Tuesday, March 23, 2010

From Pig Iron to Steel and Cast Iron

Pig iron contains up to 6% carbon (C), as well as the accompanying elements silicon (Si, 3%) and manganese (Mn up to 6%). Apart from these, it also contains traces of sulfur and phosphorus. The high content of carbon, sulfur and phosphorus make pig iron very brittle, and incapable of being forged and welded.

When the Si content is high, the carbon in pig iron is deposited as graphite during cooling. The fracture are therefore grey (grey pig iron). If the effect of manganese is predominant, the carbon combines with the iron while cooling to form iron carbide (Fe3C). this gives rise to white, radiating fracture (white pig iron).

Steel must be forgeable, weld able and also hardenable if possible. The purpose of steel production is to reduce the content of carbon and accompanying elements. The conversion of pig iron into steel is known as refining. The refining processes are, the oxygen conversion process, the Siemens-Martin process (open hearth process) and the electric process.

The different types of steel are classified, according to their use into structural steel (automobile manufacture, steel constructions, machines parts) and tool steels (cutting tools chucking and holding tools, machine tools). The steel in these groups may be alloyed or unalloyed. An alloy steel is one to which metals such as chromium, nickel, manganese or vanadium have been added to improve its properties.

Cast iron are iron casting materials with carbon contents between 2.5% and 4.5%. these materials are characterized by the fact that their melting points are lower than steel, making them suitable for casting. Casting is the most economical method of producing pieces whose shapes are complicated. Iron-carbon materials are cast iron, annealed cast iron and chilled iron. Cast steel is steel which has been cast in molds.

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