Surface Reactions During Water Atomisation and Sintering of Austenitic Stainless Steel Powder

The surface oxides formed during water atomisation and sintering of austenitic stainless steel were determined using electron spectroscopy for chemical analysis (ESCA) and Auger electron spectroscopy (AES). Optical microscopy and electron microscopy (SEM and TEM) were used for structural analysis of powder and sintered material. The materials studied were 304L, 304L + Si, 304L + Al, and 304L + C. All powders were prealloyed except for 304L + C, which was obtained by admixture with graphite. Sintering was carried out in dissociated ammonia and in a vacuum. It is shown that the surface oxidation is strongly affected by the change in cooling rate with particle size. The average oxide thickness increases significantly with increasing particle size, while the surface oxide changes from a silicon rich oxide to an oxide containing more iron and chromium. A strong correlation between the average oxide thickness and the secondary dendrite arm spacing (i.e. the cooling rate) is observed. It was not possible to distinguish any clear effect of increasing the silicon content above 1 % on the surface oxidation. During sintering, the iron and chromium oxides formed during water atomisation are reduced. The silicon oxide forms a continuous layer at 1120°C, while it is broken up into discrete particles at 1250°C. The reduction favours neck growth resulting in improved mechanical properties. For equal final density, the impact strength can be correlated to the relative neck radius. Admixture with carbon before sintering can further enhance the oxide reduction. As a result of a higher sintering temperature or the addition of carbon, sintering is enhanced and improved mechanical properties are obtained. Prealloying with aluminium leads to a highly inferior mechanical strength due to the formation of aluminium oxide on the powder surfaces