On the Effect of Filling Rate on Positive Macrosegregation Patterns in Large Size Cast Steel Ingots

Chunping Zhang,Abdelhalim Loucif,Mohammad Jahazi,Rami Tremblay,Louis-Philippe Lapierre

doi:10.3390/app8101878

Chunping Zhang, Abdelhalim Loucif + Show 3 more

Open Access

https://doi.org/10.3390/app8101878

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Abstract

The effect of filling velocity on positive macrosegregation in large size steel ingots was studied. Macrosegregation and macro/microstructures were characterized on the hot-tops and a portion of the upper section of two ingots. The measurements revealed that segregation features in the two ingots varied as a function of the alloying elements, and that the severity of positive macrosegregation in the casting body was reduced when the filling rate was increased. It was also found that at the higher filling rate, grain morphologies in the first solidified zones of the ingot changed from columnar to equiaxe, and secondary dendrite arm spacing (SDAS) became slightly smaller in the intermediate and final solidified zones. The experimental findings were analyzed in the framework of diffusion and convection-controlled solidification, as well as liquid metal flow theories. The solute dependence of segregation features was related to the difference in the solid-liquid partition coefficient and diffusion capability of each element in the liquid iron. Calculation of Reynolds numbers (Re) during the filling process, for both ingots, showed that higher filling velocity caused more instable movement of the liquid metal in the initial solidification stage, resulting in the modification of grain morphology, as well as accelerated solidification rate.

Highlights

Ingot casting is the only method for the production of large size mono-block high-strength steels used in the energy and transportation industries
The effect of filling rate on positive macrosegregation characteristics in large size steel ingots was studied based on the analysis of the hot-top and a small portion of the main ingot body
General macrosegregation patterns presented the solute dependence of segregation features, which was related to the difference in the solid-liquid partition coefficient and diffusion capability of each element in the liquid iron

Summary

Introduction

Ingot casting is the only method for the production of large size mono-block high-strength steels used in the energy and transportation industries. Macrosegregation, defined as chemical heterogeneity on the scale of the entire ingot, is one of the most important, in high alloyed steels Such compositional variation over large distances results in local changes in mechanical properties, which may lead to reduced ingot quality and sometimes even to ingot rejection [1,2]. A better understanding of macrosegregation mechanisms, with the aim to control its extent in heavy castings, is an important scientific challenge with direct industrial implications This is true when it comes to high value-added products, such as large size casting of high strength steels used for turbine shaft applications. The results could be used as an input data in numerical simulation codes which still suffer from lack of accurate determination of material characteristics [15]

Materials and Experimental Process

General Macrosegregation Patterns

Effect of Filling Rate on Segregation Severity

Conclusions