Processes in PM steel compacts during the initial stages of sintering

Abstract

Abstract In classical sintering procedures of ferrous precision part, important processes occur during the heating period and/or during presintering stages. Apart from the de-waxing process, which is not discussed here, changes in surface chemistry, formation of metallic bridges, and dissolution of alloy elements added as powders take place. In this work, desorption of gases, reduction of surface oxides, and neck formation have been studied as well as the dissolution of carbon and of metallic alloy elements. It showed that surface chemistry is changed within fairly narrow temperature intervals the position of which depends on the composition of the powder particles and in part on pretreatment. Dissolution of carbon in iron and steel matrices is a slower process than might be anticipated from the thermodynamic viewpoint, in plain Fe complete dissolution occurring only at T >900°C and the atmosphere also playing a major role. Formation of stable necks in steel–graphite compacts is retarded until dissolution of graphite has occurred to a significant extent. Finally, dissolution of metallic alloying elements, also if promoted by transient liquid phase, starts at still higher temperatures, in the case of Mo even T >1200°C being necessary, and full homogenization is complete only after extended isothermal sintering.