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Impurities in Steel

Describes the major Impurities within Steel

Metallurgy - Impurities In Steel.

The following are impurities commonly found in steel and the impact these have on quality:

  • Found in small quantities in all Steel ( 0.1% - 0.3%). In Specia; Steels it may be as low as 0.03% or as high as 1,0%.
  • Forms a Solid Solution in Iron.
  • Slightly raises the Strength and Hardness of Steel.
  • Raises the Critical Points.
  • A de-oxidizing Agent.

  • A VERY harmful element. Efforts are made to keep it to a minimum ( About 0.015% but it may be as high as 0.6% in cheap Steel)
  • Forms Iron Sulphide which is a very brittle substance. This forms as a thin film separating the Pearlite or Ferrite grains, thus greatly reducing the Strength of the Steel ( Gives rise "Hot Short or Cold Short")
  • Manganese Sulphide is stronger than Iron Sulphide and provided that there is enough Manganese present the Sulphur will all combine with it in preference to the Iron.
  • It is common practice to add eight times th Manganese required to combine with the Sulphur.
  • Iron Sulphide melts below the working temperature of Iron.

  • Combines with Iron to form a Phosphide.
  • It increases the hardness and Tensile strength of Steel.
  • It SERIOUSLY affects the ductility and resistance to shock or impact.
  • Increases grain size.
  • 0.04% is allowable in Structural Steel. 0.035% Maximum in Tool Steel.

NOTE: Both Phosphorous and Sulphur tend to segregate. i.e.These two elements tend to concentrate in the grain boundaries during freezing and in these areas they may greatly exceed the average percentage.


  • Added to all classes of Steel to improve the machinablity of the Steel.
  • It is supposed to improve tool life,

  • A powerful and most effective de-oxidant.
  • Has a good effect on Sulphur.
  • A good element for producing perfectly sound Steel free from internal defects such as Blow Holes and gas cavities.
  • Improves the Tenacity of Steel without seriously affecting it's Ductility.
  • Increases the depth hardness of hardened Steel.
  • With between 11% and 15% manganese an about 1% Carbon Steel retains it's Austenitic condition even after quenching in water after heating to 1000 deg.C. This Steel is non- magnetic. It is also soft but posses a remarkable resistance to abrasion.

  • This metal should be avoided. It forms a low melting point brittle film round the grain boundaries making the Steel practically useless.

Gaseous Impurities
  • Oxygen, In general has a bad influence on the properties of steel. It is rarely present in quantities above 0.025%
  • Nitrogen. Found in quantities from 0.005% to 0.3% in welds, were it is undesirable. It has a hardening and em-brittling effect. Intensionally added to the surface layer in case hardening to produce an abrasive resistant surface.
  • Hydrogen. This element has a bad effect. It produces gas cavities near the skin of castings and ingots. Also "flakes" and hair line cracks can be attributed too it'd presence.

Last Modified: 11 Feb 09 @ 23:45     Page Rendered: 2022-03-14 15:42:32