Macrosegregation Behavior of 8Cr Tool Steel with Induced Bridging of Solidification in Laboratory-scale Ingot

Abstract

Prediction and control of a macrosegregation of cold tool steel in a large scale ingot is very important. In order to investigate macrosegregation behavior of 8Cr type tool steel (Fe-1C-8Cr-2mass%Mo), model ingots which have a steel chill ring in the middle region of a mullite mold with two different diameters were prepared. 8Cr steels were cast into the molds, thereafter 5 kg and 30 kg ingots were made. In both sizes of the ingot, the dendrite arm spacings in the chill rings were finer than other region which indicate the chill ring induced relatively rapid cooling than other region, results in inhomogeneous solidification. An enriched solute concentration was observed near the center of the chill ring where the top of the shrinkage cavity. The macrosegregation profiles were similar in both ingots, however the segregation ratio of the larger ingot was larger than in the small ingot. Computational simulations were also carried out and the solidification profiles of each ingot was investigated. The result shows that in smaller ingot, the duration time after the mushy zone formed to closure is shorter than in larger ingot. At every liquid fraction, the permeability for the larger ingot is higher than those of the smaller ingot, which suggests that suction of concentrated liquid phase from the upper region of chill through the mushy zone in the chill center is more preferabliy occur in the larger ingot.