Metallographic study on the influence of the morphology and distribution of graphite on the solid state transformations of grey and ductile cast iron

Abstract

The present research focuses on carbon migration during the processes of carburisation (carbon diffusion from graphite particles to the matrix) and graphitisation (carbon diffusion from the matrix to graphite particles) in cast iron with spheroidal (or nodular) and flake graphite, and its effect on the kinetics of solid state transformations.The experimental methodology is based on metallographic observation of a large number of samples corresponding to different stages of the heat treatment cycles of ferritising, austenitising and austempering. This was carried out on pieces of three different sizes of both graphite morphologies. Quantitative metallographic techniques were used to measure the amount of phases present.The transformation kinetics are related to the distances for carbon diffusion, and to the graphite-matrix interface area. Hypotheses on the probable mechanisms of carbon migration, and on the way in which carbon atoms are incorporated into the primary graphite particles are discussed.The results indicate that for ferritising and austenitising, and also for the beginning of austempering, reactions take place more rapidly in grey cast iron than in ductile cast iron, and for identical graphite morphology the transformation rate is greater in smaller pieces.In processes of graphitisation, all the carbon present in the matrix is added to the primary graphite particles and no secondary graphite is nucleated, despite of the existence of Cu and Ni acting as perlitising alloying elements. In this process the incorporation of carbon atoms into existing graphite particles is not necessarily made through the exposed basal planes, as proposed in the literature.