FACTORS AFFECTING GRAPHITE MORPHOLOGY, MATRIX STRUCTURE AND TENSILE PROPERTIES OF COMPACTED GRAPHITE CAST IRONS

The scanning electron microscopic observations indicate that graphite morphologies can be changed from one form to another over a wide range of graphite formations by altering the solidification cooling rate and/or the amount of nodulizing elements present such as magnesium or rare earths.; It was also observed that those changes occur by a gradual change in the graphite morphology and do not occur in an interrupted manner which would be characteristic of unique nucleation events required for each graphite form.; Increasing cooling rate and/or increasing the effective presence of nodulizing elements resulted in the following change in graphite formations: type A flake, type B flake, type D undercooled flake, coral, compacted, deteriorated forms of spheroidal, and spheroidal graphite.; Observations also indicate compacted graphite, like flake graphite is interconnected within a eutectic cell, but predominant growth of these individual stubby lamellae, as seen under the microscope on a flat surface, take place along the C axis like spheroidal graphite. However, it is noted that clear eutectic cell boundaries on macroscopically etched samples were revealed in samples only exhibiting fully compacted graphite morphology and no clear eutectic cell boundaries were observed when the graphite form was predominantly either degenerated spheroidal or spheroidal. This suggests that eutectic cell growth characteristics of compacted graphite are similar to that of flake graphite.; It was observed that increasing silicon content upto 2.70% increased the percent compacted graphite and decreased the amount of pearlite content present, but further increase of silicon upto 2.93% resulted in decrease of percent compacted graphite and the amount of pearlite content present further.; A linear equation, Y = -0.38X + 4.46 (+OR-) 0.17, where Y = %C and X = %Si, was found for the determination of C-Si ratio and CE to produce "acceptable" compacted graphite cast iron. It was observed that the graphite floatation was inevitable in high CE (over 4.60) heats.; Optimum Mg-S, and Ti-Mg ratios to produce "acceptable" compacted graphite cast iron were determined from 2/3 to 7/1 and from 10/1 to 4/1, respectively, in the range of Mg content at 0.013-0.023%.; The successive increase of titanium additions increased the chill depth, and percent compacted graphite slightly revealing no less than 90% compacted graphite structure in both rare earth treated and lanthanum ferrosilicon treated heats. It was also observed that compacted graphite structures became thinner with increasing Ti additions and that a nearly negligible change in matrix structure resulted.; The successive increase of aluminum additions decreased chill depth and resulted in negligible change in both percent compacted graphite and matrix structure. However, it was observed that compacted graphite structures became thicker with increasing aluminum content.; The tensile properties of compacted graphite cast irons made with either MgFeSiTi alloy or rare earth alloy can be controlled by not only matrix structure, but also by graphite morphology. The tensile properties of compacted graphite cast irons may be reported as intermediate between those of ductile iron and those of gray iron at the equivalent BHN and matrix structure.; It was found that a good correlation between tensile strength and hardness exists satisfying the equation; TS = 0.30 x BHN + 5.08 with a correlation coefficient of 0.96.