Influences of cooling rates on solidification and segregation characteristics of Fe-Cr-Ni-Mo-N super austenitic stainless steel

Abstract

Solidification and segregation characteristics of Fe-Cr-Ni-Mo-N super austenitic stainless steel were quantitatively analyzed at cooling rate from 0.1 to 90 ℃/s by means of differential scanning calorimetry (DSC) and confocal scanning laser microscopy (CSLM). The results show that phase-transition characteristics in DSC curves correspond to the in-situ microstructures evolution in CSLM experiments. As the cooling rate varied from low to high, the value of crystallization temperature, the size of dendrite arm spacing and the volume fraction of eutectics were characterized as functions of cooling rate, in which corresponding expressions were established. All of these variables decreased with the increase of cooling rate. By applying the conclusions draw from CSLM experiments, the solidification and segregation characteristics in ingot casting (IC) and twin-roll strip casting (TRSC) were predicted and compared to the actual experimental microstructures. The results show good matches between predicted and experimental values except a small error in predicting the volume fraction of eutectics in cast strip (CS). The specific reasons were analyzed from casting processes. It is concluded that the available sub-rapid cooling rate and the loaded rolling force in TRSC take effect concurrently for inhibiting the formation of eutectics in CS. In addition, solute partition coefficients of alloying elements in different conditions, i.e. IC, CSLM and TRSC, show that the segregation degree was relieved with increasing cooling rate and thus the formation of eutectics in the interdendritic regions was inhibited.