Rare-earth Modification Mechanism and Duplex Modification of Low Alloy White Cast Iron

In a brasion resistant low alloy white cast iron just as in unalloyed white cast iron, ledeburitic carbide morphology can be changed by RE modification to platelike carbide morphology having discontinuous distribution. The toughness of the alloy can thus be increased (Table I and Fig.1 and 2).
Experiments showed that the effects of RE modification on the crystallization of white cast iron include the following. Most of RE is concentrated, in sulfide inclusions (Fig. 3), which can serve as the substrate for the crystallization of primary austenite and thus volumetric nucleation of austenite is effected (Fig. 4).When crystallized in RE enriched melt, the order of austenitic dendrite axis can be increased thru the effect of constitutional undercooling(Fig. 5). Both effects cause obstruction to the continuous growth of eutectic carbide.RE modification causes the lowering of eutectic freezing temperature which favors the growth of divorced auste nite-carbide eutectic(fig.6) and it also enlarges the eutectic freezing temperature range, favoring volumetric nucleation of eutectics.(Fig.7). Selective absorption of RE on eutectic carbide obstructs the continuous preferential edgewise growth of carbide and favors its more or less uniform growth in different directions as platelike carbide.Based on the above assumption of RE modification mechanism, the effect of RE modification can be improved by the following techniques. Lowering of C content causes the lowering of carbide content and the incresing of primary austenite in white iron(Fig. 8).Slight increase of Si content causes the lowering of eutectic freezing temperature.Inoculation treatment before RE modification can help nucleate primary austenite,which is called duplex treatment. Use of complex inoculant and complex modifier can develop their advantageous effects with less unfavorable by effects.with this duplex modification technology,better and more reproducible results of changing carbide morphology in low alloy white cast iron can be obtained with much less RE modifier addition (Fig. 9).