What is heat treatment?
“The method by which the internal structure of metals is changed and changed into mechanical properties is called heat treatment method.”
Metals are heated and cooled at different temperatures to change the mechanical properties of metals. Heat treatment of steel or metals to produce the required properties.
purpose of heat treatment
- Improvement in grain size of metals.
- To increase the hardness or strength of metals.
- To remove the internal stresses of metals due to cold working, forging, welding or casting.
- Increasing the machining of metals.
- Making round and small grains of martensite in steel.
- To improve the mechanical properties of metals.
A metal that does not have any effect on cooling or heating. That metal is wrought iron or cast iron. It is a ductile and soft metal. It has a carbon content of 0.5%. If the carbon content in it is increased to more than 0.5%. After that, when it is heated to a particular temperature and cooled rapidly, it acquires hardness property.
Steel is a metal made from a mixture of iron and carbon. In which the carbon content is at least 0.15% and maximum 1.7%. And the steel is heat treated to get the mechanical properties as per the requirement.
An important thing is that during heat treatment, the changes or changes in the internal particles of the metal can be seen only through a microscope.
types of steel structure
The structure of steel is described on the internal structure of the steel itself. which is as follows-
This steel structure is the softest. The carbon content in it ranges from 0.008% to 0.025%. And the internal structure of its or ferrite is fibrous. And the property of magnet is also found in it.
This structure is the hardest and most brittle. And the tensile strength of this structure is very less, about 350 kg/sq cm. It has a higher percentage of carbon than other structures. And carbon combines with iron in chemical form to form cementite. The chemical name of cementite is iron carbide.
3. Pearlite/Eutectoid Steel
Cementite combines with ferrite to form pearlite. As we increase the amount of carbon in iron, the amount of pyralite increases and the amount of ferrite decreases. When the carbon content is approximately 0.80% to 0.83%, the steel is 100% pearlite at this point.
Pearlite/Eutectoid Steel Types
Eutectoid steels are of the following types:
(i) Hyper-eutectoid steel
It is a mixture of cementite and pearlite. The carbon content in it varies from 1.8% to 2.1%.
(ii) hypo-eutectoid steel
It is a mixture of pearlite and ferrite. And it has a carbon content of 0.8%.
When steel is heated to a high critical temperature, austenite is obtained. It has a rigid structure, but is not brittle. The magnetism is lost in the steel of this structure.
If the rate of cooling of steel from the austenite state is increased too much, the steel does not return to its original state and changes into a new material, which is called martensite. Martensite is a hard substance, due to which the property of hardness develops in steel.
If the martensite structure is heated to 400°C, a more rigid structure is obtained, which is called truestite. this structure Wally Steel has the ability to bear vibration, but machining work cannot be done easily on it.
The structure obtained when steel is heated to about 700°C after tampering and cooling. It is called a sorbite. It contains ferrite and cementite.
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