Heat treatments are performed to alter the properties, chemicals and physical appearance of the steel grade. However, steel is heated due to many factors, such as the grade and the desired application.
When performed by a professional, heat treatment is a highly controlled process that allows for many benefits. For example, increased hardness, durability, corrosion and temperature resistance properties to be added to the steel.
There are countless methods and ways to heat steel; although put simply, the process involves heating, holding at that temperature and cooling until it reaches its desired state.
Why should you heat treat steel?
Performing a heat treatment on your steel product carries a range of benefits when performed correctly. The main advantage is that you’re able to alter the elements of steel, increasing or decreasing the percentage of a particular component.
The end application will always define the heat treatment process used. For example, industries such as automotive require metal to have high corrosion resistance, so they may need to undergo several different heat treatments to reach the desired state.
Other industries require increased wear resistance, improved brittleness and the ability to be compatible with other materials by increasing electrical and magnetic components.
Overall, conducting a heat treatment would be used to alter and strengthen the structure of the metal.
The process for heat treatments
You can simplify the process when defining heat treatments using layman’s terms. First, the steel is heated up, held at a specific temperature and then cooled.
When put like this, heat treatments sound simple. However, complicated chemical changes happen to the metal and its properties while this process happens.
The outcome of the steel product depends on a range of factors. Including the time spent during heating, holding and the speed at which it is cooled. Also, cooling, which we will get into shortly, can be done using different mediums (quenchants) such as oil, air and water.
Stage 1 – Heating Steel
The first stage of the process is the heating stage. During this stage, the steel should be heated to the recommended temperature set by the steel manufacturer. It’s crucial to heat the object uniformly to reach the temperature evenly and at a slow rate.
Depending on the size of the object that is being heated, a furnace may need to be used. If, when heating the element, you do not keep heat consistent throughout, you run the risk of distortion or cracking taking place within the steel.
Other factors such as the heat conductivity of the metal will determine how much and how fast you can heat. Finally, the condition will play a significant factor, as parts that have not been subject to heat in the past should be heated slower than those that have been.
Stage 2 – Holding the steel
The next step when heat treating steel involves holding the steel at the correct temperature for a period of time.
Once the steel has been heated up uniformly, you should hold it at that temperature for the recommended time set by the manufacturer.
The amount of time that you should hold the steel changes depending on the size of the steel and the tool steel grade you are treating.
When holding, you should follow the idea that the whole part should be held in two steps. First, uniformly hold at a consistent temperature just below the desired. Otherwise known as “preheating,” you’ll then be able to raise the temperature to the total final temperature without risking cracking and distortion.
Stage 3 – Cooling the steel
Once the steel has been held for the desired amount of time, the final step of the process would be to bring the steel back to room temperature.
Known as the cooling stage, there are many different ways that steel can be cooled. For example, oil, water, air, brine and gas can bring the temperature back down. Each of these mediums will have a different impact on the chemical structure and achieved hardness.
The cooling process is often referred to as quenching steel. However, quenching is still the same process of cooling the item back down using one of the mediums mentioned above.
Learn more: what is quenching?
Other methods are used when heat treating steel. However, the correct technique will depend on all the factors mentioned throughout this article. Some of the different methods used include annealing, normalising, tempering, hardening, ageing and stress relieving.
Annealing vs Tempering steel
Both annealing and tempering are methods used to treat the steel. Although they are similar, the outcome of the treated steel will be different.
Annealing is the process of heating the steel to a specific temperature. Then, proceeding to cool at a prolonged and controlled rate. When annealing, the main goal should be to restore ductility in the metal. So you can effectively use it for cold working projects.
The cooling process when you should do annealing steel at a prolonged rate. For this reason, if you are heating within a furnace, simply switching off the furnace and allowing the piece to come back to room temperature naturally will help.
The difference between annealing and tempering is that annealing should increase ductility. Whereas tempering should improve the toughness and remove excess hardness from the finished project. Furthermore, tempering steel happens after the heat-treated steel has been cooled and solid again.
Once the steel has been annealed and is in an untempered state, it is usually too brittle for end-use. Therefore, tempering will allow for excess hardness to be removed.
When tempering, you should increase the temperature of your steel product to below the lower critical point. The temperature that you’re able to raise the steel to depends on the application.
Normalising involves raising the steel temperature to 40° C above the hardening temperature and then cooling in air. The reached temperature will be higher than when annealing to help promote uniform grain size throughout the part when cooled.
Normalising aims to combat casting, forging and rolling, which causes stress on the steel.
If you’re using the finished product for an application requiring extreme impacts, it is recommended to perform normalising heat treatment.
Stress-relieving is often performed as a heat treatment technique when the product requires reduced internal stress.
The temperature should be raised below the critical temperature and then cooled uniformly.
Typically, stress-relieving will only be performed on products that have become stressed by the welding process, such as boiler tanks and pressure vessels.
Heat treatment of steel
It’s common for multiple heat treatment techniques to be used when changing the properties of the steel. Therefore, in industries such as automotive and aerospace, steel may undergo five or more of the different techniques mentioned above.
To recap on what we’ve talked about throughout this article. First, all steel must be heat-treated to be useable in a commercial setting. Finally, the treatment techniques always involve annealing, quenching and tempering.