Cast Iron Model

One of the main concerns of a foundry engineer is excessive shrinkage porosity that may form during solidification. Most of the volumetric changes in cast irons take place during cooling of the liquid alloy from the pouring temperature to the solidus and, more significantly, during eutectic solidification, when gamma-iron, graphite and carbide form. The placing of risers (or risering) provides additional metal to feed the shrinkage. Optimum risering is important for achieving good quality castings at minimum cost. The amount of shrinkage can also be controlled by proper alloying and cooling of the metal. The cast iron model in FLOW-3D CAST takes into account all these factors to predict the formation of porosity and the development of the phases during eutectic solidification.

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Overview of the Cast Iron Model

Cast iron is a near-eutectic iron alloyed with carbon and silicon. Carbon is typically present in the range from 2.5 wt % to 4.5 wt % and silicon in the range from 1 wt % to 3 wt %. Silicon is added to stabilize graphite and to reduce &#;chilling&#; tendency (i.e., the formation of carbidic iron). Other elements and compounds are present in trace amounts and typically either control the graphite shape (e.g., magnesium in ductile iron), act as additional deoxidizers (e.g., phosphorous), or serve as inoculants of graphite (e.g., ferrosilicon).

FLOW-3D CAST&#;s cast iron model accounts for the volume changes occurring from the pouring temperature to the solidus: shrinkage during cooling in the liquid state; further shrinkage during pre-eutectic gamma iron formation; subsequent shrinkage or expansion during the eutectic reaction; and the secondary shrinkage from the end of the eutectic reaction to the solidus. Since cast iron typically contains non-iron phases that may affect the formation of carbide, a heuristic allowance&#;in the form of the chilling susceptibility parameter&#;is made for the effect of these phases on the density of the solidified metal.

The latent heat release in the cast iron solidification model is computed as a function of temperature (the so-called freezing path) determined from the Fe-C phase diagram [1], using the concentration of carbon and silicon in the initial melt. The model can be used together with the general solidification model, with or without flow. However, volume changes associated with the formation of different phases are only coupled to the simplified shrinkage model that does not include flow.

The effect of mold wall movement during iron expansion is not included in the present model. Any net volume expansion that cannot be accommodated by the available space in the mold is ignored.

In the eutectic region, the speed of the eutectic front is used to compute the local chilling tendency, and, therefore, the local amount of carbide, so modeling chill zones near mold walls is possible. No attempt is made to track further transitions of the eutectic phases during the solid-state eutectoid transformation, that is, the final as-cast microstructure is not predicted.

For hyper-eutectic cast irons it is assumed that only graphite forms during the initial pre-eutectic stage of solidification, as in gray and ductile irons. In other words, the model does not include the solidification of hyper-eutectic white irons during the pre-eutectic stage in which primarily carbide forms.

Cast Iron Freezing Path

The cast iron freezing path is that of a eutectic alloy. It can be characterized by the liquidus temperature, eutectic temperature, the eutectic-start and eutectic-end solid fractions and the solidus temperature. All, but the last two quantities are computed from the equilibrium ternary Fe-C-Si phase diagram [1].

The carbon solubility in the gamma phase depends on the Si content, in wt %, according to:

(1)     $latex \displaystyle {{C}_{{\gamma ,mx}}}=2.07-0.098Si,$

which is in close agreement with the solubility reported by Stefanescu [2]. The liquidus temperature of the alloy, in degrees centigrade, follows from either the hypo-eutectic liquidus plane:

(2)     $latex \displaystyle {{T}_{i}}=-113\left( {C+0.25Si} \right)$

or, the hyper-eutectic liquidus plane [2]:

(3)     $latex \displaystyle {{T}_{i}}=-505.8+389.1\left( {C+0.31Si} \right),$

and the eutectic compositions and temperatures are given by the intersection of these planes:

(4)     $latex \displaystyle {{C}_{e}}=4.26-0.296Si,$     $latex \displaystyle {{T}_{e}}=.6+5.2Si$

The beginning of the eutectic reaction is a derived quantity given by the lever rule:

(5)     $latex \displaystyle {{f}_{e}}=\frac{{c-{{c}_{\varepsilon }}}}{{{{c}_{{\gamma ,mx}}}-{{c}_{\varepsilon }}}}.$

The measurements in [3] suggest this approximation is adequate for a number of cast irons.

The end of graphitic eutectic reaction, fee , and the solidus Ts, are left as user-defined quantities. If one considers positive segregation of phosphorous in the liquid, the actual solidus temperature is below the graphitic eutectic temperature, and is as low as °C. For this case, it is assumed that graphite precipitation is complete before the end of freezing, and that the last fraction of metal to freeze, 1-fee, does so at a density ρei different from the eutectic density.

Density Changes

Typically, superheat in cast iron castings is large and shrinkage during cooling is significant before any solidification even begins. The temperature dependence of the liquid iron density is modeled either in a linear form:

(6)     $latex \displaystyle \rho \left( T \right)={{\rho }_{0}}\left[ {1-\alpha \left( {T-{{T}_{0}}} \right)} \right]$

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or by defining the function ρ(T) in a tabular form.

Once in the freezing range, gamma iron forms until fe solid fraction is reached. The density value of this phase, ρϒ, is a 7.2 g/cc [4,5,6]. Upon reaching fe solid fraction, the eutectic reaction begins during which a regular (white) eutectic and an irregular grey eutectic grow competitively. At high freezing rates and high eutectic-freezing-front speeds the white eutectic is stable in part due to shallower carbon concentration gradients ahead of the eutectic front. At lower eutectic-front speeds the grey eutectic is stable.

A simple approach is used to account for chill formation. In a range of eutectic freezing front speeds,

(7)     $latex \displaystyle {{\nu }_{e}}\in \left[ {\frac{{\nu -}}{{{{X}_{{eut}}}}},\frac{{\nu +}}{{{{X}_{{eut}}}}}} \right]$

the amount of chill formed varies from zero to the maximum allowed for a given carbon composition. The parameters ν-=30 μ/ms, and ν+=60 μ/ms, and Xeut is the chilling susceptibility criterion, a user-defined parameter, with values in the range from 0.0 to 1.0 with the default of one. For well-inoculated iron, or for a grey eutectic with a high specific surface area, Xeut is close to zero, and no chill will form. On the other hand, if the iron is un-inoculated the default value of one should be more appropriate. The actual value of Xeut must be determined experimentally, for example, from an ASTM chill-wedge test (Fig 1.).

The term cast iron, like the term steel, identifies a large family of ferrous alloys. Cast irons are multicomponent ferrous alloys. They contain major (iron, carbon, silicon), minor (<0.01%), and often alloying (>0.01%) elements. Cast iron has higher carbon and silicon contents than steel. Because of the higher carbon content, the structure of cast iron, as opposed to that of steel, exhibits a rich carbon phase. Depending primarily in composition, cooling rate and melt treatment, cast iron can solidify according to the thermodynamically metastable Fe-Fe3C system or the stable Fe-Gr system.

The term cast iron, like the term steel, identifies a large family of ferrous alloys. Cast irons are multicomponent ferrous alloys. They contain major (iron, carbon, silicon), minor (<0.01%), and often alloying (>0.01%) elements.

Cast iron has higher carbon and silicon contents than steel. Because of the higher carbon content, the structure of cast iron, as opposed to that of steel, exhibits a rich carbon phase. Depending primarily on composition, cooling rate and melt treatment, cast iron can solidify according to the thermodynamically metastable Fe-Fe3C system or the stable Fe-Gr system.

When the metastable path is followed, the rich carbon phase in the eutectic is the iron carbide; when the stable solidification path is followed, the rich carbon phase is graphite. Referring only to the binary Fe-Fe3C or Fe-Gr system, cast iron can be defined as an iron-carbon alloy with more than 2% C. Important notice is that silicon and other alloying elements may considerably change the maximum solubility of carbon in austenite (g). Therefore, in exceptional cases, alloys with less than 2% C can solidify with a eutectic structure and therefore still belong to the family of cast iron.

The formation of stable or metastable eutectic is a function of many factors including the nucleation potential of the liquid, chemical composition, and cooling rate. The first two factors determine the graphitization potential of the iron. A high graphitization potential will result in irons with graphite as the rich carbon phase, while a low graphitization potential will result in irons with iron carbide.

The two basic types of eutectics - the stable austenite-graphite or the metastable austenite-iron carbide (Fe3C) - have wide differences in their mechanical properties, such as strength, hardness, toughness, and ductility. Therefore, the basic scope of the metallurgical processing of cast iron is to manipulate the type, amount, and morphology of the eutectic in order to achieve the desired mechanical properties.

Classification

Historically, the first classification of cast iron was based on its fracture. Two types of iron were initially recognised:

• White iron: Exhibits a white, crystalline fracture surface because fracture occurs along the iron carbide plates; it is the result of metastable solidification (Fe3C eutectic)
• Gray iron: Exhibits a gray fracture surface because fracture occurs along the graphite plates (flakes); it is the result of stable solidification (Gr eutectic).

With the advent of metallography, and as the body of knowledge pertinent to cast iron increased, other classifications based on microstructural features became possible:

• Graphite shape: Lamellar (flake) graphite (FG), spheroidal (nodular) graphite (SG), compacted (vermicular) graphite (CG), and temper graphite (TG); temper graphite results from ? solid-state reaction (malleabilization.)
• Matrix: Ferritic, pearlitic, austenitic, martensitic, bainitic (austempered).

This classification is seldom used by the floor foundryman. The most widely used terminology is the commercial one. A first division can be made in two categories:

• Common cast irons: For general-purpose applications, they are unalloyed or low alloyed
• Special cast irons: For special applications, generally high alloyed.

The correspondence between commercial and microstructural classification, as well as the final processing stage in obtaining common cast irons, is given in Fig. 2.

Special cast irons differ from the common cast irons mainly in the higher content of alloying elements (>3%), which promote microstructures having special properties for elevated-temperature applications, corrosion resistance, and wear resistance. A classification of the main types of special cast irons is shown in Fig. 1.

Fig. 1. Classification of special high - alloy cast iron

Fig.2. Basic microstructures and processing for obtaining common commercial cast irons

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