Heat Treating and Tempering of Steel

Heat treating is all about obtaining specific material properties. Often this involves increasing hardness, but other times toughness and ductility are the goal.

Quenching is a rapid cooling process that can be done using water, oil or air. It leaves the steel in a hard, brittle state, so tempering reduces this hardness to improve both toughness and ductility.

Hardening

Steel and many iron-based alloys need to be hardened as a heat treatment to increase strength, wear resistance and resistance to deformation. This is achieved by heating the material above its critical transformation temperature and then rapidly cooling it (Quenching) in a controlled way to convert the soft initial material into a much harder, stronger structure. This is a tightly controlled process, with the precise heating temperature, cooling method and cooling substance all dependent on exactly what sort of hardness you need to achieve.

The rapid cooling of the hardened steel causes the creation of a martensite structure that is much harder than the retained austenite. This gives you a much tougher and stronger metal, quenching and tempering of steel but one that will not easily bend or be cut with files. In order to get the right balance between hardness and ductility, the hardened steel is then tempered.

Tempering involves heating the quenched steel to a temperature lower than its A1 temperature, soaking it for a short period of time and then cooling it slowly again. This process is very important in achieving the correct balance between hardness and ductility, as it allows the crystal lattice to relax back into its original, softer state. The tempering process also removes internal stresses caused by the hardening and fabrication processes. The exact heating temperature and time required varies depending on the composition of your steel, but the results are always a better blend of hardness and ductility.

Strength

Strength is the resistance of a material to permanent deformation. It can be measured by tensile, compressive or shear tests. Strength is usually a combination of two properties: ductility and hardness.

To achieve high strength, a material must be sufficiently hot and then rapidly cooled. This is called quenching, and the process can be very precise. It is critical that the exact temperature and cooling method be carefully chosen to match exactly how hard a material needs to be.

Rapid cooling is achieved by immersing the metal in water, oil or another liquid. The liquid choice will depend on the alloying elements used and final mechanical properties to be achieved. Once quenched, materials are often tempered to improve their ductility and toughness.

The tempering process can be done at a variety of temperatures and for different durations. Tempering is meant to reduce the hardness of the martensite that has formed during quenching. It does so by transforming the needle or tetragonal martensite microstructure into a cubic martensite structure. This reduces the internal stresses caused by the formation of martensite, and allows the dislocations to move more easily, reducing stress cracking. In this way, a balance between hardness and toughness is achieved. The resulting tempered steel is much stronger and more durable than untreated, hardened steel. This makes it ideal for heavy duty applications that require a great deal of force to be applied.

Ductility

Often, we need to improve the strength/toughness of iron-based alloys to allow them to withstand impacts and stresses. This is where tempering comes in. It is a heat treatment that takes place after quenching, to reduce some of the hardness of the steel while increasing its ductility.

The process of tempering involves heating the material to a temperature below its critical transformation (austenitizing) temperature and then cooling it rapidly. Depending on the material, it can be cooled by air, oil or water.

Slow cooling would only restore the original state of the austenite microstructure and prevent the desired changes from taking place, so the materials need to be cooled quickly in order to create this effect.

Tempering the steel after quenching transforms the needle or tetragonal martensite into a cubic structure, reducing the hardness and improving ductility. The precipitation of spheroidal carbides is also induced during this process.

The combination of quenching and tempering allows us to produce high-performance steels with a specific combination of properties. We can then use this steel in applications like gear wheels and drills where toughness and wear resistance is important. Talk to a Clifton specialist if you need help selecting the right steel for your application. The best choice for your specific needs will depend on your environment and the requirements of your product.

Durability

The hardening process can make steel and other iron based alloys extremely tough, but it is not enough for some applications. When a high level of both strength and toughness Tinplate steel coils Manufacturer is required, the final process of tempering is performed. This step allows the dislocations that occur during the hardening process to move around more easily, preventing crack formation and improving the fatigue performance of the steel under load.

Tempering is a highly precise process in which the quenched metal is reheated at low temperatures and rapidly cooled. The cooling process is dependent on the kind of material being treated and must be carefully controlled to ensure that the resulting microstructure meets specific requirements. For example, a rapid cool down will produce a much more austenitic structure, while a slower low-temperature cooling would result in a lenticular or needle-like martensite microstructure.

The tempering process can take place in a number of stages, each with their own unique temperature and duration. In general, tempering reduces the hardness of the tempered martensite microstructure to improve the ductility and toughness of the material, while also decreasing the stress levels in the microstructure. This will prevent crack formation under load and significantly improve the fatigue performance of the finished product. This type of steel is widely used in the construction, mining and earthmoving industries for equipment such as gear wheels, cutting edges, cranes and dump truck wear liners.