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What is Stainless Steel?

Stainless steel is basically an alloy (combination of two or more chemical elements) of iron-chromium that has high resistance to corrosion, impact and abrasion, and durability.

It is 100% recyclable and has low maintenance costs. Its use in household items (such as cutlery, pans and appliances – refrigerators and stoves, for example), is well known for having a smooth, non-porous surface that makes it difficult for bacteria to accumulate.

For this same reason, it is also widely used in hospital facilities and industrial kitchens. But “stainless” steel can also be used in civil construction, industries, buses, cars and street furniture and has great visual appeal.

Its main characteristic, resistance to corrosion, is due to the presence of chromium which, when reacting with oxygen in the atmosphere, forms a surface layer that protects the steel from oxidizing agents.

This very thin and resistant protective film is made of chromium oxide and, if it is scratched or undone for any reason, it is quickly rebuilt, only for that there is oxygen.

Types and Characteristics of Stainless Steel

Stainless steels are alloys that have at least 10.5% chromium in their composition, a maximum of 30% nickel (stainless steel core), in addition to other elements such as molybdenum, titanium and niobium that can be added to their composition. structure to obtain certain characteristics.

Nickel improves the resistance of the alloy to high temperatures, its ductility and weldability, improving its resistance in general, but chromium is the main responsible for conferring its resistance to corrosion.

Stainless steel can also receive: molybdenum and copper to increase its resistance to wet corrosion; silicon or aluminum, which increases its resistance to oxidation at high temperatures; and titanium or niobium that stabilize austenitic steel by preventing the precipitation of chromium in the form of carbide during heating or slow cooling around 700 ° C.

In addition to these, other elements can also be added to the alloy: nitrogen, cobalt, boron, manganese and rare earths. (Piping system)

Stainless steels can be classified into three types according to their composition and structure:

  • Ferritics (Normative Family 430, 409 and 410S): they have between 11 and 17% chromium (Stainless steel core) and less than 0.3% carbon. They are nickel free and cheaper.

    Ferritic steels have high resistance to stress corrosion cracking and their strength can be increased by cold working. It presents easy conformation, they are magnetic and weldable with special care.

  • Martensitics (Normative Family 420): contain 12% to 18% chromium (Stainless steel core) and 1% to 1.5% carbon. They can receive tempering treatment, acquiring high levels of hardness and mechanical resistance. They are magnetic, not very weldable, and have low resistance to corrosion.
  • Austenitic (normative families 301, 304, 304L, 306 and 306L): have 17% to 25% chromium and 7% to 20% nickel (stainless steel core). They have high ductility and weldability and are the most widely used type of stainless steel for presenting better resistance to corrosion, especially if elements such as molybdenum are added or their carbon content is reduced.

    They are not magnetic and can be used to work at very low temperatures (below 0 ° C) or very high temperatures (up to 925 ° C).

Besides these main groups there are duplex stainless steel and PH. The first is a steel formed by a double ferritic matrix structure with austenite islands and which presents characteristics of high mechanical resistance and corrosion. The second is precipitation-hardenable (PH) stainless steel.

With a martensitic structure, it is magnetic iron and its hardness is increased by a different process than the martensitic ones, reaching a tensile strength of the order of 1700 MPa.

They have good ductility and toughness and their corrosion resistance is comparable to austenitic 304 steel. PH steel is widely used in the aerospace industry, while duplex has wide application in the food, chemical, petrochemical, paper and cellulose industries, among other.

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