Structure of laser remelted surface of cast irons

Abstract

Grey and white cast irons were surface remelted using a high power CO2 laser with a scanning velocity in the range 0·5–200 mm s-1 and the resulting surface structures were investigated using optical microscopy and X-ray diffraction. The remelted surface of the white iron was composed of austenite dendrites, which decompose to pearlite at low cooling rates and martensite at medium cooling rates. The maximum volume fraction of martensite was obtained at 10 mm s-1. The lattice parameter of the austenite increased with increasing solidification velocity owing to solute trapping of carbon. The remelted surface of the grey iron was composed of cellular ledeburite and X-ray diffraction analyses of the grey specimen indicated the presence of austenite, martensite, graphite, cementite, and ɛ phases. The initial austenite partially transformed to martensite at high cooling rates and to ɛ phase at low cooling rates. The appearance of the ɛ phase is therefore linked to the decomposition of supersaturated austenite.