Microstructure Selection in Laser Remelted Fe-C-Si Alloys

Laser surface remelting at various velocities has been employed to study the selection of microstructures of high-purity Fe-C-Si alloys containing nominally 3.2 to 4.2 wt.% C and 1 to 3 wt.% Si. The microstructure of the remelted region consisted of metastable Fe-Fe3C eutectic (ledeburite) or austenite dendrites, with interdendritic eutectic. Furthermore, ledeburite presents two solid-liquid interface morphologies: planar and cellular. The competition between the austenite dendrites and the ledeburite eutectic as a function of solidification rate has been experimentally determined. The critical velocity which destabilizes the planar Fe-Fe3C eutectic with respect to primary austenite dendrites, was of the order of several mm/s and depends on the initial composition of the alloy. The critical velocity for the destabilization of the eutectic interface leading to two-phase cells was 0.44 mm/s, for the alloy containing 4.2 wt.% C and 1 wt.% Si. Theoretical calculations of the coupled zone have been performed using current microstructure selection models. The results of the simulation were then used to construct a microstructure map which was compared with experimental results.