Hardened and Tempered Strip Steel
Hardened and tempered strip steel offers superior toughness, suitable for many applications such as saw blades and industrial knives. It also offers good spring properties.
The steel is heated to the hardening temperature, quenched and then reheated during tempering to temperatures that are lower than the hardening temperature. This is designed to give the steel a degree of softness to allow it to absorb energy and avoid damage.
Hardening
The first step in transforming cold rolled strip steel into high-strength spring steel is the hardening process. Hardening is done by heating the steel to temperatures above the GSK line drawing in the iron-carbon diagram and then cooling it. This heat treatment can be done by either air or gas quenching or – in the case of some grades — by marquenching in molten salt or hot oil.
It takes a lot of heat to harden steel, and the more the piece is, the more it needs. That’s why many manufacturers rely on a magnetic test to ensure their steel is hot enough for hardening. If a magnet no longer attracts the steel, it’s ready for hardening. The hardening process is usually done in a continuous heat-treating furnace. This kind of equipment is commonly found in contract heat treating plants.
The hardening and tempering plant at voestalpine Precision Strip WI in Pleasant Priarie has state-of-the-art continuous furnaces manufactured by EBNER, the world’s leading manufacturer of this type of equipment. These furnaces are designed to accommodate a wide range of gauges and strip widths. The plant can handle items ranging from a few grams in size to components weighing several tonnes. The resulting parts have mechanical properties that are appropriate for a wide range of applications, most of them being spring-type components like automobile clutch plates, washers and clock springs.
Tempering
As hardened steel is very brittle, it must be tempered to reduce this brittleness. This is done by reheating the steel to lower temperatures and leaving it there for a time. This allows the microstructure to be transformed to various forms of tempered martensite which are much more ductile and tough than the original quenched structure. Tempering also reduces the hardness of the material, which is good for some applications.
This heat treatment is normally carried out immediately Quenched and tempered steel strip after the hardening process but can be delayed in some cases. The tempering temperature used will vary between the grades of steel used and the specific mechanical properties required for each application. This is why a good understanding of the properties and performance of each grade is essential in selecting the appropriate treatment for any given task.
The treatment is usually carried out on continuous production lines and involves heating the strip to a temperature above that at which it will become fully hardened, cooling and then tempering. This is a long process and requires careful control to minimise distortion, cracking and residual stress. In particular, the tempering furnace is matched in length to the passage speed of the strip so that it only reaches the tempering temperature at the very end of its travel. This avoids the occurrence of embrittlement, which is a significant problem that can be caused by overheating during tempering.
Heat Treatment
Heat treatment is a key process that can significantly affect the tensile strength, yield strength and elongation of strip steel. It involves heating metals to a Tinplate Sheet Manufacturer high temperature for a long time, then letting them cool in still air. The exact temperature depends on the alloy’s composition and on the desired properties of the finished product. For example, very hard tools are often tempered at low temperatures while springs are tempered at much higher ones.
The Company’s hardening and tempering plant is designed to deliver consistent, repeatable results across a wide range of gauges, mechanical properties and coil weights. This consistency ensures that customers receive parts with uniform hardness and flatness that are easier to machinate and form into their required shapes.
In addition to ensuring consistency, the plant is also able to handle large volume orders quickly and efficiently. It can process up to 2000kg of coils and a large range of widths.
The plant can be used for both case hardening and martempering. In case hardening, preheated material is dipped into a heated cyanide bath and allowed to soak before being quenched and drained. This is a fast, effective method of case hardening but requires a very careful control of the process to avoid decarburisation (black layers) and scaling. This is a common problem with components supplied from multiple batches of material and can result in expensive rework for the customer.
Hardness
The metallurgical property referred to as hardness is a measure of the resistance to permanent deformation and tearing. It is the result of a combination of yield strength (the stress beyond which deformation becomes permanent) and work-hardening characteristics. It is not the same as tensile strength (the load required to cause a fracture) or shear strength (resistance to transverse forces, such as cutting). It is often possible to estimate these other properties from hardness tests, although they depend upon specific test data and cannot be extrapolated to materials which have not been tested.
The hardening and tempering process transforms steel strip into martensite, a microstructure which is very hard. Tempering reduces the hardness of the martensite, and the overall tensile and shear strength and toughness of the finished steel are improved.
In order to achieve the required hardness, the alloy is heated to a specified temperature and held there until the work-piece has reached the desired level of hardness. This can be done in open furnaces (“open hardening”), in a protective atmosphere (gaseous or inert), or by quenching in water, oil, molten salt or fluidised bed.
The engineering staff at voestalpine Precision Strip WI in Pleasant Priarie has a combined total of over 60 years’ experience in the special niche of the high-strength strip steel industry. Their expertise in continuous heat-treating allows them to supply hardened and tempered spring strip of the highest quality.