HEAT TREATMENT OF METAL
Heat treatment is process in which the metal is heated and cooled. This alters the properties of material significantly.
The process of heating and cooling can be controlled to obtain desired properties of material, depending on industrial need.
These are fallowing reason for which heat treatment is done:-- Improve mechanical properties of material.
- Reduce the stress from the material by heating and cooling process.
- Improve the corrosion properties.
- Magnetic properties are enhanced.
- For better machinability.
Important heat treatment processes- Annealing
- Normalising
- Hardening
- Tempering
ANNEALING
CLEAR CLASSIFICATION OF ANNEALING-CLICK HERE
The purpose of annealing is to soften the material and to relive the stresses for the inside of the material. This help to improve the machinability. The tensile stress, hardness decreases in this case.
Annealing is of four type:-
Full annealing
Heated above critical temperature and held for some time and then cooled slowly.
Incomplete annealing
Imparts machinability, heated above room temperature and cooled down slowly.
Spherodising
lowers down hardness and strength,improves machinability. Also it improves ductility.
Malleabilising
It is controlled annealing process where combined form of carbon in steel splits into carbon. Helps to improve ductility and machinability.
Normalising
It helps to restore the material to its original composition.
During the metal working and casting process the micro structure may get distorted. By this process, mechanical properties are refined and good machining characters can be achieved. It is done 80 degrees above the upper critical temperature and short holding at that temperature.
This process increases tensile strength but ductility decreases.
HARDENING:-
It is mechanical metalworking process which is employed to increase the hardness of material. Since hardness is one of the important property of material as it deals with resistance to plastic deformation. It is process accomplished by heating the metal above the critical temperature of material and then quenching it in water pool.
Hardening process in carried out by heating the steel or other material above the critical temperature and keeping it at that temperature for sometime. Then it cooled at the rate faster then normal cooling. This makes the material stronger and resistant to plastic deformation. There are various hardening process known to us. These are work hardening, age hardening, quenching and tempering.
WORK HARDENING
When material is strained beyond yield strength point then due to permanent straining new dislocation are formed. Since new dislocations are obtained, the density increases. With increase in density the strength increases.
The various types of quenching process are:-- Single quenching
- Double quenching
- Steeped quenching
- Isothermal quenching
SINGLE QUENCHING
The specimen is heated above the critical temperature and then suddenly cooled at room temperature. High degree of hardness can be obtained. Due to sudden temperature drop between the core of material and outer surface, cracks may appear.
DOUBLE QUENCHING
The specimen is heated above the room temperature and cooled in two segments. Firstly it is cooled to 320 degrees. Secondly it is cooled to room temperature in less intense medium. Cutters,millers etc are produced by this process.
STEPPED QUENCHING
It is accomplished by cooling the material in hot metal bath. Under such conditions transformation takes place.In this process cooling takes place step by step till the temperature of core and outer surface are same.
ISOTHERMAL QUENCHING
Such quenching gives better ductility to material. Here transformation takes place from austenite to bainite (intermediate form) cracking of grains is minimum here.
TEMPERING
It is simple heat treatment process where the metal is heated below the critical temperature of metal and holding it for sometime at that temperature and then cooling it at room temperature. Such process eliminates the brittle character of the metal. In other words the martensile structure (brittle form) is converted to more stable form and becomes less brittle.
Reasons why tempering is done:-- Reduce thermal stresses which gets induced while heating.
- Reducing induced strain.
- Improve mechanical properties like toughness, grain size.
- Relive the excess of hardness.
Classification of tempering process:-
LOW TEMPERATURE TEMPERING
- Heating takes place up to 160 degree to 280 degrees.
- Retains hardness while heating.
- Reduces internal stresses.
MEDIUM TEMPERATURE TEMPERING
- Heating is carried out between 310 degrees to 460 degrees.
- High toughness is induced.
HIGH TEMPERATURE TEMPERING- Temperature range is from 500 degrees to 660 degrees.
- Internal stresses are relieved.
- Toughness is enhanced.
- Brittleness is reduced.
FORGING
CHARACTERISTICS
The process of deformation of material by application of continuous or uniform force.
The material is kept in between the two dies.
Impact force is applied on the material to be forged.
On the basis of force applied forging can be classified as:-
HAND FORGING in which force is applied by hammer.
MACHINE FORGING in which force is applied by hydraulic/mechanical press.
On the basis of die used forging can be classified as:-- Open die forging
- Closed die forging
- Semi-closed die forging
OPEN DIE FORGING:-
In open die forging the as shown in above figure the position of specimen is changed continuously and force is imparted by hammer to obtain desired shape.
2. CLOSED DIE FORGING:-
Here the size and shape of material produced is similar to the die shape. The material is held in between the two dies and force is applied by hammer. Even mechanical press can be used to apply force, hence shape is obtained.
3. SEMI CLOSED DIE FORGING:-
Here force is applied in repeated manner by number of blows. The half of die provides the shape of material. By changing the position of body required shape can be obtained.
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