Heat Treatment Processes: Everything You Need to Know
Heat treatment is a critical aspect of material creation. High temperatures alter the microstructure of metal during fabrication, and this alteration plays an essential role in the mechanical properties of the material.
The treatment process heats a metal without letting it reach the melting point or molten stage. Afterward, the metal is cooled in a controlled manner to achieve desired mechanical properties, such as making the metal stronger, more malleable, more resistant to abrasion, or more pliable.
Heat Treatment Theory
All heat treatments involve the heating and cooling of metals, which require particular tools and circumstances to do correctly. Equipment is important because it affects the product’s properties.
For example, your heat treatment furnace must be the right size and type for proper temperature control. You must also have the appropriate gas mixture in the heating chamber and the right quenching media to cool the metal properly.
There are different types of heat treatment for different kinds of metals, which vary in the following factors:
- Heating temperatures
- Cooling rates
- Quenching types
All heat treatment methods also follow the same stages. We’ve created an infographic that will supply everything you need to know about these different heat treatment methods and their uses.
3 Stages of Heat Treatment
- Heating stage
The metal is heated to a specific temperature and cooled to harden the material in this stage. Depending on what the material needs to perform properly, heat treatment can be used at different stages in the manufacturing process. By doing so, the metal undergoes certain changes in its properties.
Uneven heating can cause one section of the metal to expand faster, resulting in cracks and deformities. To ensure that this doesn’t happen, the metal must be heated uniformly but slowly using the right heating rate. You choose the heating rate according to these factors:
- Heat conductivity
- Condition of the metal
- Size and cross-section of the metal
- Soaking stage
Also called the holding stage, the soak time of a metal refers to how long it is heated. The length of soak time plays an integral part in heat treatment, ultimately changing the metal’s physical properties. It is heated and kept at a certain temperature, then cooled to achieve the desired effect. During heating, the metal’s physical structure, also called the microstructure, changes, altering its physical properties.
To determine the correct soaking duration, you’ll need the mass of the material and its chemical analysis. You can determine the soaking period for uneven cross-sections using the largest section. Heat the metal slowly to just below the temperature where structural change happens. Then, hold that temperature until the temperature of the whole piece is consistent throughout.
- Cooling stage
After the soaking stage, the metal must be cooled in a certain manner. The cooling stage can also cause structural changes. You can use different media to control the cooling rate, such as brine, oil, water, or forced air.
Brine absorbs heat fastest, while air is the slowest. The general guidelines depending on the material are to use water to harden carbon steels, oil to harden alloy steels, and water to quench nonferrous metals.
Knowing what temperatures to cool and heat metals at and for how long is a complicated process that requires professional help and guidance. Lab furnaces are a good investment if you frequently perform heat treatment. The controlled environment allows for high precision when slow cooling is necessary.
Common Heat Treatment Methods
- Annealing
Annealing is the process of heating the metal beyond its upper critical temperature and then cooling it at a slow rate. This process softens the metal and makes it more suitable for cold working and forming. Furthermore, annealing enhances the material’s workability by increasing its toughness, reducing hardness, and improving ductility and machinability.
Metals tend to lose ductility or pliability while gaining yield strength after a certain amount of cold working. Annealing is necessary because it helps restore the metal’s original properties.
- Hardening
This is the most common heat treatment process. It is used to increase the hardness of a metal by heating it to a specific temperature and then cooling it rapidly by submerging it into a cooling medium. The cooling medium could be oil, water, or brine. After submersion, the material would have increased hardness and strength, but the brittleness would increase, too.
Hardening can help reduce the potential for cracking along the surface of a metal. Since you can reverse the process, the strength is easily measured, making it possible to choose a suitable material.
- Aging
Also called precipitation hardening, aging is a heat treatment used to increase a material’s yield strength. It produces uniformly dispersed particles in a grain’s structure, changing the material’s properties. Precipitation hardening usually comes after a heat treatment process has reached hotter temperatures. Aging only raises the temperature to medium levels and quickly brings it down.
The aging of materials may differ. Some materials may age naturally at room temperature, while others age artificially at elevated temperatures.
- Normalizing
This type of heat treatment relieves a material of its internal stresses caused by different processes, such as quenching, casting, and welding. During normalizing, the metal is heated to a temperature that’s 40°C above its upper critical temperature. The material is then cooled using air after it holds the upper critical temperature for a period.
Materials that have been normalized are harder and stronger than those annealed. In fact, steel is tougher when normalized than in any other condition.
- Stress relieving
Stress relieving is a heat treatment process wherein metal is subjected to a constant temperature below its critical temperature, followed by controlled cooling. Materials also need stress relief because drawing, forming, machining, and other procedures induce stress. These stresses, when not addressed, can cause loss of tolerance, distortion, cracking, and in-service failures.
- Tempering
This heat treatment process reduces excess hardness and relieves internal stresses. Tempering is vital to metallurgy because, without it, the workpiece becomes more prone to brittleness when there is excess hardness in a material. By doing this, a metal becomes suitable for many applications.
- Carburization
This process is one of the most common heat treatments. Carburization involves the absorption of carbon into steel or metal alloy during operation in high temperatures typically above 593°C. Carbon enters the material by diffusion and causes the metal to become embrittle and lose toughness and creep resistance.
