SURFACE-REACTIONS DURING WATER ATOMIZATION AND SINTERING OF AUSTENITIC STAINLESS-STEEL POWDER

The surface oxides formed during water atomisation and sintering of au stenitic stainless steel were determined using electron spectroscopy f or chemical analysis (ESCA) and Auger electron spectroscopy (AES). Opt ical microscopy and electron microscopy (SEM and TEM) were used for st ructural analysis of powder and sintered material. The materials studi ed were 304L, 304L + Si, 304L + Al, and 304L + C. All powders were pre alloyed except for 304L + C, which was obtained by admixture with grap hite. Sintering was carried out in dissociated ammonia and in a vacuum . It is shown that the surface oxidation is strongly affected by the c hange in cooling rate with particle size. The average oxide thickness increases significantly with increasing particle size, while the surfa ce oxide changes from a silicon rich oxide to an oxide containing more iron and chromium. A strong correlation between the average oxide thi ckness and the secondary dendrite arm spacing (i.e. the cooling rate) is observed. It was not possible to distinguish any clear effect of in creasing the silicon content above 1% on the surface oxidation. During sintering, the iron and chromium oxides formed during water atomisati on are reduced. The silicon oxide forms a continuous layer at 1120 deg rees C, while it is broken up into discrete particles at 1250 degrees C. The reduction favours neck growth resulting in improved mechanical properties. For equal final density, the impact strength can be correl ated to the relative neck radius. Admixture with carbon before sinteri ng can further enhance the oxide reduction. As a result of a higher si ntering temperature or the addition of carbon, sintering is enhanced a nd improved mechanical properties are obtained. Prealloying with alumi nium leads to a highly inferior mechanical strength due to the of alum inium oxide on the powder surfaces.