Quenched and Tempered Steel Strip

Quenching leaves the steel in a hard, but brittle state. To reduce the hardness and increase ductility, the strip is tempered.

The temperature pattern within the strip during the cooling process was predicted with a thermal imaging camera and compared to the actual results. It turns out that localized turbulence at the edges of the strip lead to higher cooling rates.

Hardening

Hardening is the thermal treatment that transforms a soft or weak steel into a strong and tough one. The process heats the strip material to temperatures above the critical (austenitic) temperature and quenches it in a liquid medium that suppresses the transformation of austenite into pearlite; this leaves behind an extremely hard microstructure called martensite.

The mechanical properties of hardened steel are governed by its strength, toughness and ductility. Quenched and tempered steel strip The strength of a hardened steel is related to its initial microstructure and can be expressed as yield strength (strength beyond which deformation becomes permanent), tensile strength or shear strength. Toughness, on the other hand, is a function of both the degree to which a hardened metal can be deformed before breaking and the energy required to cause deformation.

There is a limit to the maximum quenching severity that can be achieved, which depends on the steel analysis and geometry of the component, as well as the quenching medium used. Higher quench severity increases the risk of distortion and cracking during cooling due to high thermal stresses within the workpiece.

The brittle nature of the hardened steel can be reduced by tempering, which involves heating it to a lower temperature for a specified time and then cooling it in still air. This reduces the hardness of the martensite and enables the carbon atoms in the tetragonal phase to partially diffuse out, which decreases lattice distortion and increases ductility.

Tempering

Quenching and tempering are important to strengthening materials like steel and iron-based alloys, and improving their toughness. The process heats up the material in a precise manner, then rapidly cools it. This alters its microstructure, giving it specific and valuable mechanical properties. It’s used to strengthen steel products so they can meet design specifications that are often rigorously high.

The quenching and tempering processes can be done before, after or in between manufacturing steps. The heat treatment increases the strength of the material to create hardened steel, then it’s tempered to reduce its rigidity. This enables it to have more ductility, while still being very tough.

During tempering, the unstable martensite decomposes into ferrite and unstable carbides and eventually into stable cementite. The end result is a microstructure that consists of laths (strips) or plates that can appear acicular (needle-like) or lenticular (lens-shaped). Tempering also reduces the risk of cracking and distortion in the final product, as well as other mechanical properties.

This is a tightly controlled process, with the heating temperature, cooling method and cooling substance all depending on the type of steel and the desired hardness. In the continuous tempering process, the strip material is heated to the hardening temperature and then cooled with forced air that’s primarily composed of nitrogen. The duration of the strip material’s exposure to the tempering temperature varies along its length, but it always reaches it no later than just before it leaves the last temperature zone.

Strength

During tempering, the high-energy defects in the steel are rearranged to produce a more uniform microstructure. This reorganization increases the strength of the material, allowing for higher stress levels before failure. This is a key advantage in applications like shafts and axles where the load cycle is frequent and failure would be catastrophic. The process also improves the fatigue performance of the materials by increasing toughness.

The temperature and cooling time that is applied to the steel during tempering has a direct effect on its properties. The precise temperature and duration of tempering can be adjusted to achieve specific property values. For instance, tempering the steel at a lower temperature than it was quenched results in increased toughness and ductility while maintaining its hardness.

The final product is a tough, abrasion-resistant, weldable Steel with excellent fabrication qualities. It is used in numerous engineering applications, including gear wheels, dump truck wear liners and earthmoving buckets. Hardened & Tempered Steel Strip Supplier Typically, it has a medium carbon content and is hard and wear-resistant.

Durability

Quenched and tempered steel strip is very hard but also tough, which gives it excellent resistance to impact. This makes it suitable for heavy engineering applications such as gear wheels, earthmoving buckets and dump truck wear liners.

In general, alloying elements (solutes) increase the hardenability of the steel by lowering both the martensite start temperature and the temperature at which austenite transforms into ferrite and cementite. This enables higher quenching speeds to be achieved for achieving full hardening, which leads to a more uniform distribution of strength throughout the workpiece.

Tempering reduces the hardness of the martensite by transforming it into various forms of tempered martensite. The tempering process does not change the crystallographic texture or dislocation density but influences the size of the carbides, resulting in differences in bendability and impact toughness transition temperatures.

After the hardening process the steel strip is sheared and slit to varying widths and lengths, depending on the application. In most cases the strip is supplied with a blue-black (also known as blue-grey) oxidised surface from the heat treatment furnaces, but a non-oxidized and brightly polished surface is available on request, which enhances the appearance of the finished product. Edge-dressed strip is also available, which is material that has been subjected to a very effective in-line machining process before or after the heat treatment to remove sheared edges with high levels of micro-cracks and roughness.