Hardened and Tempered Steel Strips
Hardened and tempered steel strips are used in a wide variety of engineering applications. The process involves heating to a high temperature, rapid cooling and tempering.
To ensure a uniform tempering of strip material with a thickness that varies along its length, the invention relates to a method for continuous hardening and tempering. The method comprises dividing the tempering zone into several temperature zones and arranging the steel strip to pass through the tempering temperature-causing temperature zone last.
Tensile Strength
The tensile strength of hardened and tempered steel strip is its ability to resist bending forces without breaking. This metallurgical process is carried out by heating the metal to a high temperature until it becomes hard and then quenching it quickly to prevent cracking and distortion. The resulting martensite grain structure is very strong but also quite brittle. hardened and tempered steel strips The tempering step is then used to rebalance the mechanical properties of the material so that it will bend elastically before breaking. This process can be carried out at the same time as hardening in order to save on heat treatment costs, but it will reduce the tensile strength and toughness of the product.
A typical heat treatment line for hardened and tempered steel strip comprises a furnace in which the metal is heated to white heat, then quenched quickly. This makes the metal very hard but also extremely brittle. Tempering then rebalances the material by heating it to a lower temperature until it reaches its softening point where it can once again deform elastically before breaking. This is the main reason why hardened and tempered steel is so widely used in applications like wood cutting saws, industrial knives, sheet metal components and springs such as automobile clutch plate segments, washers and traditional clock springs. The speed at which the strip passes through the tempering zone is controlled so that it only reaches this tempering point immediately before leaving the last temperature zone.
Toughness
Hardened and tempered steel strip is the basic raw material from which various different engineering components can be made, particularly those that require high levels of both internal toughness and shock resistance. These conditions are typically achieved through the heat treatment process that involves quenching and tempering of the plain carbon steel.
The first step in the heat treatment, which is the quenching phase, produces a hard microstructure known as martensite. This is very tough but also quite brittle. Tempering, the second stage in the heat treatment, reduces this brittleness by reheating and cooling the steel strip. This allows the development of some ductility and a degree of shock resistance.
These two stages, together with the initial cold rolling, can be used to achieve a wide range of tensile strengths/hardnesses to suit specific subsequent component-manufacturing processes and end uses. This range can be extended even further by the Company through the application of edge machining which is very effective at removing the sheared edges that naturally contain a certain amount of stress and, as a result, some level of micro-cracking.
The Company’s hardening and tempering lines are designed to guarantee that the hardened and tempered strip leaves the plant with sheared edges that are free of stress, with an excellent tolerance on edge camber and flatness, and which is entirely free from what is known as ‘waviness’. These properties are important for a number of different applications such as tool manufacture and the production of springs and clockwork mechanisms.
Ductility
The ductility of metals is the ability of the material to deform under stress without failing by cracking or breaking. This is a property that is associated with the ability of metals to be pounded into a thin sheet or stretched into wire without the material fracturing. It is also the property that allows metals to be drawn into springs.
This metallurgical property can be improved by tempering which is a process that involves heating the steel to temperatures below its critical point, a temperature level at which the material fails in a brittle manner. The reheating of the strip during the tempering process allows the material to redistribute its internal stresses and to recover some of its ductility.
In the continuous production lines that are used by the Company for the manufacture of hardened and tempered steel strip, the material is typically exposed to the tempering process twice as it passes through each long continuous heat treatment furnace. This provides a more uniformly hard and ductile product than can be achieved with batch tempering.
The ductility of the strip is also enhanced by the Company’s use of a process that allows it to be edge-dressed prior to or after the tempering stage. This is an effective means of removing the sheared edges that naturally occur on strip during the slitting process and that have a high tendency to form micro-cracks in the surface. This material can then be supplied in a dressed condition which is more readily acceptable for engineering components.
Chemical Composition
We are dedicated to the production of cold rolled and alloyed steel strips on Customer’s specifications. Our manufacturing process is focused on quality & efficiency. We Annealed Steel Strip produce a wide range of grades as per International standards like BS, DIN,ISI etc.
These steels are used in a range of engineering components including springs. In the hardened and tempered condition they can offer an attractive combination of high surface hardness with good internal toughness and shock resistance. These characteristics are primarily achieved by heat treatment.
The heat treatment process for these grades involves a controlled cycle of heating, cooling and tempering. The strip is normally cooled in air and, for alloy containing grades, it can also be cooled in gaseous atmospheres, molten salt or vacuum depending on the requirement. Non-standard treatments such as faster quench rates and lower temperature tempers are not recommended. Faster quench rates increase the risk of distortion and lower temper temperatures seriously reduce mechanical properties such as toughness.
To achieve a short residence time at the tempering temperature the steel strip is preheated to a number of temperature zones before reaching the last tempering zone. In the preferred embodiment of this method the last temperature zone is matched in length to the strip passage speed so that the strip material passes through the tempering zone at the very end.