Heat treatment is a process applied to metals to achieve certain desired properties. It is carried out by heating to a definite temperature, soaking for a specific period and cooling the metal at a prescribed rate as well as in a prescribed media. Terms pertaining to heat treatment are defined below.

Key Terms With Their Definition

  1. Eutectic.The word Eutectic is used to indicate the lowest possible freezing or melting point of any combination of two metals or elements. It corresponds to a definite proportion of two substances and has a fixed freezing point.
  2. Eutectoid.The term Eutectoid is always associated with a particular composition of the involved alloying elements that has the lowest freezing point. The eutectoid of steel contains 0.87% carbon (pearlite)
  3. Hypo-Eutectoid Steel. Steels that contain less than 0.87% carbon are known as hypo-eutectoid steels.
  4. Hyper-Eutectoid Steel.Steels that contain more than 0.87% carbon are known as hyper-eutectoid steels.
  5. Solution.Certain substances will dissolve in other substances. If two substances are mixed together upto maximum extent in such a way that their separate existence cannot be seen by microscope, it is called a “solution”.
  6. Solid Solution.A solution that possesses all the characteristics of a solution and remains in the solid state is known as “solid solution”.
  7. Saturated Solution.A solution in which no more solute can be dissolved by the solvent at room temperature is called a saturated solution.
  8. Mixture.If two substances are mixed together upto maximum extent, and if their separate existences are seen through a microscope or with the naked eye, then it is said to be a mixture.
  9. Compound.Many substances when mixed or heated together combine chemically to form compounds which may or may not have any of the properties of the constituent substances. Water (H2O) is a chemical compound of hydrogen and oxygen. Hydrogen is inflammable and oxygen supports combustion, but water extinguishes fire.
  10. Critical Points.When steel is heated, the rise in temperature is not regular. Those points at which, the temperature does not rise in spite of the heat being constant are known as critical points.
  11. In cooling down the steel undergoes the same changes but in reverse and at temperatures about 30lower than those on heating.
  12. Critical Range.The temperature range covered by the lower critical point and upper critical point for any given carbon content is called critical range or transformation range.
  13. Upper Critical Point.It is the point at which the iron and the carbon merge completely into one another by dissolving the solute carbon in the solvent iron to form a solid solution.
  14. Intermediate Critical Point.It is a point between the upper and lower critical points, sometimes referred to as the second critical point (for steels with less than 0.4% carbon). At this point the steel becomes non-magnetic.
  15. Lower Critical Point.It is the first arrest point while heating and the last arrest point while cooling.
  16. Recallescence Point.When steel is heated above its upper critical point is allowed to cool, it reaches a point, where the iron carbide falls out of solid solution to form pearlite. This point is called the “Recallescence Point”.
  17. Decalescence Point.It is that temperature at which the iron in the steel dissolves the iron carbide and forms a solid solution. At this point, the steel has an added glow.
  18. Heating.For any heat treatment, heating is carried out slowly to a specified temperature that varies according to the carbon percentage and alloying elements. The object of heating is to allow the desired changes in the steel or alloy.
  19. Soaking.Soaking means retaining the temperature for a considerable time. This is very important, as the uniform transformation of the steel is mainly due to the penetration of heat towards the core of the metal. Whenever a large solid block is heated, it is soaked not less than one hour for every square inch of the cross section.
  20. Rate of Cooling.The rate of cooling plays an important role in the heat treatment process. The hardness, toughness and softness vary according to the rate of cooling. High carbon steel that is heated to cherry red colour and cooled in still air will be soft and tough. The same metal if cooled in hot ashes will be still softer.
  21. Rapid cooling is termed as ‘quenching’. The medium that is used for quenching will also affect the properties of the quenched metal. The rate of cooling depends upon the latent heat of vaporisation of the liquid.
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