Interaction of N with white-solidified cast iron model alloys - the effect of Mn and Cu on the formation of Fe and Si nitrides

Abstract

Surface remelting and subsequent nitriding improves the surface properties of cast irons. Upon remelting, a white-solidified surface layer forms, which contains coarse Si-free eutectic cementite ($\theta$) and Si-enriched ferrite, pearlite or martensite in the intercarbidic regions between the eutectic $\theta$. Nitriding produces a compound layer at the surface, which is composed of $\epsilon$ and $\gamma \prime$-iron (carbo)nitrides and enhances the corrosion resistance. Nitriding of white-solidified Fe-C-Si alloys, being model materials for remelted low-alloy ferritic cast irons, has shown that Si dissolved in $\alpha$-Fe notably affects the formation of $\epsilon$ and $\gamma \prime$ in intercarbidic regions while Si simultaneously precipitates as amorphous nitride, X. Under process conditions only allowing for the formation of $\gamma \prime$ in pure Fe, Si dissolved in $\alpha$-Fe promotes the formation of $\epsilon$ over the formation $\gamma \prime$, whereas Si-free eutectic $\theta$ transforms into nitride following the sequence $\theta \to \epsilon \to \gamma \prime$. The present work studies the nitriding of white-solidified Fe-3.5wt.%C-3wt.%-M alloys with additions of M = 1 wt.% Mn, 1 wt.% Cu or 1 wt.% Mn + 1 wt.% Cu, serving as model materials for remelted pearlitic cast irons. The presence of Mn and/or Cu causes notable deviations from the nitriding behavior known from Fe-C-Si alloys. Mn accelerates the precipitation of X in intercarbidic regions and obstructs the transformation of $\epsilon$ formed from Si-free $\theta$ into $\gamma \prime$. Cu promotes the formation of $\gamma \prime$ in Si-rich intercarbidic regions, surpassing the $\epsilon$-promoting effect of Si.