Quenched and Tempered Steel Strip
Quenched and tempered steel strip has higher strength and toughness than untreated steel. This is achieved by heat treatment – quenching and tempering – that alters a steel’s microstructure.
The hard and brittle state of pure martensite cannot be used for many applications. Therefore tempering is needed: it increases ductility and toughness by decreasing the hardness.
Hardening
Some metalworking applications require a high degree of hardness. This can be seen in file blades that have to be especially wear-resistant or for gear wheels that have to withstand very high loads. This is why hardening and tempering are used in these instances.
This process involves heating the material to a temperature above its critical point and then immediately cooling it. The speed at which the steel is cooled down also affects its mechanical properties. The desired hardness can be achieved with different cooling methods — water, oil or air – or with inert gases like nitrogen. For effective hardening, the temperatures at which the material is heated and cooled down, as well as the alloys it contains, need to be carefully chosen.
The result of the hardening process is a highly strained, but extremely strong martensite microstructure. This distortion severely restricts the movement of dislocations and significantly increases the tensile strength and impact strength of the steel.
However, these higher mechanical properties come at a price: if the component is quenched to such a high level, it may develop cracks and distortions in later use. To avoid this, it is important to consider the possible effects of the heat treatment at an early stage. This can be done, for example, by selecting the appropriate open furnace treatment or by careful design of components (avoiding stress-raising features).
Tempering
After being quenched, the steel is in a hard but brittle state. It must be tempered to reduce its hardness, improve its toughness and relieve internal stresses. Tempering is accomplished by heating the steel to a temperature lower than that of the hardening process. Then, it is quickly cooled in still air. This process allows defects in the martensite crystal structure to rearrange, making the steel less brittle and more ductile.
The tempering process also improves the fatigue performance of the steel by reducing its susceptibility to stress corrosion and fracture. This is especially important Quenched and tempered steel strip in high-stress applications like shafts and axles that experience repeated loading cycles.
Tempering is a continuous process that takes place in a special heat treatment furnace, which includes a hardening Hardened & Tempered Steel Strip Supplier zone, a cooling bath and a tempering zone. The drive device guiding the steel strip through the system controls its speed such that it only reaches the tempering temperature just before leaving the last tempering zone.
This process is most often used for construction and manufacturing equipment that must withstand extreme impact or abrasion. Typical applications include drill bits, automotive components and structural materials. It’s also ideal for mining, quarrying and earthmoving equipment, such as dump truck wear liners and buckets. Contact a Clifton specialist to learn more about the benefits of using tempered steel for your project.
Heat Treatment
The tempering process reduces the brittleness of quenched metal. It allows the material to absorb stress and impacts without breaking, improving its durability and allowing it to be used in applications where impact resistance is critical. Tempering also relieves internal stresses that can cause distortion during processing and production.
In the tempering process, the strip is heated to temperatures above the GSK line drawing in the iron-carbon diagram and left there for a dwell time before it is placed into a cooling bath with a slightly lower cooling rate than the critical one. The strip passes through several temperature zones in succession and is only exposed to the tempering temperature zone immediately before it exits this zone.
Tempering reduces hardness while retaining the high tensile strength of the steel. This is essential to achieve an optimal combination of toughness and tensile strength in particular for tool steels, which need both high hardness for cutting edges to form and toughness to allow them to withstand stress from continuous use.
Tempering also increases the ductility of the steel. This means it can deform more easily without breaking, allowing for more precise and effective shaping during manufacturing processes. It also helps to reduce internal stress within the metal that can lead to premature fatigue and failure in dynamically stressed parts of equipment, such as construction and manufacturing equipment.
Applications
The tempering process can improve the hardness and strength of steel, while reducing brittleness that could develop from the quenching process. It’s a type of heat treatment that involves heating the steel to just under its critical point temperature, then cooling it again. When done properly, this allows carbon to diffuse throughout the metal’s microstructure, preventing problems with brittleness and toughness that can occur on either side of the critical point. It can also boost formability and machinability, as well as improve the resistance of the metal to internal stresses that could cause it to crack or break.
The resulting hardened and tempered steel is perfect for dynamically stressed parts in mechanical engineering, especially when it needs to be able to withstand impact. It’s used in machinery and structures like dump truck wear liners, gear wheels, earthmoving buckets, cutting edges, and construction components.
Quenching and tempering is a complex and precise process that requires expert handling to produce the desired results. The correct temperature and timing of steps is key to achieving the right balance between hardness, strength, and ductility. A leading heat treatment provider will be able to advise you on the best combination of techniques and temperatures for your specific project’s requirements. For more information, contact a specialist today.
