Table of Contents
Introduction of Heat Treatment Processes
The heat treatment process in simple words can be defined as the heating and cooling of the metal. In this process, the metal is heated up to a certain degree temperature & then cooled down. This ‘certain degree temperature‘ up to which a metal is supposed to be heated before cooling can vary for different processes. The ‘Heat Treatment’ process is done to change some specific property of the metal. It can either be physical or chemical properties.
For example – If you want to soften the metal or alloy through the heat treatment process, then you have to heat the metal up to 40°C (this temperature in the case of Stainless Steel). It depends on the alloy or metal which you are using. For the same process, the standard PCR is supposed to be heated at 55°C to 60°C. So, the kinds of materials we are using play an important in deciding the heating temperature in the heat treatment process.
The cooling technique also differs according to the property we want to change in the metal by the heat treatment process.
Let’s take the above-discussed example. The metal which we want to soften should be cooled down at a slow rate. While the metal (In this case stainless steel) which we want to harden should be cooled down in the air. So, the cooling method depends on what kind of heat treatment we are performing.
Types Of Heat Treatment Processes
- Annealing
- Normalizing
- Hardening
- Stress Relieving
- Tempering
- Ageing
- Quenching
A brief description of each type of heat treatment process is as follows.
1. Annealing
Annealing is the heat treatment process that is done to soften the metal. In the annealing process, the metal is heated above its recrystallization temperature but below its melting point and then cooled down at a slow rate.
This process increases the ductility of the metal while reducing hardness.
2. Normalizing
Normalizing is a heat treatment method that is used for reducing the internal stress of a metal. Internal stress might have been produced due to various sheet metal work.
In this process, the metal is heated between 20°C to 50°C which is above its critical temperature. While heating the metal at its upper critical temperature it is supposed to be kept at that temperature for a certain period and then cooled in the air.
The normalizing process is done to increase the toughness of the metal.
3. Hardening
From the name itself, we can say that this heat treatment process is done to increase the hardness of the metal.
In this process, the metal is heated at a specific temperature that depends on the metal but above the normalizing temperature. While heating, it is kept at that temperature for some time and then subjected to rapid cooling. Like submerging in water, oil, brine, etc.
4. Stress Relieving
From the name of this process, we can get an idea that this heat treatment method is used for reducing the stress of the metal.
In this method, the metal is heated below its lower critical temperature. After keeping the metal at that temperature for some time. It is cooled at a very slow rate.
The slow cooling reduced all stress produced due to any type of machining.
5. Tempering
Most of the metals are very hard and that’s why they show a high brittle property. To overcome this problem tempering is done. Tempering reduced the hardness of the metal.
In the tempering method, the metal is heated below the lower critical temperature. Then it left to cool down in the air. In this process, the heating of metal is done at a much lower temperature compared to hardening.
6. Ageing
Ageing is the process of precipitation hardening. This heat treatment method is used for preventing the metal from getting strain hardening. Ageing is done by heating the metal at a specific temperature which depends on the metal and then cooling it quickly.
7. Quenching
In the quenching method, the cooling down of metal is done at a very high rate compared to other heat treatment processes, after heating it above the critical temperature. The forced or rapid cooling in this method is faster. Generally, cooling agents are used like nitrogen, oil, water, etc.
This method also helps to increase the hardness of the metal to a certain degree.
That’s the end of my article on the heat-treatment process. I would also recommend you to read Forging Process.