Hello everyone welcome to our site Mechical, in this topic we are going to learn about What are the types of heat treatment process for steel and their application.
In which we will know that purpose of heat treatment process and the types of heat treatment process for steel like annealing, normalising, hardness, tempering in detail.
The first question is what is Heat treatment process, It is heating and cooling of metals at the controlled rate to get desired improvement in mechanical property first of all that is the purpose of its process is used to increase Mechanical properties like ductility, strength of metals, etc.
Heat treatment process consists of three steps first the metal is heated to a specific temperature and it is kept at that temperature for the specific period of time in this time the material rearranges grain structure depending upon the temperature.
And after that material is cooled at the specific rate so as to get the desired improvement in its properties.
Heat treatment is an operation involving heating and cooling of metal or alloy so as to obtain certain desirable properties.
Purpose of Heat Treatment Process for Steel
- To soften the steel for machining easily.
- To harden the steel for ensuring wears and tears resistance.
- To refine the grain structure.
- To remove internal stresses.
- To reduce the brittleness.
- To change electrical and magnetic properties.
Types of Heat Treatment Process for Steel
There are following types of heat treatment process for steel involve like
- Annealing
- Normalizing
- Hardening
- Tempering
Annealing
Annealing is the types of heat treatment process for steel to change the hard steel parts suitable for machining into soft steel.
Annealing process is mostly used for high carbon steel or alloy steel to make them soft.
It is applied to the parts, those have been either cold worked, hot worked or cast.
In this process heating the steel into an above its recrystallization temperature and then hold it that at a particular temperature which is known as soaking time after that cool the steel slowly.
It involves heating the steel to a suitable temperature depending upon the percentage of carbon in the steel which is given below.
Annealing Temperature with % of Carbon Content
Purpose of Annealing Process
- To obtain softness.
- To increase ductility.
- To reduce or eliminate structural inhomogeneity
- To improve machinability.
- To relieve internal stresses.
- To refine the grain size.
Normalizing
In this process steel heated above its critical temperature after heating then left steel part at room temperature which is slowly cooling in air.
Normalizing is done by heating the material 40 to 50 degree Celsius above its critical temperature then it is hold at the temperature around 15 minutes and then it is cold slowly normalizing is done on cold war parts to remove internal stresses and restructure the material green.
Normalizing is used to eliminate internal stresses it is also used to increase strength.
Purpose of Normalizing Process
- To increase the toughness of a metals.
- To relieve internal stresses.
- To refine the grain size.
Difference Between Annealing and Normalizing Process
As we discussed in above about annealing and normalizing process the methods of these process are almost same but only difference is that during annealing process cooling is done inside the oven or furnace that means material is taken out after complition of cooling.
While in normalizing process cooling is done outside the oven or furnace that means material is taken out from oven or furnace and then cooling is done at room temperature through atmospheric air.
Must read : Forging Process
Hardening
Hardening process is used for increasing the hardness of a steel or any type of metal.
In this process firstly heated the steel above its critical temperature when temperature reach 10 to 38 degree above critical temperature on which the structure changes after that at this particular constant temperature hold it in a particular time and then remove the steel part by quenching method after removing steel part it cooled in an oil, water or brine quickly.
Hardening is the heat treatment process of heating steel to above critical temperature and holding it to that temperature for specific time and very importantly dipping the material directly into the birth of oil water or polymer as the name implies it is used to increase hardness depending on the temperature of heating and rate of printing Mechanical properties like strength and toughness and elasticity are varied.
Hardening Temperature with % of Carbon Content
Types of Hardening Process
There are following methods used in hardening
- Case hardening
- Surface hardening
Case Hardening
It is a method used to harden the outer surface of low carbon steel while leaving the centre or case soft and ductile.
Generally, it involves heating the metal to critical its carbonaceous material.
Methods are commonly used in Case hardening are
- Carburizing
- Cyaniding
- Nitriding
Carburizing
In this method Steel are heated near its critical temperature with charcoal or Carbon around its surface at such high temperature.
And then surface of steels converted into a high carbon steel.
Cyaniding
In this method steel is immersed in bath sodium carbonate and sodium cyanide at a temperature of 800 to 900 degree Celsius.
And then it is dipped in the cold bath for quenching.
This type of case hardening process is always the uniform and we can easily get grid surface finish.
Nitriding
In Nitriding method steel is heated above 100 degree Celsius of temperature in the presence of ammonia this forms nitride on a surface, nitrates are hard in nature which forms which increase the hardness of the surface.
Surface Hardening
It is frequently desirable to harden only the surface of steels by simply changing their micro chemical compositions of the surface layer.
The following methods are commonly used for Surface hardening
- Induction Hardening
- Flame Hardening
Induction Hardening
In this method the surface of the steel component is red-heated by passing through a high frequency induction coil.
The steel component is then passed through the quenching jet.
The speed at which the coil passes the steel component, and the frequency of the current, determine the depth of the hardness.
Flame Hardening
Flame hardening method as the name suggests the surface is heated by flame in this process the surfacer required to harden is heated by oxyacetylene flame and then it is quenched.
Tempering
Tempering process always be done after hardening process because after hardening process steel become brittle which may crake.
So, after hardening, metal is usually re-heated to a suitable temperature below the lower critical point to improve its toughness and ductility but it is done at the expense of hardness and strength.
It is done in order to make the steel more suitable for service requirement.
Tempering is done to release internal stresses produced due to rapid cooling in hardening.
Uses in tempering the part is repeated to temperature less than hardening temperature and it is folded at the temperature for some time and cold slowly.
In this process temperature is judged by the colour of the oxide film produced upon heating.
Tempering Temperature with Colour
Purpose of Tempering Process
- To improve the toughness of metal.
- To improve the ductility of metal at the expense of hardness and strength.
Heat Treatment Furnace
In heat treatment processes, we are generally used two distinct types of furnaces which are given below
- Hearth Type Furnace
- Bath Type Furnace
Hearth Type Furnace
In this type of furnace the heating may be by gas, oil or electricity. The hearth furnaces for gas or oil fuels are built with heating chamber of combustion space.
Bath Type Furnace
ln this furnace, heating may be through the gas, oil or electricity.
Generally, it is used in which a large variety of salts and salt mixture are used.
e.g., Sodium, cyanide, sodium chloride, etc.
Temperature Measuring Instruments
In present time various measuring instruments are using to measure the accurate temperature substance in which one of them is pyrometer.
Pyrometers are used to measure the temperature furnace.
These are of two types which are following
- Thermometric Pyrometer
- Optical Pyrometer
Application of heat treatment Process for Steel
The application of various types of heat treatment process for steel used in manufacturing industry like automobile, aeroplanes and also used in hydraulic parts, pump parts shaft.
Various bridges, dice are also heat treated.
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