Acronyms commercial AISI
The various stainless steels differ according to the percentage by weight of the alloy constituents.
Among the most commonly used steels distinguish:
304 - Cr (18%) Ni (10%) C (0.05%);
304 L - (Low Carbon): Cr (18%) Ni (10%) C (<0.03%);
316 - Cr (16%) Ni (11/03/13%) Mo (2 / 3%)
316 L - (Low Carbon): Cr (16.5 / 18.5%) Ni (10.5 / 13.5%) Mo (2 / 2, 25%) C (<0.03%);
LN 316 - (Low Carbon Nitrogen) (presence of nitrogen dissolved in the crystal lattice of the material);
316 LN ESR (electro slag remelting-);
430: Cr (16/18%) C (0.08%).
These materials can also be stabilized with titanium or niobium as:
316 I
316 Nb
430 Ti.
The location of iron within the alloy influences various characteristics of the material, of high importance for its use.
The main one is magnetism:
available in a body-centered shows the material properties and therefore magnetic ferritic;
in the face-centered and therefore austenitic steel is nonmagnetic.
As mentioned previously, the AISI 304 and 316 belong to the family of steels with an austenitic structure as the AISI 420 is a martensitic structure.
The difference between the 304 and 316 stainless steel, except for the higher cost and the presence of Mo in 316, is given by the second highest austenicità with the highest percentage of nickel.
Although these steels retain the austenitic structure, in some cases remain in the ground "islands" that have a ferritic structure, derived from δ ferrite.
UV you need a type of austenitic steel, because it has a structure closely linked and therefore less vulnerable chemically.
The presence of refractory metals such as molybdenum, electro-chemically helps to bind the iron atoms, giving it more inertia and a greater degree of hardness (about 180 degrees Vickers).
The austenitic steel alloy can also use the nell'UHV, since the nonmagnetic structural inertia gives the almost total interactions "weak" ensuring a vacuum cleaner.
The presence of chromium, despite its ferriticizzanti characteristics, stability and elasticity gives the steel, thereby ensuring ductility and malleability.
The fact remains that, in this technology, the most widely used is the austenitic steel.
Its melting temperature is 1435 ° C, however, we must consider that, during welding, in the temperature range between 600 and 800 ° C, is transformed, or rather ten days, from austenitic to ferritic (as shown in the diagram awareness of Schaeffler).
Its decay is more rapid and permanent than for steels 304 to 316.
Shelf Awareness:
304: 10 minutes;
304 L: 30 minutes;
316 L: one hour.
More extensive is this period (the extension is proportional to the presence of nickel), the material is more reliable.
To further reduce the degassing of alloy 316 is made the process of electro slag remelting, where the same has been recast in a microwave radio frequency spectrum so as to eliminate the microscorie oxides and carbides, which, in addition to "dirty" vacuum , making it more ferrite. The ESR 316 LN, since very expensive, is limited and mainly used in particle accelerators.
Steel is a constituent of the vacuum chambers, flanges and any other elements such as bolts and nuts, in any case, a stainless steel vacuum chamber requires further treatment aimed at decreasing the constant venting of hydrogen from its walls. One of the key is the vacuum firing, with which the steel is first heated to 1400 ° C and then rapidly cooled rapidly to cross the area without raising decay into ferrite. Thus, in addition to the decrease in the percentage of nitrogen on the surface, you get an increase in its austenitic.
