Containerless measurement and thermodynamic prediction of the physical properties of liquid steels

Abstract

Incomplete control over macrosegregation during steel solidification hinders the development of novel steel alloys and applications by limiting compositions which can be successfully cast. Analysis of macrosegregation at the solidification front is aided by study of the liquid state via fluid mechanics, which can place bounds on when macrosegregation can occur. This non-dimensional analysis requires knowledge of the physical properties of the liquid steel: density, surface tension, viscosity- and how they change with composition as solutes are rejected from solid at the solidification front. Macrosegregation is most pronounced in ferrous liquids containing light, non-metallic species, i.e. carbon, oxygen, and sulfur. Yet, existing literature models for predicting the physical properties of liquid alloys are incapable of accounting for these interstitial species inside an iron lattice. Additionally, direct experimental measurement of these properties is hindered by the requisite high temperature, high reactivity of the melt, and the vast composition space of steel alloys.

Herein, both the experimental and modelling challenges are introduced and addressed. An experimental technique using a floating zone furnace, pendant drop geometry, high-speed camera, and video segmentation was developed for simultaneous, containerless, high-throughput measurement of the physical properties of liquid steel samples. The central atom model, a multicomponent solution model, is extended to investigate the statistical structure of alloys consistent with their energetics and solution thermodynamics. This allows liquid structure determination from thermochemical measurements, bypassing structural and atomistic modeling challenges of high-temperature liquid systems.

These methods and models are explored on the binary systems of iron-nickel, the major substitutional alloying element in steel, and iron-carbon, the major interstitial species. Results demonstrate successful liquid property measurement at experimental rates far exceeding traditional high-temperature research and introduce a basis for a unified treatment of thermodynamic and physical properties in multicomponent alloy melts.