Manufacturing industries always aim to produce metal products with advanced properties. With the development and application of several new techniques in this sector, the heat treatment method has its own place. It’s not a method that entirely alters the shape and composition of the metal; instead, it only changes the internal microstructure and chemical composition of the material’s surface.
Thus, by implementing this process, technicians can improve the intrinsic quality of their products. However, there are various heat treatment methods, each with specific benefits and applications.
Let’s delve into the article for a better understanding!
What is Heat Treatment?
As the name implies, heat treatment is the heating process in which metals or alloys are held at a particular temperature and then cooled to room temperature. As a result, the workpiece becomes hard, and its microstructure changes.
Heat treatment aims to bring out a specific material’s required chemical, mechanical, and physical properties. The changes may occur in the form of increased strength, surface hardness, flexibility, corrosion resistance, etc. However, the outcome mainly depends upon the temperature, heating period, rate of cooling, and external conditions. Normally, manufacturers decide these factors based on the treatment method and size or type of the metal part.
Usually, most manufacturing industries deal with the heat treatment of steel and some other metals for various applications.
What Steps are Involved in the Heat Treatment Process?
The heat treatment process can be performed by following three significant steps, regardless of your preferred method.
- Heat the workpiece gradually
- Hold it at a particular temperature for the required time
- Cool it to room temperature
Now, let’s discuss the significance and role of each step in detail:
The principle of this stage is to ensure the uniform heating of the metal. In the case of uneven heating, one area of the metal may expand faster than another. Ultimately, cracks may appear on the surface of the workpiece. Hence, you should heat the material slowly to aim for even heating.
Yet, here are some factors you must consider for the ideal heating rate:
- Always determine the heat conductivity of the part. If the metal has high heat conductivity, it will heat faster, whereas the one with low heat conductivity will comparatively take more time.
- Secondly, find out the condition of the workpiece. The parts already followed by hardening or stressing must be heated slower than standard parts.
- Also, measure the dimensions of the metal. In order to maintain the inside and surface temperature of the more significant parts, they should be heated slower than small components. Otherwise, the probability of excessive wrapping or cracking may increase.
Holding or soaking steps in the heat treatment technique is essential to achieve the required internal structure of the workpiece. Often, the soaking or holding period indicates the time at which a specific material has to be heated at the required temperature.
However, to find the appropriate holding time, you need to know the mass of the metal and its chemical analysis. Make sure you don’t increase the temperature directly from room temperature to the required soaking temperature. Thus, slowly preheat the metal first to obtain a high final temperature.
The cooling step involves the dropping of temperature. But, it can be done in several ways depending on the metal type and medium of cooling. The cooling mediums may exist in a gas, liquid, or solid. Whatever you follow in cooling the metal lasts a profound effect on its desired properties.
Quenching is a popular cooling technique that can be used to cool the material rapidly. Primarily, air, water, brine, oil, or other mediums play a significant role in cooling. You can associate the quenching process with hardening.
What are Common Heat Treatment Methods Used in the Manufacturing Industries?
As discussed earlier, every surface treatment method involves heating and cooling the materials. Here are some common heat treatment methods used in industries for different applications.
During annealing, the metal is heated at a critical temperature for a specific time and then cooled gradually at a slower rate. Let’s take steal annealing as an example. It is heated at a temperature of 20 to 30°C.
The primary function of annealing is to soften the materials. Therefore, in addition to making the materials perfect for cold processing, it also modifies ductility, toughness, and machinability. Moreover, you can get rid of internal stress and reduce the chances of cracking and deformation.
The applications of the annealing process include the use of hypereutectoid steel and medium and low carbon alloy steels. Copper, silver, brass, and aluminum also make use of this annealing process.
Unlike annealing, the normalizing method uses a temperature of 40°C, which is higher than the annealing temperature. You need to hold the specific material above upper critical temperature till its transformation. Once the holding period is completed, you can let it cool in the air.
As a result of air cooling, the formation of ferritic grains that are uniform and refined throughout the workpiece. Additionally, this process homogenizes carbide distribution, eliminates stress, and increases the hardness of the metal. Moreover, normalizing benefits you by providing a faster cooling rate as compared to annealing.
Applications of normalizing are limited to only carbon steel and low or medium-alloy steel. It is not at all suitable for high alloy steel as its cooling may lead to a martensitic structure.
● Stress Relieving
The stress-relieving process takes place below the critical temperature. The cooling stage is relatively slow, due to which uniform heating is offered. Typically, the stress developed in the materials as a result of forming, rolling, machining, or straightening is relieved during this heat treatment method. If the stress is left untreated, it may cause unwanted dimension changes.
The process is mainly used to deal with products, including air bottles, boiler parts, accumulators, etc. Plus, copper and brass parts can be easily heat treated by following this method.
Hardening is a method of heating material at the temperature at which the elements in the workpiece start flowing toward the solution. The solution of fine particles is prepared before starting the process. Later, the manufacturers bring the metal workpiece in the solution to provide the temperature.
Once it is heated, it should be quenched instantly to trap the metal particles within the solution. You may add impurities to it for further strengthening. Apart from that, hardening decreases metal’s ductility, making it more brittle. Further, some types of hardening include differential hardening, induction hardening, and flame hardening.
Hardening is done on various ferrous metals for different applications.
Another heat treatment method is aging which elevates the temperature of metals or alloys to medium levels and alters their properties. This process commonly deals with malleable metals to increase their yield strength. Aging leads to the formation of uniformly dispersed particles inside the grain structure of a metal. Consequently, specific characteristics of the metal workpiece change.
Aging may occur naturally at room temperature as well as artificially at higher temperatures. Typically, the process ends in increasing the strength, stability, and coercivity of the metal workpiece.
Aging is usually used for some aluminum alloys, superalloys, and non-ferrous metals. The aluminum alloys include 2000, 3000, and 7000 series.
Tempering is the opposite of the hardening process because it reduces excess hardness and brittleness resulting from the hardening of the metal. Plus, the temperature is also comparatively kept lower. Whereas the rate of cooling doesn’t affect the structure of the metal in this situation.
You can prefer this technique to stabilize the size and structure of the workpiece to ensure its accuracy. It also improves the processing performance of the material.
This method is ideal for several stress-bearing structural components, like bolts, connecting rods, gears, and shaft parts.
Working of the carburization technique is different from all other heat treatment methods as it heats the workpiece by using another material. This specific material releases carbon after decomposition, which is absorbed into the metal surface.
When the carbon content of the surface of a metal workpiece increases, it will make it harder than its inner core.
Carburization is ideal for machined low-carbon steel components. Besides, high alloy steel gears, bearings, etc., also use carburization as the heat treatment.
In a nutshell, heat treatment offers endless benefits to manufacturers. The process enables you to produce the parts with the desired properties. For instance, you can control the metal parts’ softness, hardness, ductility, stress, and malleability. Besides the physical and mechanical properties of metals, it also improves electrical and magnetic properties.
Finally, you are aware of the best possible heat treatment methods to add to your manufacturing process. But, you must have the best hands to deal with these methods to get the perfect outcome.