What is inoculation?
Inoculation is a common and necessary practice used in foundries that produce gray and ductile iron castings. Often performed just prior to pouring, inoculation refers to a procedural step undertaken to improve the solidified structure of the metal, and therefore its mechanical properties. The process:
* promotes the formation of small and uniformly dispersed Type A graphite in gray iron and increases the nodule count in ductile iron. Type A graphite of the correct size provides the best mechanical properties of the iron;
* minimizes the formation of primary iron carbides (also called chill or white iron). These carbides create hard edges on iron castings that make machining difficult and are a contributing factor to tool breakage;
* reduces the non-uniform properties within a casting of varying section sizes. Thinner sections solidify at a faster rate than thicker sections. As a result, the properties (such as tensile strength) of these sections will be different. Inoculation provides more uniform properties within the casting by reducing the solidification rate in thinner sections;
* improves the tensile strength, impact strength, toughness, wear resistance and machinability of the casting.
Solidification of the molten metal takes place by a process of nucleation and growth. A nucleus is a "seed" that provides the starting point for solidification. There are natural homogeneous nucleation sites in the metal and others (heterogeneous nucleation sites) that are the result of inoculation. When molten iron is poured into a mold, heat from the metal is quickly transferred to the mold and the metal temperature falls. Solidification starts at the nucleation sites (both homogeneous and heterogeneous) present in the liquid. As the metal cools, the nucleated crystals grow to form metal cells or grains. These grains continue to grow until all the remaining liquid has solidified.
During solidification, latent heat of fusion is released as the molten metal becomes solid. The release of this heat slows the rate of solidification. When iron solidifies too quickly, the carbon atoms in the iron don't have sufficient time to collect and bond together to form the preferred structure, which is graphite. Instead, the carbon atoms combine with iron atoms to form primary iron carbides, which are hard and brittle. The graphite that does result irony not have the desired Type A shape (Fig. 1). Type B and Type D graphite may result, reducing the mechanical properties of the iron.
[FIGURE 1 OMITTED]
If the iron is inoculated before pouring, many more nucleation sites are created. When many nucleation sites are present, solidification occurs at a slower, more controlled rate throughout the casting. Therefore, the formation of primary carbides is reduced, Type A graphite is produced and more uniform properties are found in varying section sizes.
Aluminum and calcium are the two main elements used to produce nucleation sites in iron, but strontium, zirconium, titanium, barium and cerium are also used. Inoculants used for ductile iron production may also contain magnesium. These elements are not added to the metal in their pure form, but rather as alloys, primarily of ferrosilicon. A typical inoculant might contain 75% silicon, 0.60-1.25% aluminum, 0.50-1.00% calcium and the remainder as iron.
Methods of Inoculation
The iron may be inoculated by several methods:
* Ladle Inoculation--The iron is inoculated by adding the inoculant to the metal as it is transferred from the furnace or bull ladle to the pouring ladle (the method described in this article's opening paragraph). The turbulence encountered in filling the pouring ladle quickly dissolves the inoculant and evenly disperses it throughout the molten bath.
* In-stream Inoculation--In many automatic pouring operations, inoculation is done in-the-stream, or as the iron is poured from the pouring de vice into the mold.
* In-the-mold Inoculation--Inoculants may also be added as a preformed insert placed in the pouring basin of a mold or as a granulated inoculant placed in the gating system.
The inoculation effect is best directly following that addition of the inoculant to the metal. Over time, the nucleation sites created by the inoculants begin to degrade--which is known as inoculation fade. In-stream and in-the-mold inoculation techniques offer little inoculation fade, and generally require less inoculant material to provide the desired results.
Ian Kay, Cast Metals Institute, Des Plaines, Illinois
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|Title Annotation:||Casting Facts|
|Date:||Jan 1, 2004|
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