Composition of Steels

• The different uses of steel demand very different properties from it; i.e. strength in a bridge, yet corrosion prevention for cutlery.
• One of the major factors leading to the different properties required is the composition of the steel.
  
  

  Element	Effect on properties of steel
  Nickel	Promotes toughness at low temperatures; reduces chances of brittle fractures. Also improves corrosion resistance of steel (up to 8% used in stainless steel).
  Niobium	Forms a stable carbide; used in stainless steels and in abrasion-resistant steels or high temperature steels.
  Nitrogen	Makes the steel less ductile; undesirable.
  Carbon	Metal from blast furnace (4% C) is very brittle. High carbon steel (1% C) is strong and resistant to hard wear. Low carbon steel (0.008% C) is weaker, but easier to mould.
  Aluminium	Forms fine precipitate of aluminium nitride as steel solidifies; limits size of crystals formed in cooling metal. This results in a steel that is easily moulded, tough and impact resistant.
  Chromium	Resistance to corrosion (12-25% in stainless steel). Also makes it water resistant, and helps steel to retain its strength at high temperatures.
  Manganese	Manganese combines with sulphur to form small crystals. This improves quality of steel; makes it easier to work with when steel is hot, increasing hardness as metal cools.
  Sulphur	Increases corrosion rate, promotes stress corrosion cracking, and increases brittleness at low temperatures (generally as much is removed as possible). Small amounts of sulphur (0.08-0.13%) make it easier to machine the metal.
  Silicon	Improve hardness and strength of the alloy.
  Phosphorous	Small amounts reduce quality of steel (reducing ductility). Most steels have less the 0.04%, but some other steels, where cracking would be a serious problem, have as little as 0.005% phosphorous.
  Hydrogen	Hydrogen in steel can migrate to internal defects, where is forms hydrogen gas and builds up high pressures; causes flaking of steel and ever fracturing.
  Vanadium	Promotes small crystals in the steel and thus increases its strength. It is expensive to use and so is only added to steels that will experience severe service (tools, military vehicles, and rails etc).
  Lead	Improves the machinability of the steel.
  Molybdenum	Hardens the metal; retards the softening of steel at high temperatures and improves corrosion resistance of stainless steels.
  Tellurium	Improves machinability of steel.
  Tungsten	Forms hard carbides which have high abrasion resistance; used in high speed tools and fire-resistant construction steels.

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