Thermodynamic and experimental study of role of sintering atmospheres and graphite additions on oxide reduction in Astaloy CrM powder compacts

Abstract

Several atmospheres based on N₂–H₂ gaseous mixtures, with occasional additions of CH₄ were used to study the sintering behaviour of Astaloy CrM at temperatures of 1100 and 1240°C. Theoretical thermodynamic calculations and sintering experiments, with and without admixed graphite additions, were carried out devoting particular attention to the oxide reduction reactions. The role of carbon in the reduction of oxides is discussed with reference to Boudouard's reaction, proposing the indirect carbothermal reduction as the controlling mechanism. The microstructural characterisation of the material included inspection of the powder particles as well as sintered specimens. It was established that the Astaloy CrM particles contain two distinct types of oxides. One associated with the particle surface and another, mainly constituted by Cr, forming a dispersion of internal oxides. These internal oxides were microstructurally characterised, both directly and by carbon extraction replicas. A selection of powder mixtures containing 0 and 0.4%C additions were used for obtaining tensile specimens in order to assess neck development, by the strength and elongation obtained under various atmospheres and temperature combinations.