Tribological behaviour of carbide-free bainitic steel under dry rolling/sliding conditions

The dry rolling/sliding wear behaviour of Si alloyed carbide free bainitic steel austempered at different temperatures and sliding distances has been evaluated. 60SiCr7 spring steel samples were austempered in a salt bath maintained at 250, 300 and 350 °C respectively for 1 h. Rolling with 5% sliding wear tests were performed using self mated discs for three different test cycles, namely 6000, 18,000 and 30,000 cycles. The aim was to study the wear performance of the 60SiCr7 steel with a carbide-free microstructure containing different amounts of retained austenite. An in-depth microstructural characterization has been carried out before and after the wear tests in order to link the wear behaviour to the microstructure of each sample. The wear resistance has been expressed by means of the specific wear calculated from the mass loss after the tests. The worn surfaces were analysed by scanning electron microscopy and X-ray diffraction. Microhardness profiles were also obtained in order to analyse strain-hardening effects beneath the contact surfaces. The results indicate that the material with highest hardness—the one austempered at 250 °C—exhibited the lowest wear rate in every case. It was also observed that the hardness increment and thickness of the hardened layer increases with increasing the austempering temperature and number of test cycles. Finally, the results appear to indicate that the initial roughness of the samples has no major effect in the wear rate of the samples above 2500 cycles. The higher wear performance of the sample austempered at 250 °C has been attributed to its superior mechanical properties provided by its finer microstructure. It has been evidenced that all samples undergo the TRIP phenomenon since, after wear; no retained austenite could be detected by XRD.