On the mechanisms of structure formation in nodular cast iron

Abstract: The mechanisms of structure formation in nodular cast ironhave been examined by experiments and by physical modelling. Byincorporation of models for nucleation and growth for allrelevant phases into a heat flow calculation. it is possible topredict the as-cast structure by computer simulation. Mainly,this work has focused on the eutectoid transformation and onnucleation of graphite during solidification.From experimental observations it was concluded that a purediffusion model was not sufficient to describe thetransformation rate of austenite into ferrite and graphite aspreviously proposed in the literature. By introducing aninterfacial mass transfer resistance at the graphit/ferriteinterface, to control the incorporation rate of carbon atoms onthe graphite nodules, it is possible to reproduce the ferritegrowth experimentally observed by numerical simulation.Silicon, copper and manganese are three alloying elementswhich are frequently used in nodular cast iron. Silicon isknown to increase the ferrite content while copper andmanganese have the opposite effect, thus promoting pearliteformation. The mechanisms responsible for the different actionof these elements have been studied by thermal analysis ofcooling curves in plate castinags (4 to 50 mm) at differentalloying levels(1.7<%Si<4.9,0.03<%Cu<0.88.0.16<%Mn<0.99).The tendency for copper to decrease the section sizesensitivity can be explained by the new theories.The effectiveness of inoculants used in nodular cast iron isto a large extent dependent on the Mg-treatment and the meltpreparation. In a detailed examination of two inoculants it wasfound that the nodule counts obtained were linearlyproportional to the maximum eutectic supercooling.The models have been implemented as a subroutine into afinite difference (FDM) program in order to be able to relateheat flow calculations and structure evolution in complex threedimensional castings.Key words: Nodular cast iron, ductie iron, SG ironferrite, modelling, eutecroid transformation, silicon, copper,manganese, thermal analysisis, interface control, pearlite,diffusion control, casting simulation.