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SOLUTIONS: Cast Iron

Grey cast iron

Grey cast iron is the least expensive and most commonly used of all cast metals. The raw materials – pig iron, cast iron scrap, steel scrap, limestone and coke – are all relatively inexpensive. We estimate that approximately 90% of all cast iron is in the form of gray iron.

When using casting molds of sand the molten metal gets a slow rate of cooling. This gives most of the carbon time to collect as graphite flakes between the iron crystal borders. In small or thin-walled parts the graphite is evenly dispersed throughout the material. In large-dimension castings the material near the surface will contain comparatively small graphite flakes, as this is the part of the casting, which will cool off first. In the middle the material will cool slower and contain fewer, but larger flakes. (illustration).

Due to the presence of free graphite a fracture will have a gray appearance, which gives the alloy its name. The wide range of strengths in gray irons, from 137 MPa to more than 390 MPa, plus manufacturing economy, explains the extensive use of these irons where high resistance to dynamic stress is not a governing factor.

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Graphite flakes

General analysis of gray cast iron:

The presence of the graphite flakes promotes ready machinability, useful damping properties and resistance to wear. However, the flakes also serve as crack-indicators, and gray cast iron cannot be bent or forged to shape, neither in cold nor in red-hot condition.

Carbon 3.0–3,25% Silicon 2,0–2,4% Sulphur 0,2% maximum Phosphorous 0,2% maximum Manganese 0,6–0,7% Ferrum Rest.

Characteristics: Soft.

Less brittle than white cast iron. High machinability.

Good weldability (Unitor NICKEL-333N or NIFE-334N).

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