Water atomized steel powder particles are covered by heterogeneous surface oxide, formed by thin (~ 6 to 8 nm) iron oxide layer covering most of the powder surface, and particulate features formed by thermodynamically stable oxides containing, for example, Cr and Mn with surface coverage about 5%. Development of sufficiently strong interparticle necks requires as minimum full removal of the iron surface oxide layer that can be achieved by gaseous reducing agents as CO and H2 as well as by carbon typically admixed in the form of graphite. The study evaluates the effect of concentration of reactive components of the sintering atmosphere, with special focus on carbon monoxide, on the reduction/oxidation and carburization/decarburization processes taking place during the whole sintering process. Results of the thermal analysis, SEM analysis of oxide characteristics, metallographic, and chemical analysis of the sintered compacts were correlated with thermodynamic simulation of the oxide stability in applied sintering atmospheres. High oxidation potential of the CO‐containing atmospheres in case of Cr‐alloyed PM steels was detected during heating stage until ~1000°C. Oxidation potential is linearly increasing with the increasing content of the carbon monoxide in the processing atmospheres and rather severe oxidation is observed if CO content exceeds 1 vol%.
The initial oxide state of powder is essential to the robust additive manufacturing of metal components using powder bed fusion processes. However, the variation of the powder surface oxide composition as a function of the atomizing medium is not clear. This work summarizes a detailed surface characterization of three 316L powders, produced using water atomization (WA), vacuum melting inert gas atomization (VIGA), and nitrogen atomization (GA). X‐ray photoelectron spectroscopy (XPS) and scanning electron microscopy analyses were combined to characterize the surface state of the powders. The results showed that the surface oxides consisted of a thin (~4 nm) iron oxide (Fe2O3) layer with particulate oxide phases rich in Cr, Mn, and Si, with a varying composition. XPS analysis combined with depth‐profiling showed that the VIGA powder had the lowest surface coverage of particulate compounds, followed by the GA powder, whereas the WA powder had the largest fraction of particulate surface oxides. The composition of the oxides was evaluated based on the XPS analysis of the oxide standards. Effects of Ar sputtering on the peak positions of the oxide standards were evaluated with the aim of providing an accurate analysis of the oxide characteristics at different etch depths.
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