The inoculation process involves an addition of between 0.05 to 1% of a specialized FeSi alloy containing controlled amounts of one or more carefully selected elements to further refine the graphite morphology. Today, higher amounts of lower carbon steel can be accommodated by adding a ferrosilicon (FeSi) inoculants. With the switch to inductive melting, foundries carefully charge theire induction ladles with carefully weighed amounts of scrap steel, scrap iron and the more expensive pig iron. However, as the iron foundry industry transitions from cupola to inductive melting with similar high additions of steel scrap in the charge, alternative ladle, in-stream or in-mold additions to achieve Type A flakes in Gray iron are required. Carbon raising additions depend heavily on the melting method (cupola melting with coke will elevate carbon), the amount of silicon used and the availability of low-cost graphite. Since there is very little carbon in the scrap steel charge materials, the metallurgist needs to take into account all of the metallic charge materials (steel, scrap iron, pig iron) in the main furnace. With scrap steel being a large part of the melt charge for an iron foundry, carbon usually has to be added at some point in the process, either in the main charge or after the iron is in the molten state. Carbon & SiliconĬompared to common steel grades, the carbon content in gray iron is about ten times higher. Using the CE calculation and confirming the appropriate amount of Ferro-silicate additions, a wedge block is used to confirm the gray iron is at the desired CE level. When the objective is to cast the iron at the eutectic to prevent iron carbides from forming, it is possible to use ladle additions of Silicon to modify the molten alloy so that a eutectic equivalent is always achieved. The most effective elements for Gray Iron are Carbon and Silicon. The tensile strength decreases rapidly with the increasing Carbon Equivalent. The strength and hardness of gray iron is varied by at least four main factors: In addition, each automotive OEM has their own set of standard to govern gray iron properties and specific test methods required to measure the material quality. The Society of Automotive Engineers (SAE) published a material standard for Cast Iron, SAE J431, to account for both tensile and hardness properties. Another way of specifying gray iron is by the hardness of the material. With the YTS and UTS being so close (less than 1% elongtion), gray iron is classified as a “brittle material”. Because gray iron has very low ductility, yield strength and percent elongation, these properties are rarely measured or specified. For example, a class 30 gray iron, which was typical for an engine block, has a nominal tensile strength of 30,000 psi (207 MPa). Gray iron is specified by a class number, which corresponds to the nominal tensile strength of the alloy. This makes it very easy for cracks to propagate through the material, which inhibits strength, ductility and impact strength (fracture toughness). The graphite flakes act as tiny internal cracks which create stress intensification. Generally, gray iron has low strength and very low ductility. The five types of graphite in gray cast iron are classified b ASTM. The most common form, as described in the preceding paragraph, is is referred to as Type A. Graphite shape and the size can vary markedly due to the cooling rate and the alloy content. The form of graphite in gray cast iron is an important factor in determining the properties of the alloy. The strength of the iron is improved with finer cell sizes. Eutectic cells are somewhat analogous to grains in other metals. Each cluster of flakes defines a eutectic cell in the gray iron. The central point is the original graphite nucleus. The clustered shapes of the graphite has been compared to potato chips being glued together at a central location. In gray cast iron, the graphite solidifies as interconnected flakes, as illustrated above in the 3D microscopy with a scanning electron microscope. The name derives from the appearance of the fracture surface, which is gray. In the 1st century of the of the automobile industry, it was the material of choice for cylinder blocks, heads and many other power-train components. Next to steel, it is the most widely used engineering alloy. Gray cast iron is by far the most common of the cast irons.
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