EVOLUTION OF MICROSTRUCTURES RESULTING FROM DENDRITE GROWTH IN UNDERCOOLED FE-CR-NI MELTS

Experiments have been carried out For two alloys Fe69Cr1Ni30 and Fe69C r7Ni24 to investigate the evolution of microstructures of steels grown from undercooled melts. The microstructures undergo several critical transitions as the undercooling at which solidification starts is incr eased. Firstly, grain size can change by up to an order of magnitude f rom small to large and back to small at critical undercoolings. Second ly, the segregation pattern changes from dendrilic/equiaxed to dendrit ic and finally to equiaxed with increasing undercooling. Measurements from the segregation patterns indicate that the primary dendrites had undergone considerable Ostwald ripening depending on the processing co nditions (heat-extraction rate). Relating the measured trunk sizes to an original trunk size through ripening and then applying this value t o a recent model for grain refinement, which is based on dendrite frag mentation, yields a rough approximation to the critical transitions of the grain size. (C) 1998 Acta Metallurgica Inc.