1-1-1 Concepts and basics of heat treatment:
The history of heat treatment of metals goes back thousands of years. Ancient civilizations such as the Egyptians and… used early heat treatment techniques to improve the properties of their tools and weapons.In the Middle Ages, blacksmiths observed the effects of heating and cooling on steel and developed more advanced techniques.
Heat treatment is a process that changes the crystal structure of iron alloys and improves its mechanical properties by heating below the melting point and controlled cooling. This process has played a vital role in the production of various products for centuries. Heat treatment is very important in various industries such as automotive, aerospace, machinery and tool and mold making.Using this process, the properties of steel can be significantly improved and stronger and more durable products can be produced.
Heat treatment has a great effect on the crystal structure of steel and can change its mechanical properties. Here are some of the results of studies in this field:
2-1-1 Effect on crystal structure:
Heat treatment can lead to changes in the crystal structure of steel.For example, with controlled heating and cooling, austenite, and ferrite and pearlite or martensite are formed.
The crystal structure determines the mechanical properties of steel. For example, martensite has high hardness, while ferrite is a soft phase.
3-1-1 Influence on mechanical properties:
Hardness, strength, flexibility and corrosion resistance are among the mechanical properties of steel.
By changing the crystal structure, these properties also change.For example, martensite has high hardness and higher tensile strength.
In general, heat treatment can improve or change the mechanical properties of steel. These changes depend on the type of steel, temperature, time and type of microstructure that is obtained after tempering.
Some of the main microstructures in steels are:
Martensite:
With controlled heating and cooling, the steel cools quickly.
The structure of martensite has a crystal structure (BCT), which increases hardness and strength.
Austenite:
The structure of steels and cast irons at high temperatures turns into austenite structure, which is the basic condition of most heat treatment processes of steels and cast irons.
The structure of austenite has a cubic structure with filled center faces (FCC) and according to the cooling speed, it turns into different phases of martensite, bainite or ferlite-pearlite.
Ferrite:
By cooling steel or cast iron at a very low speed, a ferritic structure is formed.
The ferrite structure has a filled center cubic (BCC) structure, which is the most balanced structure of steel and cast iron.
This structure is very soft and has high flexibility
2-1 Types of heat treatment:
1-2-1 Volume hardening process
Volumetric heat treatment is a process that changes the structure and properties of materials by controlled heating and cooling. In this process, parts are heated to certain temperatures and then cooled rapidly. These changes in the structure can improve the mechanical and physical properties of the parts. Some other volume heat treatment techniques are:
Annealing or complete annealing and isotherm (Annealing):
In this process, parts are heated to high temperatures and then slowly cooled.
Annealing reduces stresses, improves machinability and flexibility.
Normalizing:
In this operation, materials are heated to high temperatures and then cooled at a moderate rate.
Normalization makes the grains smaller and improves the mechanical properties.
Tempering:
After casting, welding or forging, the parts are heated to moderate temperatures and then slowly cooled.
The process reduces additional stresses, also reduces the change in the dimensions of the parts and increases the resistance to failure.
2-2-1 Induction hardening process (surface)
Induction heat treatment is a more complex process than other heat processes. In this process, the workpiece is locally heated by induction current. This induced current passes through the induction coils that surround the part and the change of the magnetic fields caused by it increases the temperature in the parts. Induction heat treatment works well on parts with magnetic properties.This process is usually used for surface induction hardening, local induction annealing or induction tempering, for this purpose, induction furnaces with the ability to control the temperature at the desired penetration depth are used.
3-2-1 Carboride and nitrocarbide process
Carbide and nitrocarbide heat treatment are two important methods in surface hardening of steel. These processes lead to changes in the chemical composition of the steel surface and improve its mechanical properties:
Carburizing:
In this process, we place low carbon steel in a carbon rich environment.
Carbon penetrates from the carbonaceous material into the surface of the part and a surface with high carbon is formed.
This hard surface, after heat treatment, is resistant to wear and pressure, while the core of the piece has high flexibility, and as a result, the piece has high toughness.
Nitrocarburizing:
In this process, we place the steel in an environment rich in nitrogen and carbon.
Nitrogen and carbon penetrate the surface of the part and a surface with a combination of nitrides and carbides is formed.
This surface is resistant to wear, corrosion and heat.
4-2-1 Nitriding or nitrification process (surface)
Nitriding or nitriding is a type of heat treatment that penetrates nitrogen on metal surfaces to create a hard shell. In this process, metal in nitrogen-rich environment up to temperature
