1. By: Engr. Hassaan Bin Younis
MECHANICAL ENGINEERING FUNDAMENTALS Course Code : ME Heat treatment processes and their effects
By: Engr. Hassaan Bin Younis

2. Definition
It is the process of heating and cooling metals in order to achieve desired physical and mechanical properties through modification of their crystalline structure.
The temperature, length of time and rate of cooling after heat treatment will have an impact on properties dramatically

3. Heat Treatment Process
Normalising, Annealing, Tempering
Softening
Hardening
Hardening, Case Hardening

4. Purpose of Softening
Purpose of hardening
To reduce strength
To increase strength
To remove residual stresses
To increase wear properties
To improve toughness
To restore ductility

5. Hot Working v/s Cold Working
The working of metals above the *recrystallization temperature is called hot working.
This temperature should not be too high to reach the solidus temperature, otherwise the metal will burn and become unsuitable for use.
Cold Working:
The working of metals below their recrystallization temperature is known as cold working.
Most of the cold working processes are performed at room temperature.
* The temperature at which the new grains are formed in the metal is known as recrystallization temperature

6. Hot Working Advantages Disadvantages
The porosity of the metal is largely eliminated.
The grain structure of the metal is refined.
The impurities like slag are squeezed into fibres and distributed throughout the metal.
The mechanical properties such as toughness, ductility, percentage elongation, percentage reduction in area, and resistance to shock and vibration are improved due to the refinement of grains.
Disadvantages
It requires expensive tools.
It produces poor surface finish, due to the rapid oxidation and scale formation on the metal surface.
Due to the poor surface finish, close tolerance cannot be maintained.

7. Normalising
Normalising is achieved by heating the metal above the critical temperature, soaking the metal until it is uniformly heated and cooling the metal in still air.
Aim: To improve, hardness, machinibality, ductility and certain electrical and mechanical properties, to improve stresses
Soaking: Once the metal has been heated upto the temperature at which the desired changes in its structure will take place, it must remain at that temperature until the entire part has been evenly heated
Critical Temperature: The critical temperature of a substance is the temperature at and above which vapor of the substance cannot be liquefied, no matter how much pressure is applied.
The critical temperature of a substance is the temperature at and above which vapor of the substance cannot be liquefied, no matter how much pressure is applied.

8. Annealing
Annealing is achieved by heating the metal to a specified temperature (Above recrystallization temperature) , soaking the metal until it is uniformly heated, and cooling it in furnace (30 to 200oC per hour).
(Equilibrium structure)

9. Annealing (Objectives)
To soften the metal so that it may be easily machined or cold worked.
to improve mechanical properties like strength and ductility.
To relieve internal stresses which may have been caused by hot or cold working or by unequal contraction in casting.
To alter electrical, magnetic or other physical properties.
To remove gases trapped in the metal during initial casting.

10. Hardening
A ferrous metal is normally hardened by heating the metal to the required temperature (Above recrystallization temperature) ,soaking and then cooling it rapidly by plunging the hot metal into a quenching medium, such as oil, water, or brine.
To increase the hardness of the metal so that it can resist wear.
To enable it to cut other metals i.e. to make it suitable for cutting tools.

11. Quenching (Sudden cooling)
Dipping the hot metal into a quenching medium(liquid), such as oil, water, or brine.
The greater the rate of quenching, the harder is the resulting structure of steel.
Non-Equilibrium structure

12. Problems in Quenching
The steel hardened by rapid quenching is very hard and brittle.
It also contains internal stresses which are severe and unequally distributed to cause cracks or even rupture of hardened steel.
SOLUTION:
“Tempering”

13. Tempering (Drawing)
Tempering consists of heating the metal to a specified temperature (below recrystallization temperature) and then permitting the metal to cool at desired rate.
To reduce brittleness of the hardened steel and thus to increase ductility.
To remove the internal stresses caused by rapid cooling of steel.
To make steel tough to resist shock and fatigue.

14. Case Hardening
During the case-hardening process, a metal is heated to a specific temperature in the presence of a material (solid, liquid, or gas) which decomposes and deposits more carbon into the surface of a metal.
when the part is cooled, the outer surface or case becomes hard, leaving the, inside of the piece soft but very tough.
This type of treatment is applied to gears, ball bearings, railway wheels, etc.
Steel Example:
Case hardening is a simple method of hardening steel. It is less complex than hardening and tempering. This techniques is used for steels with a low carbon content. Carbon is added to the outer surface of the steel, to a depth of approximately 0.03mm. One advantage of this method of hardening steel is that the inner core is left untouched and so still processes properties such as flexibility and is still relatively soft.