The nucleation of iron on dense wustite: A morphological study

Abstract

Dense wustite was reduced at temperatures from 430 to 1100 °C in CO-A, CO-CO2, H2-A, and H2-H2O mixtures. Most of the experiments were conducted on a hot stage microscope, providing a clear record of the sequence of events. The morphology of phases was further studied on the optical and the scanning electron microscopes. Two types of nuclei were identified: (1) dense nuclei, ranging from regular whiskers to simple protrusions, around which flat bases develop to form a protective film, and (2) porous nuclei with lenticular shapes, which remain level with the sample surface as they grow (both radially and into the wustite) to form a porous layer. Mild gases and moderate temperatures favor dense iron, whereas strong gases and extreme temperatures (either low or high) favor porous iron. The domains of the two types are represented in morphology maps. They overlap in transition zones where mixed nuclei are found; in this case the porous nuclei appear later than the dense ones but they grow much faster. The morphology maps support the view that sponge iron in iron ore reduction does not originate from porous iron nuclei but develops as a maze of dense iron growths. Experiments with cold-worked and annealed samples indicate that dislocations are essential to the growth of whiskers.