Quenching and Tempering of Steel
Quenching is done by plunging the hot steel into a cooling medium such as oil, water or forced air. This causes the carbon atoms to be locked into their martensitic state without having time to diffuse back into austenite.
Then tempering is used to achieve greater ductility and toughness by decreasing the hardness of the material. Improvements in fatigue performance are also realised due to the rearrangement of defects in the crystal lattice during tempering.
Hardening
Steel is a strong material, but it can be made even stronger with heat treatment. The main heat treating process for steel is quenching and tempering. This involves heating the material, then cooling it rapidly. The precise details of this treatment are critical, as variations can cause distortion and damage.
The hardening process starts when the steel is heated to the appropriate temperature, which depends on the kind of iron based alloy that is being treated. It is then cooled in water, oil, or some other liquid to transform it from the soft initial material into a much harder and stronger structure. The hardened materials are then tempered, which allows them to achieve the right balance of hardness and toughness for their final applications.
The hardening process can also affect the grain structure of the steel. For example, a rapid cool down might result in more austenitic grains, while a slower cooling time could result in more bainite structures. Different grain structures can offer different advantages, so it is important to work with a specialist who can advise on the best heat treatment for your particular project. The right heat treatment can make your steel components incredibly strong, while improving their fatigue performance. This is particularly beneficial for a long, continuous component like an axle or shaft.
Brittleness
When a material is hardened it becomes extremely brittle. To counter this brittleness the steel must be tempered, which is a process that adds toughness to the metal. This heat treatment increases the ability of the metal to deform elastically before it breaks, and also makes the material more resistant to cracking.
This is accomplished by reheating the metal to a lower temperature and then cooling it rapidly, typically in oil or water. The exact temperature that is used for quench and tempered steel tempering, the amount of time that the metal is heated and cooled, and the substance that it is quenched in will be determined by exactly how hard you need the steel to be.
For example, tools such as file blades need to be hard and wear-resistant, while springs need to be able to deform elastically before they break. Therefore, the tempering process is used to precisely balance these various properties so that the metal can be used for a wide range of applications.
During the tempering process, the high-energy defects that are created during quenching are reduced and the structure of the crystal lattice is relaxed. This reduces the stress on the crystal lattice, which prevents dislocations from sliding at high loads and leads to crack formation. It also makes the needle-shaped tetragonal martensite that resulted from quenching transform into a cubic structure with precipitation of fine to ultra-fine carbides.
Toughness
Quenching creates a very hard but brittle state of steel. Tempering is done to reduce this brittleness and add some ductility to the metal, which allows it to bend rather than break. The process involves heating the steel above its critical point temperature for a short period of time, then cooling it slowly in still air.
Tempering is a highly precise and complicated process that is essential for the production of high quality iron-based alloys, such as steel. The tempering process can be used to enhance the mechanical properties of a variety of steels, but it is especially effective when applied to high-stress components such as gear wheels and shafts. It also improves fatigue performance, which is the ability of the steel to withstand repeated stress without failure.
Forgings that are tempered will often be finished by grinding, so it is important to choose the right quenching method to avoid distortion and cracking. Non-standard treatment methods, such as higher quench rates and lower temperature tempers, can seriously compromise the mechanical properties of the steel and increase the risk of warping or cracking later. CFS Forge can apply this heat-treating technique to all of our steel forged parts, including those for mining, quarrying, earthmoving and construction. Talk to one of our experts if you are interested in this process for your next project.
Durability
After being hardened, steel becomes very brittle. Tempering reduces this brittleness and improves toughness by lowering the temperature of the material. It also helps to relax the grain lattice and prevent dislocations from sliding at high loads. Tempering is a critical process for ensuring that steel has the right mechanical properties to meet specific applications.
In tempering, the metal is heated to a lower temperature and then cooled rapidly in water, oil or forced air. The cooling method and timing are highly controlled to achieve the exact desired property of the steel. The temperature, heat duration and Tinplate steel coils supplier coolant used during tempering depend on what kind of toughness and strength you require for your application.
During the tempering process, the unstable martensite crystals decompose into ferrite and stable cementite. This decomposition diffuses carbon atoms within the crystal structure and reduces internal stresses, which in turn increases ductility. The length of time the steel is tempered at this temperature is critical, as it can make or break the ductility of your steel component.
For example, a file blade requires a high level of toughness and wear resistance to resist the forces that cause it to sharpen. But, in order to retain a sharp edge, it must also be able to flex and bend, so tempering is required. This process also applies to automotive parts such as crankshafts and gears, where the force of the motor is often directed at these contact points.