Microstructural evolution of injection molded gas- and water-atomized 316L stainless steel powder during sintering

The present study investigates the microstructural evolution of 316L stainless steel in order to understand the differences in the densification behavior of powder injection molded water- and gas-atomized powder. Dilatometry and quenching studies were conducted to determine the extent of densification and the corresponding microstructural changes. Results show that water-atomized powder could be sintered to a maximum of 97% of theoretical density while gas-atomized powder could be sintered to 99% of theoretical density. The difference in the ability to eliminate porosity is examined in terms of the particle morphology, initial green density and the particle chemistry. Specimen composed of water-atomized powder exhibit large sintering shrinkage compared to that of gas-atomized powder owing to the low green density associated with poor packing property of the powder. Microstructural analysis of the quenched and sintered samples reveals that pore size and porosity at the grain boundaries of gas-atomized powder decreases with time at a sintering temperature of 1350 °C, while the porosity and pore size at the grain boundaries increases for the water-atomized powder limiting its ability to achieve full density. The results are explained in terms of the thermodynamic stability of the oxides and the reduction species at sintering temperature. It is inferred that the inability of the specimens made of water-atomized powder to achieve full densification is due to the presence of oxides.