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Thursday, 25 February 2016

Nitriding

          The nature of the nitriding process used to obtain a case hardened is very different from that of the carburising process. Nitrogen, instead of carbon, is added to the surface of the steel. Carbon does not play any part in the nitriding operation but influence the machinability of steel. The temperature used in nitriding are much lower than those used in carburising and below the critical temperature of the steel.
          Simple carbon steels, which are often used for carburising are not used for nitriding. Steels used in the process are specially alloy steels. With the nitriding developing rather thin cases, a high core hardness is required to withstand any crushing loads. High tempering temperature call for a steel with a higher carbon content in order to develop this increase in core hardness. In addition to higher carbon content various alloying elements are called for in the steel to bring about an increase in the formation of these nitrides. Aluminium seems to display the strongest tendency in the formation of these nitrides. Chromium, molybdenum, vanadium and tungsten., all being nitrides former, are also used in nitriding steels. Nickel in nitriding steels hardens and strengthens the core and toughens the case but slight loss in its hardness.

Nitriding operation:

Advantages:
  1. Better retention of hardness at elevated temperature.
  2. Greater fatigue strength under corrosive conditions.
  3. Less warping or distortion of pans treated.
  4. High endurance limit under bending stresses.
  5. Greater resistance to wear and corrosion.
  6. Greater surface hardness.
Disadvantages:
  1. Necessity of using high alloy containers to resist the nitriding.
  2. High furnace costs due to the long lime of treatment.
  3. Necessity of using special alloy steels.
  4. Medium used is expensive.

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