Heat treatment methods
Heat treatment methods are used in the manufacturing industry to achieve exceptional metallic properties of a material that will undergo a wide range of stresses during its service life.
The selection of metal for an application is a crucial design step when manufacturing any product. The right selection gives us a product that will not manage but excel at service.
But rarely ever in nature can we find a product that we can just use as it is . It needs to be modified and customised to suit the application.
This is where heat treatment comes in. In this article, we explain heat treatment in the simplest way on the internet. Read on until the end to acquire a correct and long-lasting understanding of the topic.
What is heat treatment?
Heat treatment is the use of controlled heating and cooling of a material to change its mechanical properties.
We heat the material at a controlled rate in a furnace, soak it in there for a predetermined duration and then bring it down to room temperature, again at a controlled rate of cooling.
With heat treatment, we can manipulate some very important properties such as strength, ductility, hardness, machinability, etc.
Heat treatment works on pure metals, alloy steels as well as alloys of other metals. We can use
How does heat treatment work?
Heat treatment methods change the properties of metals by changing their microstructure.
The microstructure of any metal refers to the arrangement of its atoms and molecules. Different arrangements offer different properties. Some arrangements may provide greater flexibility while others may offer greater hardness.
By understanding which microstructure is available at which temperatures and how we can retain it after cooling, we can fine-tune most physical properties of metals reliably.
Types of heat treatment processes
Heat treatment is one of the oldest methods of changing a metal’s property reliably. We have been heat-treating metals for many thousands of years now.
As is expected from any science that is this old, we have achieved enviable mastery over heat treatment. Today, we can attain desired properties permanently with a very high degree of accuracy.
Some of the most common heat treatments in use are as follows:
- Hardening
- Quenching
- Annealing
- Normalising
- Tempering
- Stress relieving
- Case Hardening (Carburizing, Nitriding)
- Precipitation Hardening
Let us briefly go through each of these so we can get an overall view of the capability of heat treatment methods today.
Hardening
As the name suggests, the hardening process is used to improve a metal’s hardness. High hardness is shown by higher resistance to any change in shape. The material must hold its shape when exposed to sharp or blunt forces from other materials.
In the hardening process, the metal is heated above its critical transformation temperature and cooled relatively faster than other processes to retain a hard internal structure.
The material may be cooled in air or other media such as oil, water or polymer. The material gains hardness and a general improvement in other mechanical properties. This process may also cause the material to become more brittle.
Brittle materials are dangerous as they do not provide much flexibility and fail almost immediately after a certain threshold. This can cause catastrophic failure in many engineering applications. Thus, hardened materials usually go through further processes such as tempering to tradeoff some of the hardness for ductility.
Quenching
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The quenching process can be considered a subset of the hardening process. Quenching also improves the hardness of a material but it does so by rapidly cooling a material.
When the material undergoes rapid cooling, there is very little time for it to change its phase. Hence, it retains the microstructure that gives excellent hardness but only exists at high temperatures.
In the case of steel, this microstructure is martensite. It exists as a needle-like microstructure and provides exceptional hardness. But it is also very brittle and must be tempered for use in most applications.
Some quenching media in decreasing order of their cooling rate are brine (salt water), fresh water, polymer, oil and air. Generally, we use water or brine quenching for carbon steels whereas alloy steels are quenched in oil.
In case of air cooled quenching, the air is forced upon the material through external means.
The process of quenching can lead to the formation of internal stresses within the material. To eliminate these, further heat treatment is necessary. This heat treatment is done at much lower temperatures than the quenching temperature.
Annealing
The annealing process reduces the hardness and improves the ductility of a material by rearranging its grain structure. It is usually done after a workpiece has undergone a hardening process or work hardening because of prior processes such as welding, forging, rolling, forming and bending.
The process involves heating the metal above its upper critical temperature and encourage the formation of new grains and the realignment of the old ones. The metal is allowed to soak in the furnace and then cooled in air
Annealing a part provides benefits such as:
- Improving the machinability for future processes
- Softening of welded areas on solidification
- Eliminating internal stresses
- Better homogeneity of solid solution
- Improved wear resistance and chemical resistance
- Higher electrical conductivity
Normalizing
The normalizing process is used in the manufacturing industry similar to how an annealing process is used. Normalizing increases the ductility of the material while reducing the hardness. It may also increase the hardness depending on the initial hardness of the material used.
In the normalizing process, the material is heated above the recrystallization temperature. The final temperature is more than that in the annealing process. Once the temperature is achieved, the material is held there in the furnace for some amount of time (this is known as soaking) and then cooled in air.
In annealing, the material is allowed to cool in the furnace. However, in normalizing, the material is cooled outside the furnace. As a result, normalizing has a faster cooling rate than annealing.
In cases where the final properties are similar whether annealing or normalizing is used, normalizing can provide a more affordable alternative as it requires less furnace time.
Tempering
Tempering is one of those heat treatment processes that are very common across the industry. The tempering process reduces excessive hardness and brittleness and improves the ductility and machinability of a material.
In tempering, the material is heated to a specific temperature below the lower critical temperature where the material softens and the internal stresses are eliminated. However, the tempering temperatures are generally lower than those in annealing and normalizing as we stay below the critical point.
The heating and soaking is followed by cooling in still air. Tempering is usually done for the entire part but partial tempering in induction plants is also possible.
Stress relieving
As the name suggests, the stress relieving process is used to relieve internal stresses that may accumulate as machining operations and other processes are carried out on parts.
Stress relieving uses lower temperatures than tempering, annealing and normalizing. We stay below the lower transformation temperature. What this means is that there is no change in phase of steel.
But not just steel, we can use stress relieving for plastics and other non-ferrous metal alloys such as aluminium and copper alloys. But the temperatures are lower for non-ferrous metals and even lower for plastics.
The stress relieving temperature for steels ranges between 540 to 700 degrees Celsius and for copper alloys, it is about 500 degrees Celsius. For plastics, a temperature of about 100 degrees Celsius is sufficient.
The more time spent at the stress relieving temperature, the more effective is the process. Even metals with lower internal stresses can be improved by increasing the process duration.
At times, hardened and tempered parts may go through a stress relieving process at temperatures that are lower than tempering temperatures. Such a process relieves the internal stresses without affecting the hardness.
Case hardening
Case hardening is a special type of heat treatment process in which we only harden the outer layer of a part while the internal core retains its original properties. This gives us a combination of hard and soft material in one part. Such a part can have a unique advantage in applications that must be able to resist sudden loading in addition to having a hard surface.
Case hardening is carried out by various methods. These are as follows:
- Heating and Quenching
- Carburizing
- Nitriding
- Carbonitriding
- Ferritic nitrocarburizing
Heating and Quenching
For the first process of heating and quenching to increase the hardness, there must be sufficient carbon content in the part. If the part has low carbon content (e.g. low carbon steels), we use carburising and other methods to increase it before heating and quenching the part to obtain the desired hardness.
Since carburizing and nitriding are the two most common methods in case hardening, let us take a brief look at each of them.
Carburizing
The carburizing process refers to the addition of carbon to a metal’s surface to increase its carbon content. The increased carbon content allows it to respond much better to quenching and the subsequent creation of a martensitic structure which is high in hardness.
The process works by diffusing carbon atoms into the surface of a metal. Since the percentage of carbon is directly proportional to a steel’s hardness, the surface of the metal hardens and becomes wear resistant while the core retains properties closer to the original state.
Carburising involves heating the metal in a carbon-rich gaseous atmosphere. The carbon atoms diffuse into the surface but the hardening does not actually occur until the quenching occurs.
The amount of carbon diffusion into the surface affects the final properties. We can control the extent and depth of diffusion by changing the process parameters such as carburizing cycle time (longer for more diffusion), temperature (higher for more), and atmospheric carbon content (higher for more).
Nitriding
In nitriding, we add nitrogen to a metal’s surface instead of carbon to increase its hardness. When the metal is exposed to a nitrogen-rich atmosphere, either through gas, liquid bath, or plasma, the nitrogen diffuses into the metal.
The diffusion causes the formation of hard metallic nitrides that make the surface very hard. The hardness through nitriding can be as hard as 76 HRC.
The advantages of nitriding over carburizing are that it requires lower temperatures, less process duration, and provides greater resistance to softening at higher service temperatures.
What this means is that nitrided steel retains its hardness at temperatures of say 200 °C, whereas carburized steel will lose its hardness and become soft.
Precipitation strengthening
Precipitation strengthening is a three-step heat treatment process that can increase the strength of non-ferrous alloys. The three steps are solution heat treatment, quenching, and aging.
Solution heat treatment
Solution heat treatment is a heat treatment process that improves the strength of materials by creating a solid solution in the material.
The material is heated to a high temperature to form a single-phase solution. This temperature is above the solvus line of the iron-carbide phase diagram. Heating the material above the solvus line increases it solubility and introduced impurities are able to dissolve much better.
We hold the metal at this temperature for a set period of time to dissolve the excess phase sufficiently. This part of the process is also known as soaking.
Quenching
Once the soaking is complete, we carry out the quenching process. Quenching refers to cooling the material rapidly to room temperature. This allows it to retain the solid solution formed in step 1.
If we do not cool it fast enough it increases the likelihood of precipitation. What this means is that the impurity comes out of the single phase solution as a separate mass.
Precipitation heat treatment or Ageing
In this third step, we reheat the material to an intermediate temperature that is below the solvus line, hold it here for a certain duration and cool it back again. The cooling is done at a slower rate than quenching.
During the process, the impurities disperse in the lattice structure and increase the hardness of the material.
Ageing can be a complicated process to perform. As the precipitation heat treatment is in progress, there comes a point before the equilibrium phase where the hardness is maximum. Once we cross this point, the process is known as overaging and will actually reduce the hardness.
Generally, lower temperatures give us higher hardness but the precipitation treatment time will be very high in these cases, going up to a month or more. Higher temperatures would reduce the final hardness but allow us to reach the maximum hardness in minutes or hours.
The same goes for ductility. Precipitation hardening done at lower temperatures will require over a month to achieve the desired ductility. If we use slightly higher temperatures, we can achieve the desired ductility within hours.
The precipitation hardening process is carried out in three steps so we have a greater control over the process. If we were to quench directly to the intermediate temperature during quenching, we would have lower control because the likelihood of precipitation increases.
But when we are bringing the temperature back up from room temperature, we can select and maintain the temperature at which we want to carry out the precipitation more accurately.