Roles of dendrite tip undercooling and solid state diffusion in microsegregation of Fe-Nb welds

The contributions of dendrite tip undercooling and solid state diffusion to the final degree of microsegregation of gas tungsten arc welds in Fe-Nb alloys were experimentally determined. With a partitioning coefficient of ∼0-28, niobium is expected to highly segregate to the liquid during solidification. However, weld microsegregation modelling using a finite difference technique indicated that almost complete homogenisation occurs as a result of solid state diffusion during solidification and cooling. The final degree of microsegregation could be accounted for considering only solid state diffusion. Examination of specimens that had been liquid tin quenched during welding showed that a significant amount of tip undercooling occurred during solidification. The degree of undercooling and amount of solid state diffusion measured were compared with predicted values using solidification models. It was found that although tip undercooling resulted in a higher niobium concentration during initial solidification, its impact on the final degree of microsegregation was small because of the overwhelming effect of solid state diffusion.