Effect of Subzero and Tempering Treatments on Corrosion Resistance of Medium Carbon Steel in Acidic Medium

This study investigated the effect of subzero and tempering treatments on the corrosion resistance of hardened medium carbon steel with a view of improving its performance in service condition. Standard corrosion specimens of medium carbon steel were austenitized at 8500C, soaked for 2 hours and quenched in water to room temperature. The quenched samples were immediately subjected to cryogenic treatment in dry ice (solid CO2) at -780C for 24 hours, followed by tempering at 5000C for 1-5 hours and then air cooled to ambient temperature. The heat-treated and as-received samples were characterized with the Autolab PG STAT 204 Potentiostat & FRA32 EIS Module, X-ray diffractometer (XRD) and Smart EVO 10 Scanning Electron microscope/Energy dispersive spectroscope (SEM/EDS). The results show that tempering of hardened and subzero treated medium carbon steel generally improves its corrosion resistance in acidic medium, transformed the retained austenite to martensite, and modifies the morphology of the transformed martensite to tempered martensite. The results also depict that the retained austenite stability is different from one crystallographic plane to another and the driving force required for transformation is different from plane to plane. It can be concluded that tempering treatment can be used to improve the corrosion resistance of hardened and subzero treated medium steel in acidic environment.