Austenitic stainless steel AISI 304 316 321 347 348 317 309 310

It is a steel face-centered cubic structure, containing N i and Cr in the percentage that could preserve the structure austenitic even at room temperature. Is categorized according to the percentage of Ni and Cr (see table) in the ASTM classification is the 3XX series.

The basic composition of austenitic stainless steel is 18% Cr and 8% Ni, codified in 18 / 8 (AISI 304). A percentage of 2-3% of molybdenum allows the formation of molybdenum carbides better than those of chromium and ensures a better corrosion resistance of chlorides (such as sea water and deicing salts) (18/8/3 steel ) (AISI 316). The carbon content is low (0.08% C max), but there are also sweet austenitic stainless steels (0.03% C max). The austenitic stainless steel can be stabilized with titanium or niobium to prevent a form of corrosion in welds (see further weakness of this type of steel). Considering the large percentage of high-quality components (Ni, Cr, Ti, Nb, Ta), the austenitic stainless steels are among the most expensive among the steels commonly used.

The AISI 304 stainless steel is an alloy composed of an austenitic Cr content between 18% and 20% Ni and between 8% and 11%. The 304L is characterized by a lower content of carbon (C), less than 0,035% (for 304 allows up to 0,080%).

The AISI 316 stainless steel is an alloy composed of an austenitic Cr content between 16% and 18% of Ni between '11% and 14% Mo and Sun between 2% and 3%. The 316L is characterized by a lower content of carbon (C), less than 0,035% (the 316 allows up to 0,080%).

AISI 321 is the name by which you define the category of austenitic stainless steels obtained by the addition of titanium as a stabilizer, the latter containing up to 4%. It is widely used in applications that require a range of carbide precipitation between 450 and 850 ° C. Typical uses are manifold rings for aircraft, parts of reactors, welded constructions, equipment for the chemical industry and more.

The typical composition (expressed in percentages by weight) shows the presence of carbon (0.08%), manganese (2.00%), silicon (0.75%), phosphorus (0.045%), sulfur (0.03%), Chromium (17-19%), nickel (9-12%) and titanium (0.7%). The technical name given by the Euronorm (EN) X6CrNiTi18-10.

Its key feature is the high resistance to intergranular corrosion at the temperatures described above. At these temperatures the family of these materials exhibits high mechanical strength, resistance to fouling and corrosion in aqueous media. However it should be noted that it is also subject to corrosion pitting and crevice-type environments with chlorides hot and breaking stress corrosion above 60 ° C.

The main physical characteristics of this material are approximately as follows: density 8027 kg / m³, elastic modulus 196 GPa, Poisson's ratio 0.3, thermal conductivity 16.1 W / m / K, specific heat at constant pressure 500 J / kg / K.

The basic properties are:

excellent resistance to corrosion;

ease of cleaning and good hygiene factor;

easily workable and weldable forgeability;

If hardenable cold-worked and not by heat treatment;

not in a position of complete annealing is magnetized.

The austenitic structure (with crystal CFC) makes them immune from the ductile-brittle (which manifests itself instead with the ferritic structure, crystal ccc), and then retain their toughness to cryogenic temperatures (liquid He). The grain size, significantly higher than that of ferritic steels construction makes them resistant to creep and therefore among the steels for construction of pressure vessels, are those that can be used at higher temperatures (600 ° C) .

Since the austenite is paramagnetic, these steels can be recognized easily calibrated by placing a permanent magnet.

The use of these steels are very large: pots and domestic services, architectural finishes, slaughterhouses, breweries, beverage cans and food; liquefied gas tanks, heat exchangers, pollution control devices and fume extraction , industrial autoclaves. Their resistance to most aggressive chemicals also makes them very popular in the chemical industry. The same type of steel was used in 1929 for the construction of the spire of the Chrysler Building in New York City: the structure was built in the workshop into 4 separate sections and then assembled on top of the building within 90 minutes. The brightness of the spire, 80 years after its construction, demonstrates the high degree of rust resistance and Nirosta.

The austenitic stainless steels, however, suffer from some limitations:

the maximum temperature which can be processed is 925 ° C;

Low-temperature corrosion resistance decreases drastically: the acids break the oxide film, and this causes corrosion in these steels generic;

crevices and protected areas the amount of oxygen may not be sufficient for the conservation of oxide film, resulting in crevice corrosion;

of halide ions, especially the anion (Cl-), break the passivating films on austenitic stainless steels, causing pitting the so-called, in slang called pitting corrosion. Another effect of chlorine is the SCC (Stress Corrosion Cracking - break from stress corrosion cracking).

The only heat treatment recommended for this class of steels is the solubilization of C to 1050 ° C, with rapid cooling to avoid staying in the area between 800 and 400 ° C, where it can be the precipitation of chromium carbides. The precipitation of these carbides, which are generally Cr23C6, implies a local depletion of chromium that can drop below 12%, thus losing the properties of steels. The consequence is the possible onset of pitting corrosion.

share this

share with