Heat Treatment – Engineering Notes

Heat Treatment – Engineering Notes

Hardening

Process

  • The piece is heated to a cherry red colour and then cooled rapidly in water oils and brine.
  • At the cherry red colour the steel will have undergone structural changes (FCC to BCC, martensite) and the rapid cooling does not allow it to revert to its original condition.

Result

  • Very hard and Brittle.

Tempering

Process

  • It is done by heating the article to a suitable temperature (500°C) below the reddening point of steel and then cooling it in oil or water.
  • The higher the temperature the greater the reduction in hardness and brittleness.
  • The temperature is gauged by the colour of the oxide film exhibited on the surface of the piece.

Result

  • Allows the carbon to diffuse out of BCC Structure.
  • Removes some of the hardness but increases toughness in martensite

Annealing

Process

Steel is annealed by heating it to a cherry red and allowed to soak to achieve uniform heating. It is then cooled very slowly. This can be carried out in the furnace by stitching it off when the steel has reached the correct temperature.

Result

  • Annealing is done to soften metal and relieve internal stress in work hardened pieces.
  • Refines grain size.

Normalising

Process

  • Normalising is carried out to refine the structure of steel and remove inner hardness caused by hammering, bending and rolling.
  • The steel is heated to a cherry red colour, approximately 50°C above its upper critical point and allowed to cool naturally in still air.

Result

  • Produces harder steel than annealing.
  • Improves Ductility and Toughness.
  • Removes internal stresses.
  • Refines abnormal grain structures.

Case Hardening

Process

  • Mild steel does not contain a sufficient amount of carbon to be hardened in the same way as High Carbon Steel is.
  • It is possible to increase the carbon content at the skin which can then be hardened.
  • It is a two stage process: (1) Carburising (2) Heat Treatment.

Result

Resists wear and breakage.

Pack Carburising

Process

  • The piece is placed in a box of powdered carbon.
  • The box is placed in a furnace and heated above its upper critical temperature.
  • The longer it is left, the higher the carbon content of the steel.
  • Applied to steels containing less than .2% carbon.

Result

Increases carbon content of mild steel at the surface.

Spherodising

Process

  • Piece is heated to 700°C and soaked.
  • Iron Carbide forms as spheroids.

Re-crystalisation

  • Where new crystals begin to grow from the distorted or dislocated nuclei formed during cold working.
  • The component is heated and new crystals grow until they have completely replaced the original distorted structure.

Temperature Measurement

Optical Pyrometer

  • Compares the intensity of light from the filament of a lamp.
  • The current flow is adjusted using a variable resistor to match the light of the furnace.
  • When the filament disappears a temperature reading can be taken.

Thermo-electric Pyrometer

  • Two dissimilar metals are joined together with a galvanometer placed at the cold junction.
  • A rise in temperature at the hot junction creates a current.
  • The temperature is then read at the galvanometer.

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