The addition of nickel increases the stability of austenite as shown in the Fe-Cr phase diagram (detail).
Stainless steels are distinguished from their main phase in their crystalline structure.
Austenitic stainless steels
These are stainless steels with a main phase of austenite (γ-Fe). They contain very little carbon (usually <0.08% C, but some contain up to 0.15% C) and at least 16% Cr. The austenite is stabilized by the addition of Ni and / or Mn, and the stable phase remains throughout the temperature range from the melting point of the alloy to well below 0 ° C. Because austenite is not magnetic (it is not "ferromagnetic"), the austenitic stainless steel is not magnetic. Austenitic stainless steels are not heat-treated.
The most common stainless steels are 18/8 (18% Cr, 8% Ni) and 18/10 (18% Cr, 10% Ni), belonging to the 300 series, according to the American AISI-SAE standards. In stainless steel AISI-SAE 304 (ISO A2), the higher the nickel content, the higher the corrosion resistance. Stainless steels AISI-SAE 316 (ISO A4) show even higher corrosion resistance, as molybdenum up to 2%. Stainless steels AISI-SAE 304L and AISI-SAE 316L contain very little carbon (<0.03%) to be more easily bonded.
Besides the common austenitic steels, there are also the least resistant manganese austenitic stainless steels of the ANSI series 200, which contain Cr and Mn, and Ni in relatively low content. There are also super-stainless stainless steels with very high Ni content (> 20%) and Mo (> 6%), for high resistance to corrosion by acids, chlorine and chloride solutions. AISI-SAE 904L (UNS N08904) is stainless steel (19-23% Cr, 23-28% Ni, 4-5% Mo) and contains 1-2% copper for high resistance to acidic reducing environments Sulfuric acid.
Ferritic and martensite stainless steels
The classification of Schäffler stainless steel. On the x axis, the alloying elements given are favoring the formation of ferrite as equivalent Cr (= (Cr) + 1.5 (% Si) + (% Mo) + 0.5 (% Nb)), The alloying elements given on the y axis are favoring the formation of austenite as Ni equivalent (= (Ni Ni) + 0.5 (% Mn) + 30 (% C)).
Ferritic stainless steels are transformed into martensitic with a suitable heat treatment ("dyeing"). Martensitic stainless steels are softer than the corresponding austenitic, and are therefore more suitable for machining. Also, martensitic stainless steels can be cured by precipitation. A typical martensitic stainless steel contains 12-14% Cr, 0.2-1% Mo, & lt; 2.5% Ni and 0.1-1.2% C.
Ferritic and martensite stainless steels belong to the AISI-SAE 400 series, but martensite stainless steel which has been undergone precipitation belongs to the AISI-SAE 600 series. The most widely known stainless steel undergone precipitation is 17 / 4PH (AISI-SAE 630), which contains 15-17.5% Cr and 3-5% Ni [7].
Two-phase stainless steel
Biphasic or rheostatic stainless steels or duplex stainless steels contain austenite and ferrite in a ratio ranging from 50:50 to 40:60. Usually they contain 19-28% Cr, <5% Mo and a little Ni. They exhibit just as good corrosion resistance as austenitic stainless steels, but they are softer. The most common double-phase stainless steel is AISI-SAE 2205 (UNS S31803 / S32205).