Surface Characteristics of Water Atomized Pre-alloyed Steel Powders and their effect on Sintering
Powder Metallurgy (PM) is an efficient method, in terms of cost and raw material utilization, for the production of structural parts that have strict dimensional tolerances and complex geometries. In the common processing route, steel powder is fabricated by water atomization and then pressed and sintered into the final component. Due to the production method the powder surface is covered by a surface oxide that plays a vital role in the development of inter-particle connections during the sintering and thus on the final properties of the part. The materials under investigation are Cr-alloyed PM steels. The aim is to first establish a solid methodology for evaluating the surface characteristics of metal powders by means of surface sensitive analytical techniques like X-ray photoelectron spectroscopy, Auger electron spectroscopy and scanning electron microscopy combined with X-ray microanalysis. Focus was also put into investigating the oxide distribution in the interior of the powder. The analysis revealed that the powder particles were mainly covered by a homogeneous thin (~6 nm) Fe-oxide layer and some spherical particulate features with size up to 200 nm that were complex Fe-Cr-Mn-Si-oxides. Additionally inclusions below 1μm in size and rich in Cr and Mn were observed rarely in the interior of the powder. During sintering the surface oxide undergoes some changes which depend on the sintering temperature and time and the sintering atmosphere. The presence of hydrogen in the sintering atmosphere has a clear positive effect on the early reduction of surface iron oxide during the heating stage, which is reflected on lower amount of oxides present as well as carbon loss after sintering. In the case of vacuum pronounced development of inter-particle connections during the heating stage above 900°C leads to early enclosure of surface oxide layer inside the necks. As a consequence a larger amount of inclusions and agglomerates is observed after sintering. The morphology and size of oxide inclusions change during the heating stage and after sintering they were mainly encountered as small (<1µm) spherical inclusions and rarely coalesced into larger agglomerates. At higher temperatures inclusions are stoichiometric Cr-Mn spinel oxides that have among the highest thermodynamic stabilities for this system.