Effect of silicon on Kinetics of bainitic reaction in austempered ductile cast iron

The effect of different contents of silicon on the transformation kinetics of austenite in a ductile cast iron austempered at 400 degrees C has been investigated. The amount of silicon varied from 1.55-3.48 wt-%. The number of nodules/mm(2) and the carbon content in the original austenite C(T)o, were similar for all of the chemical compositions studied. The effect of silicon on the rate of formation of bainitic ferrite, the product morphology, and the segregation was studied. The transformation of austenite occurs in two stages: in the first stage a bainitic ferrite and retained austenite microstructure forms; in the second stage austenite decomposes, increasing the volume fraction of ferrite and carbide. It was observed that increasing silicon content greatly increased the lime for completion of the first stage; the volume fraction of bainitic ferrite decreased with increasing silicon content, whereas the volume fraction of retained austenite increased slightly. The morphology of bainitic ferrite did not change significantly. The 'processing window', i.e. the time elapsed between the end of the first stage and the beginning of the second, was closed for all the silicon contents and temperatures studied. (C) 1996 The Institute of Materials.