Surface melting of nodular cast iron by Nd-YAG laser and TIG

The effect of surface melting and rapid solidification on the structure and hardness of nodular cast iron has been investigated. Two heat sources were used for surface melting mainly, laser beam (LB) and electric arc generated between the tungsten electrode and the specimen (i.e. TIG). Optical microscope, scanning electron microscope and X-ray diffraction were used to describe the microstructure and identify the phases in the melted zone. Results showed that surface melting has led to a complete dissolution of the graphite nodules and re-solidification of a dendritic structure. The specimen melted by TIG showed dendrites of transformed austenite in a matrix of eutectic consisting of transformed austenite γ and Fe3C while the laser-melted zone exhibited finer dendrites of retained austenite surrounded by a continuous network of Fe3C; some needles of martensite within the dendrites are observed. The retained austenite dendrites have a preferred growth direction and contained a high concentration of dissolved carbon. The microhardness of the nodular cast iron was found to be significantly increased after melting.