Hardened and Tempered Strip Steel Finds Wide Use in Special Industrial Applications

Hardened and tempered strip steel is the raw material from which many engineering components are made. This product is produced by continuous heat treatment lines that can produce high levels of hardness accompanied by the excellent ductility and shock resistance end-use industries require.

The hardening process involves heating the steel to a very high temperature then cooling it, rapidly, usually in water or oil. This makes the steel very hard but brittle. Tempering reheats the steel to lower temperatures and softens it slightly.

Types

Steel is an alloy of iron and carbon that is widely used in construction, manufacturing and industry. Steel can be made stronger, more durable and more versatile by heat treating it. Heat treatment is a series of processes that involve heating the metal and cooling it at a controlled rate. The three main steps are annealing, hardening, and tempering. Depending on the metal, these steps can be done separately or together.

Hardened and tempered strip steel can be supplied in gauges from 0.011mm to 2.0mm wide in a hardened and tempered strip steel variety of finishes. The normal blue-grey oxidized finish from the heat treatment is available, or in some applications a more shear friendly bright ground or satin finished product can be provided to improve machinability.

The hardening process heats the strip to temperatures that will transform it into a martensite phase. At this stage the steel is very hard but brittle. It is tempered to reduce the brittleness and to give it the ductility that allows components to resist stress in service.

The hardening and tempering processes are carried out in continuous production lines at voestalpine Precision Strip WI in Pleasant Prairie using state of the art equipment. The response to hardening and tempering depends on the steel composition, section size and method of treatment. Guidance is given in BS970 and BS EN 1 0083-1 & -2 for the mechanical properties that can be achieved with different section sizes using recommended treatment parameters.

Applications

The hardened and tempered high carbon steel strip, also known as martensite, finds wide use in various special industrial applications. The key is the combination of high tensile strength and good toughness which is achieved by the thermal processes involved. This allows the component designer to save weight and material while achieving the mechanical properties required.

Typically, hardened and tempered strip is supplied to the customer in labeled coils packed in wooden cases. Each coil is marked with the steel grade, heat, lot and coil number allowing full material traceability. Net and gross weights are also clearly marked on the case.

High-performance strip steels are used in a variety of industry sectors including automotive, aerospace, construction and defence. These products are generally required to meet a demanding range of tolerances on flatness, edge camber and hardness. Our plant is capable of achieving these requirements on strips up to 2.00mm in width.

Using state-of-the-art continuous production lines, the strip is first heated to temperatures above which it can transform into martensite. This is known as the critical transformation temperature for a given steel grade. The steel is then rapidly cooled, a process which makes the steel extremely hard but brittle. The second furnace on the line reheats the strip to a lower temperature to temper it, softening it and developing ductility.

Hardness

The hardening and tempering of medium & high carbon steel strips develops exceptional spring properties, toughness, tensile strength and flatness. The uniform hardness enables precision blanking, which in turn rationalizes further processing by eliminating post heat treatment distortion and improves productivity.

Hardening involves heating the steel to a very high temperature, then rapidly cooling it. The result is a very hard, but also brittle material. Tempering is the process of reheating the hardened steel to a lower temperature, which refines the grain structure and reduces the brittleness.

In our continuous hardening & tempering lines, the strip is heated and quenched at a controlled rate, ensuring that it reaches the desired temperature at the end of the pass. This eliminates the risk of over-hardening, as well as distortion and cracking.

Alloys, such as the high-carbon steels we use to manufacture our hardened and tempered strip, are comprised of microscopic crystals, or grains. Each grain has a particular, thermodynamically preferred repeating arrangement of atoms that is determined by the alloy’s chemical composition.

As the steel work hardens during cold rolling, it develops a specific amount of strain that is measured as the ratio between its starting thickness and its final thickness. This ratio is called the percent reduction and is used to calculate how much work hardening has taken place.

Finish

The engineering staff at voestalpine Precision Strip WI in Pleasant Priarie has a combined 60 years of experience in this special niche of the steel industry. They use state-of-the-art, continuous heat-treating furnaces from EBNER, Austria, the world’s acknowledged leader in this type of technology. The hardened and tempered strip steel that they produce is used extensively in mechanical engineering applications, particularly springs.

The treatment involves heating the steel to its critical transformation temperature for that grade of steel, holding it at that temperature, and then rapidly cooling through quenching. As a result, the steel becomes hard and very brittle. The response of the component to this treatment depends on the steel composition, section size and method of hardening. Guidance is given in BS970 and BS EN 1 0083-1 and 2 on the mechanical properties obtainable for different grades of steel with various section sizes using recommended treatment parameters.

Smaller components are generally hardened in open furnaces (in air) whereas larger components are often treated by martingering, where they are quenched at elevated temperatures in molten salt or oil. This is to reduce distortion Annealed Steel Strip that can occur when components are subjected to rapid changes in section.

It is possible to further reduce distortion by pre-machining the sheared edges of the strip before treatment. This is known as edge-dressing. It can be performed either before or after the hardening and tempering process. It is not a substitute for good design, however, to avoid stress-raising features such as sharp corners and abrupt changes in section.