Stainless steels are a particular type of special complex steel, i.e. steel to which several alloying elements are added, such as Chromium (Cr), Nickel (Ni), Molybdenum (Mo) and, to a lesser extent, other metals, to give it certain specific characteristics.
The peculiarity of stainless steels is their resistance to corrosion, which is due to the significant presence of Chromium. In fact, the characteristic of this element is that, in contact with the oxygen contained in the air, it creates a very thin film of oxide on the surface of the steel called the passivating layer; this film is insoluble, compact and well adherent to the substrate and acts as a protective barrier against corrosion.
The nature of the passivating layer, formed essentially of chromium oxides/hydroxides, is self-healing and guarantees the protection of the metal even if abrasion or removal of the film occurs locally. In particular, the passive film can be more or less resistant depending on the concentration of chromium in the alloy and the possible presence of other elements such as nickel, molybdenum, etc.
The minimum value of chromium in order to speak of stainless steel is 11-12%, up to a maximum of 30%; if the percentage of the alloying elements is high, one no longer speaks of stainless steel but of austenitic stainless alloys.
Since the chromium oxides formed on the surface of stainless steels are very stable at temperatures higher than room temperature, the material will also show high resistance to oxidation and hot corrosion.
Typical areas of use for these steels are in the pharmaceutical, biomedical and cosmetic industries, the chemical and petrochemical industries, the food sector, construction, transport and infrastructure of high strategic importance, household appliances and furniture.
HOW TO CLASSIFY STAINLESS STEEL?
FAMILIES OF STAINLESS STEELS
Stainless steels are traditionally divided into three major families defined by their microstructure at room temperature.
We therefore have
- Martensitic stainless steels
- Ferritic stainless steels
- Austenitic stainless steels
Martensitic stainless steels offer the best mechanical strength characteristics among stainless steels when they are applied in the quenched and tempered state (quenched and tempered = hardening + tempering).
Austenitic stainless steels are not as susceptible to heat treatment as martensitic stainless steels, but they are able to increase their strength by cold work hardening, thus increasing their tensile strength, fatigue resistance and resilience, i.e. their ability to resist impact. this work hardening phenomenon is generally achieved during the cold forming process.
Ferritic stainless steels, like austenitic stainless steels, are not susceptible to an increase in their characteristics through hardening treatment and consequently their strength characteristics are not very high and cold work hardening increases their strength characteristics, although to a lesser extent than austenitic stainless steels.
In addition to these main categories, there are sub-categories whose use is growing rapidly, namely:
- Precipitation-hardened steels (PH)
- Duplex austenitic stainless steels (Duplex)
- Nickel-free austenitic stainless steels