Different grades of steel have been developed and classified over years, based on their chemical composition and physical properties by a number of standard development organisations such as SAE (Society of Automotive Engineers) steel grades, British standards, International Organization for Standardization, German steel grades, Japan steel grades and UNS. The chemical composition of steel is a key characteristic as it determines the corrosion-resisting ability of the alloy, its physical and chemical properties as well as the microstructural balance. Steel is gaining popularity because of its versatility and it canbe recycled completely. It is used in the manufacturing of various appliances and products ranging from buildings to cookware.
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Grading by Mechanical Properties
There are various ways in which grading of steel is done, based on their both physical and chemical properties, on the carbon content present and as per the classification system used. In general, steel grading falls into categorisations based on their mechanical properties and corrosion resistance.
(a) Chemical composition of steel, such as carbon type, low alloy type and stainless steel type. (b) Manufacturing methods including open hearth, electric furnace and basic oxygen process. (c) Finishing process (“i.e.” hot rolled or cold rolled products) (d) Shape of the product (“i.e.” bar, sheet or strip) (e) Deoxidation process (“i.e.” removing elemental oxygen from the product) (f) Microstructural phase. This may include different steel grades such as ferritic steel (contains 30 per cent chromium with a crystal structure called ferritic), austenitic (consists of 0.15 per cent carbon and 16 per cent chromium), duplex (mix of austenite and ferrite) and martensitic (mixture of per cent carbon and 18 per cent chromium) (g) Heat treatment such as annealing, quenching and tempering
The most widely used grades of steel are ferritic Types 409 and 430 and the austentic Types 304 and 316.
Grading by Percentage of Carbon
Strength of the steel is determined by the percentage of carbon present, as it decides the hardness of steel. Therefore, the higher the percentage of carbon content, the harder the steel and lower its ductility. Steel grades are divided into three main categories based on their carbon content: low carbon steel grades, middle carbon steel grades and high carbon steel grades.
Grading Type by Organization
Sometimes, steel grading can vary according to different grading organisations. According to SAE standards of classification, steel grades are divided into following categories-
(a) Alloy steel grades (b) Non-alloy steel grades (c) Stainless steel grades (d) Tool steel grades (e) Steel grades for sheet and strip and (f) Steel grades for electric sheet and strip
Appropriate steel grade can be selected by closely evaluating its corrosion resisting ability in the given environment and also by monitoring its mechanical properties.
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