Segregation and eutectic formation in solidification of Fe-1C-1 5-Cr steel

Abstract

The microstructure and solute segregation have been investigated in a continuously cast bloom and laboratory cast ingot of Fe–1C–1.5Cr (wt-%) steel. Eutectic carbide formation was observed only in the centreline region in the continuously cast bloom. In both specimens, the maximum chromium level detected was 3% in the columnar and 5% in the equiaxed region, while the minimum remained at 1.2% in both regions. The corresponding segregation ratios (C _max/C _min) were 2.5 and 5, in agreement with many previous studies. By numerical modelling of microsegregation it has been shown that the equilibrium partition coefficient of chromium k _Cr, which changes with carbon content, has a significant effect on chromium distribution during solidification. The carbon distribution may be taken to be in equilibrium during solidification, while that of chromium develops a concentration gradient in the solid. Numerical predictions of segregation behaviour, assuming local equilibrium at the liquid/solid interface, backdiffusion in the solid and complete mixing in the residual liquid, are consistent with experimental results in the columnar and equiaxed regions. The conclusion that eutectic carbide observed in the centreline region must have resulted from macrosegregation is supported by an estimate of the composition of the enriched liquid.