MICROSTRUCTURE SELECTION IN LASER REMELTED FE-C-SI ALLOYS

Laser surface remelting at various velocities has been employed to stu dy the selection of microstructures of high-purity Fe-C-Si alloys cont aining nominally 3.2 to 4.2 wt.% C and 1 to 3 wt.% Si. The microstruct ure of the remelted region consisted of metastable Fe-Fe3C eutectic (l edeburite) or austenite dendrites, with interdendritic eutectic. Furth ermore, ledeburite presents two solid-liquid interface morphologies: p lanar and cellular. The competition between the austenite dendrites an d the ledeburite eutectic as a function of solidification rate has bee n experimentally determined. The critical velocity which destabilizes the planar Fe-Fe3C eutectic with respect to primary austenite dendrite s, was of the order of several mm/s and depends on the initial composi tion of the alloy. The critical velocity for the destabilization of th e 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 o f the coupled zone have been performed using current microstructure se lection models. The results of the simulation were then used to constr uct a microstructure map which was compared with experimental results.