Transition element, with symbol Fe and atomic number 26.
Pure iron has a hardness between 4 and 5, its colour is silvery, and it is ductile and malleable.
Iron is a reactive element: it reacts with halogens (fluorine, chlorine, bromine, iodine, and astatine), sulphur, phosphorus, carbon, and silicon.
Heated at high temperatures, it burns in an oxygen atmosphere becoming oxide Fe3O4, which is also formed when metal particles are detached from a piece of violently wrought iron, burning in the air with the formation of sparks, or during hot forging of iron (chips oxide).
When exposed to moist air, it corrodes and gets covered with red-brown hydrated ferric oxide, commonly called rust, whose formation is due to an electrochemical process: in the presence of water, which provides the electrolytic means, a weak current is established by which the metallic iron decomposes and turns into rust, reacting with atmospheric oxygen. This reaction is accelerated by the presence of the rust previously deposited on the surface. In contact with many diluted acids, iron goes into solution, liberating hydrogen; it does not react, however, with concentrated nitric acid, because of the occurrence of a phenomenon called passivity: the metal is covered with a layer of oxide that protects it from the attack of acids.
In its raw lamination or forging state, industrial pure iron has a tensile breaking load of 30 kg/mm², a yield of 20 kg/mm², and a breaking elongation of about 25%. Its plasticity characteristics are satisfactory when cold, but sometimes they undergo significant alterations when hot and the metal has a weakness that undermines its forgeability. Due to its weldable qualities, as well as for its forgeability, drawability, and printability, iron is used for the construction of metal sheets, plates, bolts, wires, tubes, etc.. Due to its good magnetic characteristics (in particular its low coercive field strength), annealed industrial irons are used for many electromagnetic devices, for the formation of nuclei of soft iron: polar expansions of electromagnets, lamination of electrical machines, nuclei of relays, etc..
Iron and carbon alloy with a carbon content less than 1.8%, which can acquire very different characteristics with heat treatment and machining.
Pure iron is used for the manufacture of stainless steels, common or bonded, and as raw material in the melting crucible. Iron powder, by reduction or electrolytic, is industrially exploited in powder metallurgy for the construction of porous parts and in the welding industry, for the preparation of electrode coatings for welding and for cutting with oxy-acetylene flame.
Stainless steel is the popular name given to steels with a high chromium content, because they do not get rusty when exposed to air and water: chromium, oxidizes in contact with oxygen and it is transformed into chromium oxide (CrO2) that adheres to the work piece, preventing further oxidation (this phenomenon is known as passivation).
These are an extremely important class of steels, used for many different purposes.
Types of stainless steel
- Martensitic stainless steel
- Ferritic stainless steel
- Austenitic stainless steel
- Duplex stainless steel
- Stainless steel at high temperature
- Superferritic stainless steel
- Austenitic stainless steel alloys
- Steels for ultra-high vacuum and cryogenics
Stainless steel was discovered by Harry Brearly from Sheffield, England; in 1913, while experimenting with steel rods for firearms, he discovered that one of his steel specimen with 13-14% chromium and a relatively high (0.25%) carbon content, did not get rusty when exposed to the atmosphere. Subsequently, this property was explained with the passivation of chromium, which forms an extremely thin, continuous, and stable oxide film on the surface.
Types of stainless steel
The term stainless steel generally indicates high-alloy steels containing amounts of chromium between 11 and 30%. Other binders increasing corrosion resistance are nickel, molybdenum, copper, titanium, and niobium; in any case, steel should not have a total quantity of alloying elements exceeding 50%. The members of this family of steels are classified according to their microcrystalline structure that results from their different chemical composition.
Martensitic stainless steel
It has wide mechanical properties and is well workable with machines. It is best known with the American nomenclature: for example, chromium-only steel is called AISI 400 series (especially AISI 410 and 420, with 0.20% Typical elements present in this type of steel are carbon, manganese, silicon, chromium, and molybdenum, but not nickel; sulphur can be added, if it needs to be chipped (at the expense, however, of the mechanical properties). Martensitic stainless steel is self-hardening. Martensitic stainless steels are used mainly for their high creep resistance, although their weldability is extremely critical, and their corrosion resistance is lower than the one of ferritic and austenitic stainless steel.
Ferritic stainless steel
Has a lower carbon content than martensitic steel. One type particularly resistant to heat contains 26% chromium. Other elements are molybdenum, aluminium to increase resistance to hot oxidation, and sulphur to facilitate workability.
Its fundamental properties are: moderate corrosion resistance, which increases with the percentage of chromium; it is magnetizable; it cannot be tempered and should always be used after annealing; its weldability is poor, because the overheated material undergoes enlargement of the crystal grain due to the chromium.
Most common uses are for low quality crockery or cutlery, sinks, kitchen sinks, and fittings for the construction industry. In thin sheets, they are used for coating, plates for ship bridges, spillways, chain conveyors, fume extractors, and dust collectors.
Austenitic stainless steel
It is a type of steel containing Ni and Cr in such a proportion to maintain the austenitic structure, even at room temperature. It is classified according to the percentage of Ni and Cr
The basic composition of austenitic stainless steel is 18% Cr and 8% Ni, codified as 18/8. A percentage of 2-3% molybdenum ensures better resistance to corrosion (18/8/3 steel). The carbon content is low (0.08% C max), but there are also mild austenitic stainless steels (0.03% C max). Austenitic stainless steel can be stabilized with titanium or niobium to prevent corrosion in the area of the welds (see the weaknesses of this type of steel below). Considering the high proportion of high-quality components (Ni, Cr, Ti, Nb, Ta), austenitic stainless steel is among the most expensive commonly used steels.
Its fundamental properties are:
excellent resistance to corrosion;
easy to clean and with an excellent hygienic coefficient; easily workable and weldable forgeable; incrudible if cold worked and not heat treated; in condition of total annealing, it does not magnetize.
This kind of steel is used in many ways: for pots and domestic services, architectural finishes, slaughterhouses, breweries, beverage and food cans; liquefied gas tanks, heat exchangers, pollution control and fumes extraction equipment, industrial autoclaves. Its resistance to most aggressive chemicals also makes it very popular in the chemical industry.
Consonni Srl provides electroplating treatments such as Chromium plating of iron alloys.