Expansion and Contraction during Hydrogen Reduction of Green and Pre-heated Hematite Compacts Containing Foreign Oxides

Abstract

Effects of SiO₂ or Al₂O₃ added to green or pre-heated hematite compacts on the expansion/contraction of the both compacts when reduced in a hydrogen atmosphere were studied by measuring changes in the electrical resistance and thermal expansion/contraction and by examining the relationship of the degree of reduction to the electrical resistance and thermal expansion/contraction. The results obtained are as follows:
(1) Green compacts are reduced fast in the style of a uniform type where the reduction proceeds simultaneously at the inner parts as well as at the surfaces, while pre-heated compacts are reduced slowly in accordance with the core model where the reduction proceeds gradually from the outer to the inner parts.
(2) In case of green compacts, the smaller the oxide particle size is, the slower the reduction rate is and also the less the shrinkage due to reduction is.
(3) Cracking of green and pre-heated compacts containing foreign oxides must be due, among other causes, to the local stresses in compact matrix attributable to the difference in shrinkage among the contained oxides of different sizes and, in some cases, to the so-called phase-separated structure.
(4) A clear relationship of the electrical resistance and the expansion/contraction is observed over about 95% reduction in case of green compacts except the compact containing γ-alumina and, in case of pre-heated compacts, over about 90% reduction when silica is contained and over about 60% reduction when alumina is contained.