Steels resistant to corrosion in aggressive environments
The unique characteristic of stainless steel is that they have a resistance to corrosion that is significantly higher with respect to other steels; some of them also have excellent resistance to high temperatures.
These characteristics of stainless steels are obtained thanks to the addition of chromium and other alloy elements and allow use even in particularly aggressive environments such as chemical plants, marine environments and offshore platforms. These qualities of resistance to assaults make them perfect materials to be also used in architecture to increase the durability of the objects created.
Some of these steels, thanks to a chemical composition that is particularly rich in nickel and chromium, are called "refractory steels" and can withstand high temperatures with a limited loss of mechanical characteristics.
Stainless steels can be provided in their natural rolling state or annealed, normalized, quenched and tempered, or heat-treated. Bars, coils, and wires are available with a wide variety of packaging.
Austenitic, non-magnetic steels, in addition to chromium – a basic element of all stainless steel – austenitic stainless steels also contain high levels of nickel, an element that significantly improves its resistance to corrosion, as well as the possible addition of other elements such as molybdenum, titanium, and niobium, present in some versions.
The austenitic structure of these steels does not allow the execution of quenching and tempering treatments, but cold drawing deformations allow very high mechanical characteristics to be obtained, especially on small diameters.
In the solubilized state, they are completely non-magnetic, while in the drawn state, they acquire slight ferromagnetism.
The versions with the addition of copper, instead, have a decidedly improved cold deformability, making them suitable, for example, for use in formed screws and, in general, for cold forming.
The steels of the Plus series undergo special processing during casting and an analytical balancing that provides them with excellent machinability in the tool in relation to this category of materials.
The addition of sulphur, finally, allows easy machinability of the material and makes it suitable for the production of turned parts with challenging machining; in contrast, the resistance to corrosion is slightly decreased.
The ferritic stainless steels, non-temperable steels, which are characterized by a ferritic structure, cannot be tempered, but the mechanical strength characteristics can, in any case, be improved thanks to cold deformation or drawing. The chromium content ensures good resistance to corrosion, further increased in the variety with the addition of molybdenum, while the addition of sulphur allows the machinability by chip removal to be improved.
The steels of this category are often used in the white (appliances) sector for the production of grills and whenever a generic resistance to corrosion is required without very accentuated characteristics.
Duplex steels are suitable for aggressive environments, and the chemical composition of this category of steels, also called biphasically, allows a hybrid structure of austenite and ferrite to be obtained with unique corrosion resistance characteristics, especially to corrosion under voltage and to marine corrosion.
Thanks to these characteristics and to the high mechanical strength, which can be obtained with hardening and cold deformation, these steels are commonly used in offshore platforms and in any case where the action of salt and corrosive agents is particularly accentuated.
They are magnetic, can generally be welded and cannot be tempered.
Therefore, these are products with a particularly high level of performance, whose development is still underway and whose applications have yet to be fully explored.
Martensitic stainless steels, as is the case of quenched and tempered steels, can be tempered. For this reason, the products can be provided in both the annealed state (a treatment that improves their machinability and deformability) and in the quenched and tempered state (which instead enhances their mechanical strength and corrosion resistance characteristics).
In order to achieve high surface hardness values, it is finally possible to subject these steels also to induction hardening after chip removal machining.
As with the other steels, the versions with the addition of sulphur lend themselves more easily to machining by chip removal.
Steels resistant to corrosion and high temperatures are generally used for the specific needs of environments that have very high temperatures, have led to the need to create stainless steels able to cope with these very particular conditions: this is, for example, the case with the steels designed to be used within heat treatment ovens or as resistors of radiating elements.
Thanks to the addition of chromium and nickel in high quantities and to a balanced analysis, the steels in this category are particularly suitable for retaining good mechanical characteristics at high temperatures; they can therefore remain in service for a long time in environments at high temperatures and with chemical aggressiveness.
