Grey Iron

Grey cast iron is the oldest form and even today many people consider it to be the only true cast iron. The grey colouration is caused by the presence of silicone in quantities greater than 2%; at high temperatures this encourages the iron carbide the break down and the carbon comes out as nodules of graphite. That are several effects of this graphite; it acts as a lubricant which makes the finished product more suitable for machine parts, where friction could be a problem; the molten metal is more fluid, which leads to sharper, more accurate castings; and it gives a degree of corrosion resistance, after protective scales have developed.

 Thermal conductivity is increased which makes the castings more resistant to heat distortion, and the finished metal is easier to cut and shape. Finally the material has a 'damping effect which allows machinery to run with less vibration and subsequent stress. Unfortunately there are downsides too; grey iron is brittle and can shatter if over-stressed, it has a lower tensile strength and welding it can be difficult. As a result of all these factors it is usually used for machine parts where it's resistance to sliding friction is paramount, or cookware, where it's thermal properties come in useful. Possibly the main benefit of grey cast iron is it's cost - it is, in relation to most alternatives, very cheap to produce.

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