How Hardened and Tempered Steel Strips Are Made
Hardened and tempered steel strips are heat-treated on continuous lines to develop high hardness, toughness, tensile and spring properties. This produces a product that is free of waviness and capable of meeting demanding tolerances for edge camber, flatness, etc.
During tempering the strip material is heated to temperatures above the GSK line drawing in the iron-carbon diagram, left there for a dwell time and then cooled.
Hardening
A hardened and tempered steel strip is a good choice for applications that require high mechanical properties. Quenched and tempered steel plate Its excellent spring properties, flatness, and toughness make it easy to work with, while its durability makes it resistant to heavy loads. It is also able to withstand the shear forces that occur during cutting, punching, and bending.
In a conventional continuous heat-treating process, the strip is heated to an austenitizing temperature before being quenched to obtain high hardness. This is a lengthy process, which reduces productivity. The use of a reducing atmosphere in the hardening furnace can help to shorten this time. However, it is important to keep the temperature uniformly distributed. A symmetrical temperature zone design can decrease transverse temperature gradients and improve the flatness of the strip.
The hardening process may be carried out open (in air or combustion products), in a protective environment (gaseous atmosphere or molten salt) if the component is required to be free from scale and decarburisation, or by martempering (quenching in an elevated temperature bath). If non-standard treatments are used, the risk of distortion increases. Faster quench rates and lower-temperature tempers can significantly reduce the toughness of the component.
The preferred method for achieving this is to use a series of temperature zones in which the strip is passed. Each of these temperature zones is matched in length to the passage speed of the strip material. This allows the strip to reach the tempering temperature as late as possible.
Annealing
Steel is annealed to remove any stress introduced by the heat treatment process. This is done by raising the temperature of the metal and allowing the atoms to move within its microstructure. This movement allows dislocations to be resolved and reduces brittleness that would otherwise occur in the resulting hardened metal. It also relieves internal stresses that may have developed during the forming and working processes.
In addition, annealing will restore some of the softening characteristics of the steel strip and this is often an important factor in end use applications such as in clutch plates, car parts, horticulture tools, hand tools and clock springs. It is particularly useful in high carbon steel strip as it allows the product to be used without losing the valuable ductility and shock resistance that these products require.
During annealing the strip is normally heated in a continuous strand anneal line and then cold rolled to a specified reduction (for example, 8%). The lowered thickness will have built up strain energy which can be released through critical grain growth when the steel is re-heated to a higher temperature. The result is a temper which, when combined with the work hardening achieved in the previous heat treatment, will give the desired mechanical properties to the finished product. It is important to understand that every alloy has a maximum section size above which full work hardening cannot be attained and so careful consideration should be given to this aspect when designing products which require this level of treatment.
Quenching
After being heated, the metal is cooled very rapidly. This is called quenching, and the process alters the structure of the steel through a function known as diffusion. The metal is cooled in oil, air, water or brine, depending on the specific grade of steel and desired qualities.
After quenching, the steel is hard but very brittle. It is tempered to reduce the brittleness. This is done by Tinplate Sheet Manufacturer heating the steel to a temperature below its critical point for a specified amount of time. The resulting martensite microstructure is tougher and less brittle than the hard, crystalline structures formed during quenching.
Tempering also improves fatigue performance. This is important because it enables the material to withstand repeated stress without failure. Strength is also improved by tempering, and it increases the ability of the metal to bend before it breaks.
Hardened and tempered spring steel strip is used in many applications, but it is particularly useful for products that are subject to high impact. This includes gear wheels, mining equipment, earthmoving buckets and dump truck wear liners. It is also a popular choice for clutch plate segments, piston rings and traditional clock springs. This product is available in a wide range of thicknesses and mechanical properties, and it can be supplied as either rolled or cold drawn.
Tempering
Tempering is the process of running a hardened and tempered steel strip through a specific tempering temperature to achieve an optimal combination of toughness and tensile strength. This is done in order to ensure that the metal will not crack or break when it is deformed under stress. For example, tools such as hammers and wrenches require good abrasion resistance, while springs need to deform elastically and not break under pressure.
In the process, the steel strip is heated to a hardening temperature and quenched. Subsequently, it is cooled and then reheated again to reach the required tempering temperature. The duration of the residence time at the tempering temperature is extremely short, and ideally the material only reaches the tempering temperature shortly before it leaves the last tempering zone.
The exact temperature for the tempering can be determined from an analysis of the material’s chemical composition, its mechanical properties and the desired end-use. However, if non-standard treatments are used, it is important to bear in mind that the faster quench rates and lower tempering temperatures will have an adverse effect on toughness and may cause cracking of the material. Non-standard treatments can also result in a high number of machine problems during processing and lead to costly downtime. It is therefore preferable to use a standard temper.
