Surface chemical analysis of prealloyed water atomised steel powder

Abstract

Prealloyed water atomised steel powder was investigated regarding composition, morphology and thickness of the surface oxide. The materials were two varieties of Cr alloyed and one Mo alloyed. The oxides formed on powder surfaces were studied by means of the surface analytical techniques X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy, in combination with high resolution electron microscopy and X-ray microanalysis.On all powder surfaces, the oxide formed contained strong oxide forming elements such as Cr, Mn and Si. Still, the dominant oxide on the powder surface was Fe oxide, the relative cation concentration in the surface being about 80%. The surface morphology showed a heterogeneous structure with particulate compounds supposed to be rich in strong oxide formers and an Fe rich thin oxide layer. This heterogeneous surface oxide morphology was more pronounced for the Mo alloyed powder compared with the Cr alloyed varieties. For this alloy, the average thickness of the oxide layer was somewhat larger (7–87·5 nm for fraction , <63 μm and 63–106 μm) compared with that of Cr alloyed powder (6–7 nm in all cases for fraction , <63 μm and 63–106 μm). An interesting observation was that Cr oxide also appeared on the nominally Cr free Mo alloyed powder, which indicates selective oxidation of Cr also when this element is present in low quantities. Although the alloying elements are enriched by a factor of 40–50 in the surface oxide, there is little effect on depletion of them from the steel matrix.From a sintering point of view, the particulate oxide compounds should play a minor role, since the most probable contacts between individual metal particles are Fe oxide/Fe oxide.