Redistribution of alloying elements during graphitization of mottled nodular graphite cast iron

Abstract

When cast on a chill, cast irons may solidify partly in a stable system and partly in a metastable one. Stable and metastable eutectic solidification are related to different partitioning of alloying elements between the liquid and the solid phases that precipitate. Accordingly, the distribution of alloying elements as well as the amplitude of microsegregation in as-cast irons may vary significantly with the amount of cementite, i.e. the relative amount of stable and metastable eutectics. During graphitization, cementite decomposes rapidly to austenite, so that this process is certainly controlled by transfer of carbon atoms from cementite to graphite. How redistribution of substitutional species between parent cementite and product austenite proceeds, and the resulting composition of austenite remain unclear. This work presents results related to the redistribution of alloying elements (Si, Mn and Cr) during graphitization of a initially mottled nodular cast iron. Measurements have been made by mapping and line scanning. Emphasis is put on the evolution of the composition of austenite at the matrix/cementite interface and of cementite during decomposition of this latter phase.