Tuesday, May 18, 2010

Cast Iron and Malleable Cast Iron





Cast iron with lamellar graphite (GG) is virtually non-elastic. The fracture is grey, because the carbon is largely precipitated as graphite. Lamella – like veins with small radii of curvature show up in the fractures, which pit the casting and reduce the tear resistance. The chips formed in processing are brittle. The compressible strength is high, make this steel harder than other kind of steel.

Cast iron with nodular graphite (GGG)
Because small quantities of magnesium and cerium have been added to it, the graphite deposits in its structure are spherical (nodular). After heat treatment, therefore, a strength comparable to steel is obtained (up to 700 N/mm2). It is sufficiently elastic cast iron (GG). The greater strength is due to the nodular graphite which minimises pitting.

Chilled iron (GH) is produced by adding manganese and cooling the melt rapidly. As a result, the carbon throughout the cross section of the casting is deposited as iron carbide (FeC). The fractures appear white, in term of its mechanical properties, this makes chilled iron harder, stronger and more water resistant.

Malleable cast iron is an iron carbon casting material with properties similar to steel.
Un-annealed malleable iron is melted in a foundry shaft furnace and contains 2.4% to 3.4% carbon which exist in the form of iron carbide (FeC), after the iron has solidified. It is hard and relatively brittle. The cast pieces must therefore be annealed (tempered).

Decarburising annealed (“white”) malleable cast iron (GTW)





The casting are annealed at around 1000oC in an oxygen containing environment (red haematite or an oxidising gas mixture). The iron carbide breaks down into iron and carbon oxidising to CO or CO2 in the outer zones. The surface layers of the workpieces are therefore decarburised and the fractures appear white.

Non-carburising annealed cast iron (“black”) malleable cast iron (GTS)
The casting are surrounded with neutral substances (sand or gas envelope). In the annealing steel, the iron carbide breaks down into iron and flaky graphite (temper carbon). This ensures that structure is uniform throughout the cross section. The fracture appears black.

Uses: Lever arms, hubs, chain links, brake drums, parts for machines.

Cast Steel
Cast steel is a casting product from steel produced in the LD converter, the SM furnace (open-hearth furnace) or the electric furnace. It is cast in unalloyed, low alloyed and high alloyed forms.


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