Graphite nucleation control in grey cast iron

A research programme has been undertaken to achieve a more detailed understanding of graphite nucleation control in grey cast irons, at different sulphur (0.02-0.1%), residual aluminium (0.001-0.010%) and zirconium (0.001-0.015%) levels in iron melts. It was found that three groups of elements are important to sustain a three stage model for the nucleation of graphite in grey irons: (i) strong deoxidising elements (Al, Zr) to promote early formed very small microinclusions, oxide based, which will act as nucleation sites for later formed complex (Mn, X) S compounds (ii) Mn and S to sustain MnS type sulphide formation (iii) inoculating elements (Ca, Sr, etc.) which act in the first stage or/and in the second one of graphite formation, to improve the capability of (Mn, X) S compounds to nucleate graphite. It was confirmed that 0.07% S level is beneficial for graphite nucleation in grey irons with a lower incidence of carbides and undercooled graphite, compared to 0.023% S cast irons. Low residual Al level (0.001-0.003%) results in higher chill and more undercooled graphite and lower eutectic cell count, in inoculated irons. A 0.007-0.010% Al content in the melt is important to sustain type A graphite nucleation and reduced chill. Not only inoculation but also the preconditioning (Al or/and Zr) of the base iron has a strong beneficial effect on the solidification pattern of cast irons. Both Al and Zr sustain the type A graphite formation with a lower degree of undercooling and free carbides. These elements were associated in a complex alloy (FeSi based), very efficient in preconditioning of grey irons for thin wall castings, at a low addition rate.