In this paper an experimental study of the microsegregation and the microstructure in uni-directionally solidified 18Cr-BNi and 25Cr-20Ni austenitic stainless steels is described. The aim of the present investigation is to provide the quantitative data of the microsegregation and the microstructure in alloys of engineering interest in order to explain the homogenization process, and moreover to interpret the mechanism of the solute redistribution in dendritic growth.
For this purpose Cr- and Ni-isoconcentration contours around the primary dendrite arm were determined and a discussion was made on the solute distribution as a function of the fraction of solid which was given by area fraction solidified in the transverse section.
The results obtained are as follows:
1) Minimum Cr and Ni concentration at the center of primary dendrite stalk is slightly decreased with increasing distance from chill. Consequently with increasing distance from chill, the effective distribution coefficient K_E decreased and on the contrary the segregation ratio I increases.
2) There is a inflection in the solute distribution curve drawn against the fraction of solid. Andthe value of the fraction of solid at the inflection point is increased with increasing distance from chill. Therefore, it is concluded that the diffusion layer exists ahead of the solid-liquid interface near the growing dendrite tip, that is, homogenizing in liquid is never caused during dendritic growth, at least in the initial stage, and the diffusion layer has a very remarkable effect upon the solute redistribution.