Silicon- The Great Influencer In Your Steel And In Your Machine Shop

October 19, 2010
 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:

  1. Silicon helps increase the steel’s strength and hardness, but  is less effective than manganese in these functions.
  2. In electrical and magnetic steels, silicon helps to promote desired crystal orientations and electrical resistivity.
  3. In some high temperature service steels, silicon contributes to their oxidation resistance.
  4. In  alloy grades, silicon also increases strength (but not plasticity!) when quenched and tempered.
  5. 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

    Opal

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    Killed Steel

    July 20, 2010

    Some things you want to have bubbles, some you don’t.

    Usually, Bubbles are good.

    In beermaking, yeast consumes the sugars in the wort and convert them into CO2 gas bubbles- carbonation.

    In steel making the main reaction is the combination of Carbon in the melt with Oxygen to form a gas. At the high temperatures involved, this gas is very soluble in the molten bath.

    If the Oxygen that is available for this chemical reaction isn’t completely removed before the steel is cast the gases will continue to be forced out of the melt during solidification, resulting in porosity in the steel.

    Bubbles and where the gas goes can be important in your steel part.

    In order to control the evolution of gas, chemicals called deoxidizers are added to the steel. These chemicals, Silicon or Aluminum, Vanadium, Columbium, Niobium scavenge the available oxygen in the molten steel, react chemically to form solid oxide particles dispersed throughout the steel, rather than bubbles of Carbon Dioxide.

    The amount and type of deoxidizer added determines the type of steel. If sufficent deoxidizers are added, no gas is evolved from the solidifying steel, and the steel is said to be “killed.” The ingot drawing labelled number 1 shows a fully killed (deoxidized) steel showing only a shrinkage cavity, and no bubbles or porosity. ( This shrinkage cavity would be cropped off in normal rolling practice.)

    Because gas is still evolving, this beer is NOT KILLED.

    Killed steel has more uniform chemical composition and properties than rimmed, semi-killed, or non-killed steels, and generally less segregation. The uniformity of killed steel and and its freedom from porosity makes these steels more suitable for critical components and for applications involving heat treatment.

    Killed steels generally contain 0.15-.35 weight percent Silicon as a deoxidizer, and may contain  some of the other elements as mentioned above. These other elements may be used as deoxidizers or as grain refiners.

    Steel grades with a Carbon maximum of 0.30 weight % and above, and all alloy steels are typically provided as “killed steels.”

    Free machining steels such as 12L14, 1215, and some 11XX series steels are not “killed” with Silicon, Aluminum, etc., due to their deleterious effects on tool life and machinability. The high amounts of Manganese  in these steels form Manganese Sulfides to promote machinability, and also the Manganese scavenges excess Oxygen, preventing  evolution of CO2.

    Killed steel is specified so your critical parts won't have bubbles in them.

    Killed steel- for critical parts. Non-killed beer for critical  after work down time.

    Cheers!

    Beer Bubbles Photo Credit

    Ingot scan from a handout in my files originally after Making Shaping and Treating of Steel.

     Beer Head Photo Credit

    Bread with Holes

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