| A series of twenty cast iron heats were made to accomplish the following: (1) to attain an improved understanding of the mechanisms which control the growth of graphite from liquid cast iron, (2) to develop a technique to produce compacted graphite (CG) iron consistently from high sulfur base irons, and (3) to determine the mechanical and casting properties of CG iron. The influence of chemica composition, iron treatment and cooling rate on the formation of compacted graphite from near-eutectic melts was studied. Casting properties and properties of selected CG irons were determined. Compacted graphite growth was studied by interrupted solidification quench tests, scanning electron microscopy and chemical microanalysis.;Casting characteristics were determined to be intermediate between those of normal gray and ductile irons. The mechanical properties of CG iron were affected by the amount of spheroidal graphite and the pearlite content of the matrix. Wear tests showed pearlitic CG iron to have wear resistance superior to that of a pearlitic, Class 35 gray iron.;The amount of free sulfur and oxygen in the melt after treatment with magnesium and rare-earths was a critical factor in determining the graphite structure. The equation below which describes the excess sulfur, (DELTA)S, was used to etermine the effect of residual magnesium, rare-earths and sulfur contents on the graphite morphology. (DELTA)S = final %S - 0.34 (%R.E.) - 1.33 (%Mg). In magnesium and rare-earth treated melts, compacted graphite structures were produced at (DELTA)S values from -0.015 to -0.0225. Aluminum contents greater than 0.35%, titanium contents greater than 0.10% and a combination of 0.20 to 0.30% aluminum plus 0.05 to 0.10% titanium significantly increased the (DELTA)S range over which compacted graphite iron forms by a factor of 3.5 to 6.0. Zirconium and antimony additions did not promote the formation of compacted graphite. It is possible that titanium and aluminum expand the (DELTA)S range for CG iron by promoting the formation of austenite dendrites, which affect the amounts of sulfur, oxygen, magnesium and rare-earths in the interdendritic liquid. This interdendritic segregation promotes the formation of compacted graphite. |
