heat treatment fitter online note

INTRODUCTION
i)-The structure of steel becomes visible when a piece of the metal is broken.
ii)-The exact grain size and structure can be seen through a microscope.
iii)-Steel is classified according to its structure.
iv)-Steel is an alloy of iron and carbon. But the carbon content in steel does not exceed 1.7%.
v)- The heat treatment is a metal heating and cooling process.
vi)- In this process the metal structure and internal stress and properties will be changed.
Types of steel structure 
1)-Ferrite -
i)-It is almost pure from of an iron.
ii)- It structure is soft and ductile.
iii)- It has more magnetic properties.
iv)- Pig iron or steel with 0% carbon is FERRITE
2)-Cementite
i)- It is a chemical combination of iron and carbon it is called ‘iron carbide’ or CEMENTITE.
ii)-This alloy is very hard and brittle but it is not strong.
3)-Eutectoid/Pearlite steel
i)- It is a combination of ferrite and cementite
ii)- A 0.88% ferrite and 12% cementite is known as PEARLITE steel.
iii)- This is much stronger than ferrite or cementite
4)-hypereutectoid steel
i)- It is a combination of pearlite and cementite.
ii) It is more then o.88% of carbon of steel
5)-Hypoeutectoid steel
i)- It is a combination of pearlite and ferrite.
ii) It is less then o.88% of carbon of steel
6)- Aus-tenite
i)- If steel is heated, a change in its structure commences from 723°C. The new structure formed is called ‘AUS- TENITE.
ii)- Austenite is non-magnetic.
iii)-  it structure will have fine grains which is makes it easily machinable.
7)-Martensite
i)- If the hot steel is cooled rapidly the austenite changes into a new structure called ‘MARTENSITE.
ii)-This structure is very fine grained, very hard and magnetic.
ii)- It is extremely wear-resistant and can cut other metals.
Critical temperature
i)- When steel is heated for maxmum tempture then temperature rise at same speed upto certain point then it stops to increase further that point is called critical temperature.
ii)- It is two types Lower critical temperature and Upper critical temperature
Lowercritical temperature -The certain temperature point the steel structure start to changed is called lower critical temperature.
Upper critical temperature - The point where carbon completely dissollved in iron is called upper critical temperature. 
Types of heat treatment process
Annealing, Normalising, Hardening, Tempering .
Annealing-
i)- In this process steel is heated to a suitable temperature depending upon its carbon content.
ii)- The temperature held for sufficient time.
iii)- Then slowly cooled to room temperature.
Purpose of Annealing
i)- To obtain softness.
ii)- Improve machinability.
iii)- To increase  ductility property.
iv)- To refine the grain size.
Normalising-
i)- In this process steel is heated to a suitable temperature depending upon its carbon content.
ii)- The temperature held for sufficient time.
iii)- Then slowly cooled freely in air.
Purpose of Normalising
i)- To reduced ductility property
ii)- To relieve the internal stress due to hot working,cold working & machining.
iii)- To bring uniformity of structure.
iv)- To improve mechanical properties.
Hardening-
i)- The steel is heated above critical tempreture.
ii)- Then it cooled rapidly deep in to water.
Purpose of Hardening
i)- To save the soft steel from worn out.
ii)- To make cutting tool fit for cutting metal.
Tempering-
i)- After harding steel becomes hard and brittle property.
ii)- So to avoid this type of property reheating the hardened steel  to a temperature below 400°C.
Purpose of Tempering
i)-To relieve the internal stresses.
ii)-To regulate the hardness and toughness.
iii)-To decrease the brittleness.
iv)-To restore some ductility.
v)-To induce shock resistance.