Do you know that metals in raw form do not yield any such commercial benefit? That is why most industries do heat treatment to alter the mechanical properties of metals. Heat treatment is a very crucial process in the metallurgical industry. So let us learn more about what heat treatment is and the types of heat treatment in this article.
What is heat treatment?
Heat treatment is a heating and then cooling process using predefined methods to achieve desired mechanical properties like hardness, ductility, toughness, strength, etc. It is the combination of thermal, industrial, and metalworking processes to alter the mechanical properties and chemical properties of metals.
In simple words, heat treatment is a process of heating the metal, holding it there for some time, and then cooling it back. During the whole process, the mechanical properties get changed due to changes in microstructure.
All metallic metals have grains which are nothing but microstructures of crystals. The nature of those grains determines the behavior of the mechanical properties of a metal. Heat treatment changes that mechanical structure by controlling the rate of diffusion and rate of cooling within that microstructure.
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Heat Treatment Of Steel: Process steps
The properties of heat-treated materials vastly depend on the processes that it has to undergo. Below are those key processes of heat treatment.
The first step in the heat treatment process is heating the metal. The temperature depends on the types of metal and the technique used. Sometimes you need to heat the outer surfaces of the metal, and sometimes you need to heat the whole body. That depends on what kind of alteration you want in the mechanical structure.
Below are different furnaces that are used for heating metals in heat treatment process.
- Box type furnace
- Batch furnace
- Elevator type furnace
- Bell-type furnace
- Pit type furnace
- Salt bath furnace
- Fluidized bed furnace
During the holding process, the metal is kept at the achieved temperature for some period of time. The time required depends on the type of metals and the type of mechanical properties expected.
The holding time also depends on the part size. If the part is large it is kept in a holding state for more time than the same type of metals having a small part size.
After the holding process, cooling starts. The cooling must be done in a prescribed manner. During cooling, there are some structural changes occur. Different media such as water, oil, or forced air is used to aid in cooling. You can also use furnaces for cooling purposes as the control environments help inefficient cooling.
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Heat Treatment Techniques
Following are few common heat treatment technique used in industries.
Annealing is a heat treatment process that is used to soften the metal. In other words, annealing helps to improve ductility, machinability, and toughness. On the flip side, the hardness of metals gets reduced. Annealing does this by changing the microstructure of metals.
Annealing is done by heating the metals at the above critical temperature, hold them there for some time and then cool it at a very slow rate in the furnace itself. Annealing is usually done on ferrous and non-ferrous metals to reduce hardness after the cold working process. Annealing is also done to enhance the electric conductivity of the metal.
Types Of Annealing
There are two types of annealing process which are shown below.
- Process Annealing
- Full Annealing
Process annealing is done when metal is heated below the critical temperature, keep it for a suitable time, and then cool it slowly. This process is suitable for low carbon steel like sheet metal and wires. No phase transformation occurs during process annealing, and it’s considerably cheaper than full annealing.
Full annealing is done when metal is heated above the critical temperature. This process is suitable for low and high carbon steel. Phase transformation occurs during the full annealing process, and it is a costly operation than process annealing.
Why Annealing is done?
- To relieve internal stress
- To reduce the hardness of a metal
- To increase ductility, maintainability, and toughness of metal
- Refine the grain structure of metals
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Normalizing is also similar to annealing, but the metal is air-cooled instead of other mediums used in the furnace. The metal is heated above the critical temperature, kept in there for some time, and allow to cool down under open air. Normalizing is usually done for low carbon to high carbon steel.
Please note that normalizing does not make metals that much softer as in the case with annealing, but it allows to convert uniform grain structure, and internal stresses are also relieved. Normalizing is usually done on carbon steel.
Why normalizing is done?
- To get a uniform grain structure in carbon steel
- To relieve internal stress
- To improve the machinability and strength of carbon steel
Tampering is done on metals that are already hardened. We all know that sometimes our application needs metal to be hardened as well as tough. Tampering helps to achieve the required toughness by sacrificing the hardness. Tampering is a very common process for machine tools, knives, etc.
Tampering is usually done by heating the metal at a relatively low temperature. The temperature depends on the required mechanical properties of metals. If you want high ductility, then you need to heat it at a high temperature. But if you need low ductility, then the low temperature is sufficient.
Why Tampering is done?
- To improve ductility
- To reduce hardness
- To relieve internal stress
- To reduce brittleness
As the name suggests, hardening is used to increase the hardness of a metal. This is usually done by heating the metal above normalization temperature, keeping it at normalization temperature, and then rapidly quenching ( Cooling) it in water, oil, or brine solution.
The heat required depends on the size and the required mechanical properties of the metal. Often after hardening, tampering is done to increase the ductility and toughness of metals.
Why hardening is done?
- To increase the hardness of metals
- To improve the magnetizing properties
- To reduce ductility and toughness
- To reduce grain size
Types Of Hardening
- Case Hardening / Surface hardening
- Differential hardening
- Flame Hardening
Case Hardening / Surface Hardening
Case hardening or surface hardening is a hardening heat-treatment process. In the case of hardening, the complete metal piece is heated. But in the case of case hardening, only the outer surface is heat-treated to make it hardened. The inner metal is still soft and ductile.
Case hardening is widely used for the tool and die industry where the tool surface needed to hardened but the inner metal piece has to remain ductile.
Types Of case hardening
Nitriding is a case hardening process in which nitrogen gas is used to harden the outer surface of the metal. The metal is heated in an ammonia (NH3) atmosphere, and then it is cooled.
During the whole process, ammonia defuses into nascent hydrogen and nascent nitrogen. This nascent nitrogen diffuses on the outer layer of metal form nitrite which increases the surface hardness.
Cyaniding is a case hardening process in which the metal piece is immersed in a bath of molten sodium or potassium cyanide. After that, the metal piece is cooled into lime water so that cyanide salt is sticking to the outer surface of the metal. This cyanide salt is responsible for hardening the outer surface of the metal.
Differential hardening is kind of a hardening process in which different area of the metal piece gets a different heat-treatment process. This is a very popular hardening process for high-end cutting tools.
Inflame hardening, only a portion of the metal piece is hardened. This is different from differential hardening where the whole metal piece getting hardened by the different heat-treatment processes.
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In carburization, the hardness of the metal piece is increased by increasing the carbon content. The metal piece is heated below the melting point with high carbon materials such as charcoal. The heated metal piece then absorbs carbons to make it more hard and brittle.
Quenching is a process of cooling a metal piece quickly after it was heated. Quenching helps metals to become harder or softer depending upon whether it’s a ferrous or non-ferrous alloy. In the case of ferrous alloy, quenching helps to make it harder, but it becomes softer in the case of non-ferrous.
For quenching, the metal needs to be heated above the upper critical temperature and then cool rapidly under forced air, water, oil, nitrogen, etc., depending upon the type of alloy and the desired mechanical properties. Sometimes when you do quenching too quickly, metal forms crack due to excessive internal stress.
Advantages Of Heat Treatment
- Heat treatment assist in improving the ductility of metal in the annealing process
- Heat treatment helps in hardening metals
- Case hardening helps in hardening only the outer surface of the metal piece keeping the rest of the portion soft and ductile.
- Machinability of metals gets improved.
- Resistance to corrosion capability gets enhanced.
- Electrical and magnetic properties get improved
- Internal stresses are relieved
- The grain structure of metal get refined
Heat treatment is an essential process in the material science industry to improve metal properties for commercial purposes. It is one of the key processes that help gain the desired mechanical and chemical properties of metals.
That’s all I have in this article. If you do have any question or you want to add something in this article which I missed, you are always welcome to pen it down in the comment section.
Frequently Asked Questions ( FAQ)
What is the heat treatment process?
Heat treatment is a process of heating the metal to a certain temperature, keeping it there for some time, and then cool it down. During the whole process, grain crystallization happens, which helps alter the mechanical, chemical, and electric properties of metals.
What are the different types of heat treatment?
What are the three stages of heat treatment?
Why heat treatment is done?
Heat treatment is done to alter the mechanical, chemical, and electrical properties of metal to be suitable for industries to use.