Stainless steel is an alloy of steel that contains at least 10-12% chromium, although it may also contain other metals such as nickel, aluminium, tungsten or molybdenum. The chromium reacts with oxygen to create a layer of chromium oxide called a passive layer. This prevents the reaction between iron and oxygen and therefore it is a steel with a high resistance to atmospheric corrosion and oxidation.
Properties of stainless steel
- It is a material that does not destroy itself.
- It is solid.
- It is a material of great durability and hardness.
- It is ductile, so it can be deformed without breaking.
- Its melting point is 1427 °C and its density depends on the type of alloy.
Types of stainless steel
It is classified in 4 families according to its chemical composition: Austenitic, Martensitic, Ferritic and Duplex.
- Austenitic Stainless Steel: Iron + Chromium + Nickel + Carbon (less than 0.10%). There are two types which are the best known of this group. On the one hand, 18/18 steel (14304) which is 18% chromium and 18% nickel. On the other hand, 18/10 steel (14301) which has 18% chromium and 10% nickel.
- Martensitic Stainless Steel: Iron + Chromium (10-12%) + Carbon (0.10%<).
- Ferritic Stainless Steel: Iron + Carbon (<0.10%) + Chromium (16-18%) + Nickel (low content).
- Duplex Stainless Steel: Iron + Chromium (18-38%) + Nickel (4.5-8%). They are constructed in two phases: ferrite and austenite.
Stainless steel undergoes a series of transformations from the time the raw materials are melted until the desired thickness is obtained. It is divided into 3 stages: steelmaking, hot rolling and cold rolling.
In this first stage, the desired chemical composition is obtained, being the one that will define the quality starting from ferroalloys, scrap and other elements.
In hot rolling, the thickness or diameter is reduced by taking advantage of the ductility of the material at high temperatures.
Finally, cold rolling is the stage in which the final thickness or diameter is obtained. This reduction is done without preheating.
The raw materials are melted in an electric arc furnace. Excess carbon is then removed. Once this has been removed, the steel is melted and cast. It is then rolled in a hot and a cold process and then goes through the annealing process. In the annealing process, the stainless steel is softened, its ductility is improved and the grain structure is refined. As it produces flakes, it is bathed in nitric-hydrofluoric acid to de-scale it. It is then cut, usually using mechanical cutting, and refined.
Recycling of stainless steel
Stainless steel is 100% recyclable and does not lose quality. Its demand has increased a lot in the last years, that is why its recycling is very important. As these alloys are similar in appearance, advanced identification technology is used. Once the material is sorted, it is packaged and sheared. The ferrous metals are then separated from other materials. Finally, the recovered materials are melted down and shaped into ingots.
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