Sunday, June 28, 2020

Exhaust valve Materials and Treatments

Material Requirements.
  • The material should retain its greatest strength at high temperatures.
  • No tendencies to air harden.
  • Critical temperature above 800*C.
  • No tendency of high temperature scaling.
  • Hot and cold corrosion resistant.
  • Able to be forged and machined easily.
  • Capable of consistent and reliable heat treatment.
  • Most diesel engines use an Austenitic heat-resisting alloy steel. The seating surface can be stellited.
Typical heat treatment: 
  • Heat up to 950*C and cool in air to give a Brinnel Hardness of 269.
Surface Treatment:
  • Surface treatment is frequently used to improve or modify valve steel characteristics. Chrome-cobalt-tungsten alloy available in various grades of hardness is widely used.
  • The hardness when deposited is in the order of 375 to 425 Brinnel.
  • The valve head is treated to more than 430*C to reduce contraction stresses.
  • The value face is now sweated by an oxyacetylene flame and the alloy deposited continually by welding (1.02 mm to 1.52 mm).
Valve Seat Inserts:
  • Alloy Irons, with high percentage of molybdenum and Chromium with a Brinnel number of Approx. 500 are best.
  • Alloy steel with stellited seating surface are also in common use.
  • The methods employed for fitting the inserts include screwing and shrinking.
Valve Guides:
  • Valve guides are mostly made of Cast Iron.
  • To avoid scaling etc at high temperatures alloy Irons are preferred.
  • Phosphor Bronze and Gun metal have also been successfully used.
  • Alloy Iron guides with Bronze linings also are in common use.
Valve Housing:
  • Mostly made of pearlitic cast iron and provided with a chamber for cooling water.

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