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

Effects of the addition of SiO2 and Al2O3 to green and pre-heated hematite compacts on the expansion/contraction of the two compacts when reduced in a hydrogen atmosphere were studied by measuring changes in the electrical resistance and thermal expansion/contraction.The results obtained are as follows:(1) Green compacts are reduced faster than pre-heated compacts. The reduction of green compacts 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 is considered to be due mainly to the local stresses in compact matrix caused by the difference in shrinkage between the contained oxides of different sizes and, in some cases, to the so-called phase-separated structure.(4) The electrical resistance is clearly related to expansion/contraction over 95% reduction in case of green compacts except the compact containing γ-alumina; in case of pre-heated compacts, over 90% reduction when silica is contained and over about 60% reduction when alumina is contained.