Apparent density for water atomized low-carbon steel powder

Abstract: Carbon content does not affect the apparent density for water atomized iron powder monotonically. At low carbon content, around 0.1 wt %, apparent density is higher than a similar powder with 0.3 wt % carbon. But then the apparent density increases with increasing carbon content forming a U-curve. The goal of this thesis was to try to determine what causes this behaviour. Powder was atomized in a pilot atomiser to create material for the study. At carbon contents below 0.1 wt % there is a sharp decrease in apparent density, which has not been seen before. Density was then found to decrease up to a moderate carbon level, before increasing again. This leads to a local maximum at approximately 0.1 wt % carbon content, and a local minimum at approximately 0.25 wt % carbon. For carbon contents below 0.1 wt %, carbon dynamic image analysis showed that there was a decrease in the occurrence of small- (0–45 μm) and an increase in the occurrence of medium- (45–150 μm) sized particles. These size classes did not change above 0.1 wt % carbon. The fraction of large (>150μm) particles was approximately constant for all carbon contents. Particle morphology (sphericity) showed a clear increasing relationship with apparent density. A theoretical surface tension was calculated from literature, and it showed a dependence on morphology on surface tension up to a 0.1 wt % carbon content. This surface tension changes because of the oxygen content of the melt which, in turn, changes because of carbon content. At higher carbon contents, particles seem to become more irregular, while the nitrogen content of the powder also increased, but this needs to be studied further. The change in apparent density in higher than 0.1 wt % carbon could also come from a change in solidification time due to the difference between liquidus and solidus for the material. The results of this are interesting for industry, as it shows that to achieve the same apparent density with different carbon contents, some other parameters should be changed. It also shows that additions of surface-active elements have a large impact on the apparent density and this effect should be studied further.