Evaluating freckle tendency for electro-slag remelting 30CrMnSiNi2A ingots by experiments and simulation

Abstract

The mechanisms and tendency of freckle formation in industrial scale ESR ingots were studied by
thermodynamic calculation, thermo-physical property calculation, metallographic observation, and composition
analysis. The macrostructure and compositions of the freckle regions in a low alloy ultrahigh strength ESR steel
ingot were investigated to clarify the freckle formation mechanism. Combining the results with composition
analysis and thermodynamic calculation, it can be concluded that the compositions of freckles correspond
to that of liquid with a liquid fraction of 0.24 to 0.41, and the freckles were caused by the upward solute-rich
liquid flow which initiate at the lower part of mushy zone. The relative Rayleigh number (Ra), a freckle criterion
considering the effect of a tilted solidification front, was adopted to evaluate the freckle formation tendency
in the industrial scale ESR ingots. The calculated results of Ra numbers of different locations are in good
agreement with the actual distribution of the freckles in the ingot.