In Situ Observation for Abnormal Grain Coarsening in Vacuum-Carburizing Process

An in situ observation method was developed to investigate abnormal grain coarsening which occurs around the surface of steel during the vacuum-carburizing process. In this method, diffusion of carbon atoms in the vacuum carburizing was simulated by a cementite and steel diffusion couple. Abnormal grain coarsening, which appeared around the cementite and steel interface, was observed by a confocal scanning laser microscope. With this method, it was observed that when holding time was 60 seconds, the temperature at which the abnormal grain coarsening appeared in a specimen was higher when carburized than the temperature when not carburized. On the contrary, when holding time was 120 seconds, the temperature at which the abnormal grain coarsening appeared in a specimen was 10 K to 20 K lower than that in a non-carburized specimen. The validity of the observed results was confirmed by the calculated NbC fraction using Nb solubility and measured carbon content.