Formation of alumina in steel and its dissemination during mechanical working

Abstract

An examination has been made of alumina formed during the deoxidation of iron–oxygen alloys by aluminium, and also of the fracture of the alumina during mechanical working. The morphology of alumina is controlled largely by the supersaturation of the melt, high supersaturation giving dendritic alumina while low supersaturation gives liquid mixed oxides which react to form alumina and spinel reaction rims. The morphology of the alumina formed direct from the melt is controlled largely by the dissolved oxygen content which, in turn, controls the alumina/melt surface energy. Spheroidization and sintering of the dendritic alumina occurs as the dissolved oxygen of the melt decreases until dense, sintered alumina aggregates are formed. During deformation, the alumina becomes fractured and disseminated into stringers. Dendritic alumina fractures by breaking off the dendrite arms, while the polycrystalline alumina aggregates fracture intergranularly. Mechanisms are suggested for the formation of the various forms of alumina, and the processes involved in their fracture are discussed.