Effects of Nitriding and Thermal Processing on Wear and Corrosion Resistance of Vanadis 8 Steel

Abstract

Vanadis 8 steel is a tool steel manufactured by powder metallurgic processing. Its main alloy elements are V, Cr and Mo. By implementing an experimental design with five factors—all of them are related to the thermal processing of this steel and with ionic nitriding—the effects of said factors on adhesive wear resistance and corrosion resistance were studied. For this purpose, Pin-on-Disc wear tests and lineal polarization resistance tests were carried out using an aqueous solution with 3.5% NaCl by weight. The main aim was to increase this steel use in more aggressive environmental conditions, such as in coastal environments. By means of XRD, the percentage of retained austenite was determined, and by SEM-EDX, the microstructure was revealed. The conclusion is that adhesive wear resistance is improved if thermal processing parameters are at such levels that increase austenite destabilization and reduce retained austenite content. This means to destabilize austenite at 1180 °C during 1 h, with oil quenching, tempering at 520 °C during 2 h and ionic nitriding at 520 °C during 2 h. Corrosion resistance is highly improved with ionic nitriding. At the same time, to compensate for the negative effect on corrosion resistance of a high density of primary and secondary carbides, it is essential to carry out the ionic nitriding treatment. The harmful effect of electrochemical microcells that appear in the carbide/matrix interface is compensated by the passivating effect generated by the nitrided surface.