SILICON.
Silicon is of value to cast iron on account of its influence upon the carbon. Sili con of itself would add no beneficial quality as its effects are indirect and through the carbon. Pig iron generally contains from two-tenths of 1 per cent to 12 per cent silicon. The lower silicon grades are hard charcoal, basic, or oil bessemer irons, while the extremely high silicon irons are special softeners made to mix with other irons to increase the percentage of silicon. A low proportion of silicon gives a hard cast iron of white fracture: a high proportion gives a soft iron of grey fracture. Cast iron carries from 0.4 of 1 per cent to 1 per cent silicon for chilled work and from 1 to 3 per cent for different grades of grey iron.
Condition of Silicon in Cast The condition of silicon in cast iron has not been fully determined. Certain silicides of iron have been separated from cast iron, but their com position is so indefinite and they are so easily broken up that their condition exerts no import ant influence upon cast iron.
Effect of Silicon on the Physical Proper ties of Cast Silicon through its action on the carbon softens cast iron, reduces its shrinkage, increases its fluidity, and regulates its strength.
The effect of silicon as a softener is limited, and when too much silicon is present its soften ing action ceases and it begins to harden iron. The same is true with regard to its effect on fluidity and strength. Cast iron with 3'/ per cent silicon will retain almost no carbon in combination, consequently any silicon in excess of this amount exerts a direct effect upon the iron and renders it unfit for use as a cast product.
Action of Silicon on The im portance of the action of silicon on carbon is due to the fact that silicon reduces the solu bility of iron for carbon. An iron containing silicon will dissolve less carbon than an iron containing none. At a given temperature, then, the more silicon an iron contains the less carbon will the iron retain in the dissolved or harden ing condition, consequently when an iron cools from a high temperature the amount of carbon that will separate out from solution is regulated the amount of silicon present. The higher the silicon the less carbon will the iron hold in solution, and the more will separate out as free carbon.