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Due to their large specific surface, powders and powder compacts typically contain significant amounts of oxygen which has to be removed in the early stages of sintering. In the present study, it is shown that the homogeneity/heterogeneity of the oxygen affinity within the compact, which depends on the alloying route chosen, strongly affects the de-oxidation process. For several types of alloy steels, it is shown that systems alloyed through the mixing or master alloy route and containing elements with widely varying oxygen affinity exhibit oxygen transfer reactions through the internal gas phase. For pre-alloyed grades, in which the oxygen affinity is homogeneous, i.e. consistent between the powder particles, transfer reactions occur rather within the metallic particles themselves. In any case, the sintering temperature should be selected such as to grant reduction also of the most stable oxides contained.

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The use of low melting point master alloy (MA) powders contributes beneficially to sintering by increasing the distribution rate of alloying elements, enhancing homogenisation and sometimes also promoting densification. However, working with liquid phases poses important challenges like maintaining a proper dimensional control and minimising the effect of secondary porosity on the final performance of the steel. In this work, three different MA systems are compared: a low dissolutive Cu-based MA, and two systems with a higher degree of iron dissolution but different content in oxidation-sensitive elements. The combination of wetting experiments, step sintering tests and dilatometry studies show how the evolution of the microstructure, dimensional stability and overall densification are strongly affected by the characteristics of the liquid MA and in particular by its ability to dissolve the iron base particles, and by the amount of oxidationsensitive elements present in the composition of the MA powder.