Surface modification of nodular cast iron: A comparative study on graphite elimination

In this study, two surface processes, TIG surface melting and chilling, were applied to remove the negative effect of the near-surface graphite phases of nodular cast iron. Afterwards, the treated surfaces were plasma-nitrided and boronized. After graphite elimination processes, plasma nitriding and boronizing, the microstructures were compared with the one of untreated cast iron. The results showed that the chilling process that was applied during the casting process of nodular cast iron did not eliminate the graphite phases from the surface. However, an almost nodule-free surface was obtained by the TIG melting process. It was of no use in nitriding and boronizing the chilled surfaces due to the existence of superficial nodular graphite in the surfaces after completion of the chilling. On the other hand, a continuous uninterrupted compound layer and boride layer were observed on the TIG melted surface. the light of these experimental studies, the TIG surface process is proposed to eliminate the nodular graphite on flat and curved surfaces of nodular cast iron parts before thermochemical surface hardening treatments or hard coating processes are applied to the nodular cast iron surfaces.