Solidificación

Solidification of Eutectic Alloys: Cast Iron

D.M. Stefanescu, University of Alabama

Introduction

Cast iron is a binary Fe-C or a multicomponent Fe-C-X alloy that is rich in carbon and exhibits a considerable amount of eutectic in the solid state. Two such possible eutectics may result, as follows:

• If solidification occurs according to the metastable diagram, Fe-Fe3C, the white eutectic or austenitic

(γ), iron carbide (Fe3C) forms.

• If solidification follows the stable diagram iron-graphite (a significant amount of silicon is required for this to occur), the gray eutectic, austenite-graphite (Gr), results

Depending on composition, cooling rate, and liquid treatment, it is also possible to produce a mixed white-gray eutectic called mottled structure. The two basic types of eutectic are very different, with mechanical properties such as strength, ductility, and hardness varying over very large intervals as a function of the type and the amount of eutectic formed. To understand the mechanism of the solidification of cast iron, it is necessary first to discuss the structure of liquid ironcarbon alloys.

Structure of Liquid Iron-Carbon Alloys

Observations from x-ray, neutron diffraction, and sound velocity measurements on liquid binary iron-carbon alloys at temperatures approximately 20 °C (35 °F) above the liquidus indicate that for up to 1.8% C the distance between nearest iron neighbors rI, as well as the number of nearest neighbors NI in the first coordination sphere, increases (Fig. 1). Above 1.8% C, the distance remains constant, while the number of nearest neighbors continues to grow. Above 3.5% and up to 5.5% C, both the distance and the number of neighbors remain constant. Above 3.5% C, short-range order regions rich in carbon exist in the melt. This means that the melt becomes more dense with the addition of carbon. A maximum packing density is reached at 3% C, and it remains constant at higher carbon concentrations. The excess carbon forms carbon-rich regions (nonhomogeneities) in the melt.

Fig. 1 Variation of distance between nearest neighbors (rI) and the number of nearest neighbors (NI) as a function of the percentage of carbon in the iron-carbon alloy. Source: Ref 1.

Viscosity measurements (Fig. 2) show a correlation between viscosity and percentage of carbon. This correlation can be further explained in terms of increased viscosity as the interatomic distance becomes smaller.

Fig. 2 Viscosity of iron-carbon alloys as a function of carbon concentration at a temperature » 20° above liquidus. Source: Ref 2.

Liquid iron-carbon alloys with low carbon content (< 3.5% C, that is, steels and cast irons poor in carbon) are microscopically homogeneous. Liquid iron-carbon alloys with high carbon (>3.5% C, that is, cast irons rich in carbon) are colloidally dispersed systems with microgroups of carbon in liquid solution. The nature of these microgroups is not clear.

It is hypothesized that they are either Fe3C clusters (Ref 3) or Cn clusters (Ref 1, 4).

Because the nucleation energy for Fe3C is smaller than that for graphite, it is thermodynamically possible for the carbonrich regions to exist as Fe3C clusters. Other investigators consider the Cn clusters to be stable (Ref 3). Their size is supposed to be in the range of 1 to 20 μm (40 to 800 μin.), and it increases with the carbon equivalent, lower silicon content, and lower holding time and temperature. It is to be expected that the carbon-rich configurations existing in molten iron-carbon alloys are in dynamic equilibrium and that they diffuse within the melt.

References cited in this section

1. S. Steeb and U. Maier, Structure of Molten Fe-C Alloys by Means of X-ray

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