A Study of Cerium and Magnesium in the Nodularizing Process for Ductile Cast Iron.

Abstract

In 1948 in one of the most important metallurgical discoveries of the century, it was found that the addition of small amounts of Mg or Ce to liquid gray cast iron produced a material with strength and ductility approaching cast steel. The reason for the improvement in properties is the addition of Mg or Ce results in the formation of spheroidal (nodular) graphite in place of the flake graphite of typical gray cast iron. This material called ductile or nodular iron is now the second most important cast material, surpassing cast steel. Since the discovery, there has been considerable controversy regarding the relative merits of Mg and Ce in producing spheroidal graphite. Despite the large number of investigations, there has been no consistency in reporting the quality of the treatment (expressed as % nodularity). The effects of other variables such as the base analysis of the iron and the effect of section thickness have not been investigated in depth. Furthermore, there has been no generally applicable correlation of % nodularity with mechanical properties. The first portion of this investigation was devoted to a critical comparison of three methods of evaluation of % nodularity. It was found that the completely objective image analysis method gave the best reproducibility although somewhat lower average values than metallographic grid method or the visual lot estimate using st and ard photographs. The second portion of the investigation involved a comparison of the effects of Mg from 0 - 0.06% and of Ce from 0 - 0.075% (which are beyond commercial ranges) over a range of section thicknesses from 1/4 inch, (6 mm) to 3 inch, (75 mm) for eutectic and hypereutectic compositions. Above 0.30% Mg over 85% nodularity was obtained while the maximum for Ce showed 60% nodularity. The % nodularity declined above 0.055% Ce while no maximum was encountered for Mg. Minor effects from section thickness and chemical analysis were obtained. Correlation of % nodularity with % elongation has been obtained with best fit equations for 1 inch section. % Elongation = 5.97 - 0.21X + 7.36 x 10('-3)X('2) - 2.91 x 10('5)X('3) where X = % Nodularity Better correlation of the structure and mechanical properties has been obtained by developing a new structure factor W which equals (% nodularity) multiplied by (mean free path between graphite particles).