MICROSTRUCTURE CONTROL IN ALLOY-STEEL POWDERS

Microstructural developments related to solidification in two alloy st eel 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 po wders to be composed of austenite and ferrite. Generally, the two phas es occurred separately in different powder particles, but in some case s dual-phase powder particles were also observed. With increasing powd er size the amount of austenite phase was observed to increase and at the largest sizes yield a martensitic transformation product. The infl uence of powder size on the solidification morphology and phase select ion has been analyzed on the basis of relative undercooling and compet itive solidification kinetics effects. The control of microstructure t hrough powder size variation is an important process control that can be effectively utilized.