MICROSTRUCTURE CONTROL IN ALLOY-STEEL POWDERS

Microstructural developments related to solidification in two alloy steel compositions have been examined as a function of powder size. With decreasing powder size the solidification morphology was observed to change from a dendritic to a cellular mode with a refined size scale. X-ray diffraction and metallographic investigations show the finest powders to be composed of austenite and ferrite. Generally, the two phases occurred separately in different powder particles, but in some cases dual-phase powder particles were also observed. With increasing powder size the amount of austenite phase was observed to increase and at the largest sizes yield a martensitic transformation product. The influence of powder size on the solidification morphology and phase selection has been analyzed on the basis of relative undercooling and competitive solidification kinetics effects. The control of microstructure through powder size variation is an important process control that can be effectively utilized.