Surface Analysis of PM Martensitic Steel Before and After Consolidation: Part 1: Surface Analysis of Powder

Abstract

Surface analysis of Fe12Cr martensitic steel powder was performed using electron spectroscopy for chemical analysis and Auger electron spectroscopy. The analysed powder was produced by nitrogen gas atomisation, the average particle size being 190 μm. In the as atomised condition the powder is covered by particles of manganese, chromium, and iron oxides, as well as a thin (3 nm) layer mainly consisting of Fe₂O₃. The average thickness of the oxide particles is ~13 nm, whereas the total average oxide thickness is ~7 nm. The composition and thickness of the surface oxides are independent of powder particle size in spite of the difference in cooling rate between large and small metal particles. The liquid metal droplets solidify at or below 1400°C. Most of the oxide is formed during solidification. It is suggested that a mixed oxide (MnCr₂O₄) is produced. Above 1400°C no metallic oxide is formed due to the oxidation of carbon to carbon monoxide. At very high temperatures manganese evaporates. During cooling the manganese gas is condensed and oxidised on the surface. As a result, a thin layer of MnO is produced on top of the previously formed oxides. Owing to surface depletion of the alloying elements the oxide formed at lower temperatures· during handling of the powder is mainly iron oxide. PM/0440A