Effect of Back-Tempering on the Wear and Corrosion Properties of Multiple-Pass Friction Stir Processed High-Speed Steel

In this study, a scalable surface modification strategy for M2 high-speed steel was applied using multiple-pass friction stir processing (FSP) with overlapping ratios of 25%, 50%, and 75%. A comprehensive investigation of the microstructure, surface hardness, wear, and corrosion resistance was conducted to elucidate the properties of FSPed M2 as a function of the overlapping ratio. In the single-pass FSPed M2, the major phase was martensite and the minor phases included retained austenite where refined carbides (M₆C, M₂₃C₆, and MC) were detected. However, back-tempering occurred near the overlapped zone (OZ) between consecutive tracks for the multiple-pass FSPed M2. The martensite formed in the first pass was turned into tempered martensite by the thermal cycle from the subsequent pass. This back-tempering resulted in a localized decline in hardness from 900 to 650 HV_0.2. Further wear tests revealed that the wear rates of the tempered zone (TZ) of the multiple-pass FSPed M2 (FSP25%: 1.40 × 10^-5 mm³/N·m, FSP50%: 1.20 × 10^-5 mm³/N·m and FSP75%: 1.00 × 10^-5 mm³/N·m) are all higher than that of SZ of the single-pass FSPed M2 (0.75 × 10^-5 mm³/N·m), indicating lower wear resistance of the TZ. Moreover, increased carbide content in the TZ led to the depletion of passivating elements near proximity of the tempered martensite, acting as the active sites for selective corrosion attack. The corrosion potential (E_corr) and corrosion current density (I_corr) increased significantly, with values of -397.6 ± 5.6 mV and 9.5 ± 0.8 μA·cm^-2 for FSP25%, -424.4 ± 6.0 mV and 14.7 ± 1.7 μA·cm^-2 for FSP50%, and -440.9 ± 2.8 mV and 17.1 ± 1.9 μA·cm^-2 for FSP75%.