliding Tribo-Contacts under Variable Loading

The present study evaluates the effectiveness of low-temperature gas nitriding of alloy structural steels 38KhMYuA, 38KhN3MA, and 38KhN3MFA used in heavily loaded gear transmissions, and determines the applicability limits of thin nitrided layers from the standpoint of contact fatigue. The investigations were carried out after nitriding at a temperature of 580 °C for 7 h using microhardness profiling, layer-by-layer chemical analysis by high-temperature gas analysis, X-ray diffraction analysis, and contact fatigue testing of rollers simulating rolling with sliding conditions typical of helical gear engagement. It was found that lowtemperature nitriding produces a hardened surface layer with a thickness of 0.18—0.25 mm and a microhardness of 4.8—5.5 GPa, while maintaining a ductile core with a hardness of 240—260 HB. The results show that under elevated contact stresses, failure of nitrided specimens initiates in the subsurface region and develops according to a deep contact fatigue mechanism, accompanied by deep spalling and the formation of a so-called white etching layer. The obtained data indicate the limited effectiveness of thin nitrided layers in highly loaded gear systems and demonstrate that materials with thicker hardened layers (0.8—1.2 mm), or design and technological solutions aimed at reducing contact stresses, are more appropriate for improving service life under severe cyclic loading conditions.

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