Qualities and Applications
Austenitic stainless steels
The addition of nickel increases the stability of austenite, as shown in the Fe-Cr phase diagram (detail).
Stainless steels are classified based on the main phase in their crystal structure.
These are stainless steels with austenite (γ-Fe) as their main phase. They contain very little carbon (usually < 0.08% C, but some contain up to 0.15% C) and at least 16% Cr. Austenite is stabilized by the addition of Ni or Mn, and remains the stable phase throughout the temperature range from its melting point alloy to well below 0°C. Because austenite is not magnetic (it is not "ferromagnetic"), austenitic stainless steels are not magnetic. Austenitic stainless steels cannot be heat treated.
The most common stainless steels are 18/8 (18% Cr, 8% Ni) and 18/10 (18% Cr, 10% Ni), which belong to the 300 series, according to the American AISI-SAE standards. In AISI-SAE 304 (ISO A2) stainless steels, the higher the nickel content, the greater the resistance to corrosion. nickel content, the greater the corrosion resistance. AISI-SAE 316 (ISO A4) stainless steels have even higher corrosion resistance because molybdenum content of up to 2%. AISI-SAE 304L and AISI-SAE 316L stainless steels contain very little carbon (< 0.03%) to make them easier to weld.
In addition to common austenitic steels, there are also less resistant manganese austenitic stainless steels of the ANSI 200 series, which contain Cr and Mn, as well as Ni in relatively small quantities. There are also super austenitic stainless steels with very high Ni (> 20%) and Mo (> 6%) content, for high resistance to acid corrosion. chlorine and chloride solutions. AISI-SAE 904L (UNS N08904) steel is a super austenitic stainless steel (19–23% Cr, 23–28% Ni, 4–5% Mo) and contains 1–2% copper for high resistance in acidic reducing environments, such as sulfuric acid.
Ferritic and martensitic stainless steels
The classification of raw stainless steels according to Schäffler. The x-axis shows the alloying elements that favor the formation of ferrite as Cr equivalent (= (%Cr) + 1.5(%Si) + (%Mo) + 0.5(%Nb)), and the y-axis shows the alloying elements that promote the formation of austenite as Ni equivalent (= (%Ni) + 0.5(%Mn) + 30(%C)).
These are stainless steels with ferrite (α-Fe) or martensite (a metastable phase resulting from rapid cooling of austenite) as the main phase. They contain 10.5–27% chromium, but little or no nickel (< 2%). However, they contain molybdenum or titanium.
Ferritic stainless steels are transformed into martensitic steels by appropriate heat treatment ("quenching" with rapid cooling). Martensitic stainless steels are softer than their austenitic counterparts and are therefore more suitable for machining. Martensitic stainless steels can also be hardened by precipitation. A typical martensitic stainless steel contains 12–14% Cr, 0.2–1% Mo, < 2.5% Ni, and 0.1–1.2% C.
Ferritic and martensitic stainless steels belong to the AISI-SAE 400 series, but precipitation-hardened martensitic stainless steels belong to the AISI-SAE 600 series. The best-known precipitation-hardened stainless steel is 17/4PH (AISI-SAE 630) steel, which contains 15–17.5% Cr and 3–5% Ni[7].Two-phase stainless steels
Duplex or austenitic-ferritic stainless steels or duplex stainless steels contain austenite and ferrite in a ratio ranging from 50:50 to 40:60. They usually contain 19–28% Cr, < 5% Mo, and a small amount of Ni. They exhibit equally good resistance to corrosion as austenitic stainless steels, but are softer. The most common duplex stainless steel is AISI-SAE 2205 (UNS S31803/S32205).
