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| Specifications of stainless steel AISI 316 components |
Stainless steel is an iron alloy that combines mechanical properties typical of steel with peculiar corrosion strength.
Stainless steel is traditionally divided into three main categories depending on its microstructure:
1. Martensitic stainless steels
2. Ferritic stainless steels
3. Austenitic stainless steels
Stainless steel series 300 belongs to the austenitic class since it is a steel with a face-centered cubic structure, containing such a percentage of Ni and Cr able to preserve the austenitic structure even at room temperature.
The main and most used steels of series 300 are the following:
AISI 304: the progenitor of the austenitic stainless steel category. It has a good corrosion strength and good mechanical properties. Used in the pharmaceutical and the food industries.
AISI 304L: Unlike AISI 304, it has a low content of carbon (C=0,03%) that considerably increases its corrosion strength.
AISI 316: it differs from AISI 304 in the content of molybdenum present at about 2.5 % and in higher percentage of nickel. These substances ensure better mechanical properties to the steel, better corrosion strength at high temperatures. Used for the manufacturing of fabrics, synthetic fibres and sea water systems.
AISI 316L: unlike AISI 316 it has a low content of carbon (C=0,03%).
It assures even better corrosion strength keeping good mechanical properties. It is used for pipes and fittings in heat exchangers in contact with corrosive agents at high temperatures.
Considering the high percentage of valuable components ( Ni, Cr, Ti, Nb,Ta) the austenitic stainless steels are more expensive than the ordinary steels.
Main benefits:
> Excellent corrosion strength
> Easy to clean and excellent hygienic coefficient
> Forgeable and weldable
> Work-hardened if cold worked and not through thermal treatment
> In case of total annealing it does not magnetize
Their strength to most chemical attacks makes them very appreciated in the chemical industry. The austenitic stainless steels suffer from some limits:
> max. treatment temperature up to 925 ° C
> At low temperature their corrosion strength sharply decreases
Benefits against corrosion:
Stainless steels, because of their chemical composition, have the chance to self-passivate and withstand most different attack conditions.
There are several parameters that facilitate the initiation of a corrosive phenomenon:
> Chemical agent nature (type, concentration, PH)
> Chemical agent temperature
> Surface finish of the metal
> Fluid speed on the material walls
Generally speaking we can say that chlorine compounds (CI-) are the main enemies of stainless steel. The austenitic steels prove to be the best against corrosion, in particular the ones connected to chromium, nickel, molybdenum (Cr- Ni-Mo) that have a particularly resistant passive film.
According to the alloy percentage of those elements, on which the corrosion strength directly depends, it is possible to calculate a parameter value, the PREN (Pitting Resistance Equivalent Number) that supplies an approx. estimate about the capacity of stainless steel to withstand the pitting, or in general, the localized corrosion.
In the below chart, as an indication, you can find some PREN values for some stainless steels. The remarkable properties of AISI 316 compared to the 304 type are evident; AISI 304 is more popular for its better machineability and its lower cost. .
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