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STAINLESS STEEL
Stainless steels are iron based alloys gathering typical mechanical properties of steel to the distinctiveness characteristics of corrosion peculiarity. Their discover in year 1913 by Mr Harry Brearly of Sheffield while experimenting for fire gun barrels. He discovered than one of his steel tests with 13-14% Chrome together with a quite high carbon level (0,25%) would not get rust once exposed to atmosphere. This peculiarity has been explained with chrome passivity forming on surface a very tiny and stable oxide coat. Subsequent progress on metallurgy field, in years '40 and '60, have widen its development and application. Stainless steels are still improved and modified following various industrial requirements such as petroliferous/petro-chemical, mineral, energetic, nuclear and food.

 

The Italian word to distinguishes stainless steel is effectively a not an appropriate word since they are very much oxidizable while the correct English word "stainless" comes from the capacity of those materials to become oxidized but not to get rusted . This process is more precisely called "passivate in atmospheric habitats". The passivate phenomenon consists in the forming of an invisible oxide coat, composed by few atomic layer (3-5x10-7 mm), protecting the underneath metal from corrosion coupling. This happens as reaction to oxidizing environment (air, water, different solutions, others).

 

The passivate-level nature, essentially composed by chrome oxides/hydroxides is a self cicatrizant and grants the metal's protection though in some parts there could be some abrasion or membrane removal.
The passive coat can be more or less resistant depending on chrome concentration in the alloy and the eventual presence of other elements such as nickel, molybdenum, titanium, others. The minimum chrome percentage in order to consider steel stainless steel is 11-12% up to a maximum of 30%. The presence of high alloying elements make it not being considered stainless steel any more but austenitics alloys steel.

 

Stainless steels are traditionally divided into three big families:

 

- Martensite alloy steel
- Ferritic alloy steel
- Austenitic alloy steel

 

MARTENSITE STAINLESS STEEL
The Martensite stainless steels are chrome alloys (from 11% to 18%) with quite high carbon content together with small quantities of other elements such as manganese, silicon, chrome and molybdenum. Should shaving be required than some sulphur needs to be added (this inspite of mechanical characteristics). Martensite stainless steel has very high mechanical characteristics and is very well machine workable. It is the only stainless steel that can be tempered and therefore get the mechanical characteristics ( breaking point, yield point, hardness point) increased through thermic treatment. It is mostly known under American nomenclature i.e only chrome steel is AISI 400 series ( to remember AISI 410 and 420 with 0,20% < C < 0,40% and Cr = 13%, AISI 440 C = 1% circa e Cr = 17%. )
Under UNI nomenclature are called X20Cr13, X30Cr13, and X40Cr14. It is a magnetic one and it is also known as Steel "series 00".

 

FERRITIC STAINLESS STEEL

Even ferritic steels are stainless steels chrome alloys (variable from 11% to 30%). Their mechanical and corrosion resistance is a good one. They have a cubic structure with a well balanced body same as carbon steels but, they cannot get their mechanical characteristics increased through thermic treatments. Compared to Martensite stainless steel they have a lower carbon level. One alloy particularly resistant to heath has a 26% Chrome content. Other components are Molybdenum, Aluminum to increase oxidation resistance, Sulphur to easy workability.

 

AUSTENITIC STAINLESS STEEL
The austenitic steel structure can be preserved even at room temperature due to the contained Ni and Cr content. It can be classified following Ni and Cr percentage (see under-reported table)
The 18% Cr and 8% Ni is the basic composition, also called inox 18/8. The content of 2-3% di Molybdenum assures a better resistance to corrosion (inox steel 18/8/3).
The carbon content is low 0,08% max but there are even "sweet" austenitic stainless steels with 0,03% max Carbon content.
The austenitic stainless steel can be stabilized with titanium or niobium in order to void a sort of corrosion in welding area.
Considering the remarkable percentage of valuable components (Ni, Cr, Ti, Nb, Ta) the austenitics Stainless Steels are among the most costly stainless steels of common use.

The basic properties are:

- Excellent corrosion resistance
- Easy cleaning and excellent hygienic coefficient
- Easy workable, forgeable and weldable
- In case of complete re-melting it doesn't get magnetized

 

STEEL COMPOSITION

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