Silicon makes up about a quarter of the earths crust. It is mined as sand, quartz, mica, talc, feldspars, vermiculite, and others; silicon is a key ingredient in glass, computer chips, and certain gemstones- rock crystal, agate, rhinestone, amethyst. Opal.
The human body contains approximately one gram of silicon, ranging from 4 ppm in blood, 17 ppm in bone, and up to 200 ppm in various tissues. Cereal grains are our primary source of dietary silicon.
Silicon is seldom found as a pure element, because it has a high affinity for oxygen. It is this ability to scavenge oxygen that makes silicon important in steelmaking.
Silicon’s primary role in steel making is as a deoxidizer. It makes steel sound, by removing oxygen bubbles from the molten steel. The percentage of silicon in the analysis was related to the type of steel, rimmed and capped steels (made by the ingot method) had no silicon intentionally added. Semi-killed steels typically contained up to 0.10% max silicon, and fully killed steels could have up to 0.60% maximum. Commercial practice in the US and Canada throughout my career was 0.15-.35 % silicon in SAE carbon and alloy steels.
In addition to deoxidiation silicon also influences the steel five different ways:
- Silicon helps increase the steel’s strength and hardness, but is less effective than manganese in these functions.
- In electrical and magnetic steels, silicon helps to promote desired crystal orientations and electrical resistivity.
- In some high temperature service steels, silicon contributes to their oxidation resistance.
- In alloy grades, silicon also increases strength (but not plasticity!) when quenched and tempered.
- Silicon also has a moderate effect on hardenability of steel.
But there are always less desireable aspects of any element in an alloy
Bottom line, on plain carbon and alloy bar steels, silicon contents of 0.10, 0.15-.35 weight percent are typical; On resulfurized , and resufurized and rephosphorized free machining steels, silicon analysis above 0.02 wt % is cause for concern, due to potential surface quality and certain tool life issues.