Rapid solidification of martensitic stainless steel atomized droplets

The microstructure of martensitic stainless steel powders produced by inert gas atomization was investigated. Depending upon the powder particle size, the microstructure was found to exhibit a cellular, dendritic, or martensitic morphology. Relationships between the microstructure scale and the particle diameter were identified. It was found that at a critical particle diameter of 25 to 30 μm, the structure changed from cellular/dendritic (96.5 vol pct bcc and 3.5 vol pct fcc) to martensite. The solidification path of the powder particles below and above 25 to 30 μm in size was considered. High-temperature X-ray diffraction (HTXRD) measurements revealed that there is a delay in the appearance of the fcc phase for the small particle size. The delay in the appearance of the fcc phase is a result of different nucleation sites for the fcc phase between the large and the small particle size.