Wednesday, April 14, 2010

Vacuum Treatments of Casting Steel

Steel is largely freed from gases by a variety of casting process. Iron oxides make steel brittle during heat treatment, nitrogen leads to aging, hydrogen causes cracks and small cavities during forging.

Gas bubble remaining in steel are partially expelled only when it is hot worked later. Vacuum treatment is carried out to produce gas free steels. In this process, the gasses dissolved in the melt (much like the carbon dioxide dissolved in mineral water) are allowed to escape in a low pressure vessel.

In the vacuum casting process, the melt is poured from that casting ladle into ingot molds placed in a vacuum container. The gases are sucked out from the casting stream. In the vacuum circulation process, the molten steel is lifted from the casting ladle through a suction pipe. After degasification, the steel flows back through another pipe.

Open Poured Cast Steel (Rimming Steel)

When molten steel solidifies, carbon monoxide gas form as carbon reacts with oxygen:

C + FeO  ----> Fe + CO

The rising bubble of gas agitate the still molten steel, forcing carbon, phosphorus and silicon into the still molten interior of the ingot. The steel aggregates.

Accumulation of phosphorus in the steel give rise into brittleness (tendency to break during cold working). Accumulation of sulphur cause red-shortness (tendency to break when worked in the red-hot state). High local concentration of W, Ti or Mn in the steel give rise to hard spots. These cause indentation which can lead to the work piece breaking.

Used in strip steel, sheet steel and wire, where the quality of the work piece surface is more important than the core characteristic are.

Dead Cast Steel (Killed Steel)

If aluminum, silicon or manganese is added during casting, the oxygen is bound chemically to these metals. The metal oxides so formed cannot be reduced by carbon:

2 FeO + Si ----> 2 Fe + SiO2

Steel which has been killed while solidifying is quite uniform since segregation does not take place. High grade steel is always dead cast, since the alloying elements would otherwise be oxidised.

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