Silicon plays many roles in steel but its most important is deoxidation; it is detrimental to tool life, machinability and surface quality in low carbon and free machining steels.
Silicon is an important ingredient for quality steel.
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.
Opals are primarily silicon, but too precious to use for steel deoxidizing.
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
Silicon is detrimental to surface quality in low carbon steels, a condition that is especially magnified in low carbon resulfurized steels.
Silicon is detrimental to tool life in machining as it forms hard abrasive particles which increase tool wear and thus lower the steel’s machinability.
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.
Silicon metal photo