Porcelain enamel processing is performed on the inner surface of iron kettle and tea pot for export. The biggest problem of the porcelain enamel processing on cast iron is the occurrence of bubble defect. Many studies have been reported on the causes of the bubble defect, but there remain many unclear points. In this work, five kinds of annealed cast iron samples, namely flake graphite cast iron with A-type, C-type and D-type graphite structure, spheroidal graphite cast iron with ferritic and pearlitic matrix and white cast iron without and with annealing, were used. Porcelain enamel processing was performed and the relation between surface microstructure of cast iron and bubble defect was examined. The surface microstructure differed from the center microstructure in the sample used. It became clear that graphite morphogy in the surface have a relation close to bubble defect formation in the porcelain enameled sample. The flak graphite cast iron sample, having a center microstructure of D type graphite in ferrite matrix, and the so-called non-graphite phase in the surface microstructure, has the smallest amount of bubble defect. Considerable bubble defect was observed in the samples with chill or with graphite fiakes protruding to the surface. Abnormal graphite formed in the surface texture of spheroidal graphite cast iron, and considerable bubble defect was observed, irrespective of the matrix microstructure.
