Chromium forms a thin film of chromium oxide on the steel surface. This film protects the steel from further oxidation, making it stainless.
According to the AISI classification Stainless steels are divided onto groups:
Austenitic stainless steels (200 and 300 series) contain chromium and nickel (7% or more) as major alloying elements.
The steels from this group have the highest corrosion resistance, weldability and ductility.
Austenitic stainless steels retain their properties at elevated temperatures.
At the temperatures 900-1400ºF (482-760ºC) chromium carbides form along the austenite grains. This causes depletion of chromium from the grains resulting in decreasing the corrosion protective passive film.
Sensitization is depressed in low carbon steels (0.03%) designated with suffix L (304L, 316L). Formation of chromium carbides is also avoided in stabilized austenitic stainless steels containing carbide forming elements like titanium, niobium, tantalum, zirconium. Stabilization heat treatment of such steels results in preferred formation of carbides of the stabilizing elements instead of chromium carbides.
Applications of austenitic stainless steels: chemical equipment, food equipment, kitchen sinks, medical devices, heat exchangers, parts of furnaces and ovens.
Ferritic stainless steels (400 series) contain chromium only as alloying element.
The steels from this group are low cost and have the best machinability. The steels are ferromagnetic. Ductility and formability of ferritic steels are low. Corrosion resistance and weldability are moderate. Resistance to the stress corrosion cracking is high.
Applications of ferritic steels: decorative and architectural parts, automotive trims and exhausting systems, computer floppy disc hubs, hot water tanks.
Martensitic stainless steels (400 and 500 series) contain chromium as alloying element and increased (as compared to ferritic grade) amount of carbon.
Due to increased concentration of carbon the steels from this group are heat treatable. The steels have austenitic structure (FCC) at high temperature, which transforms to martensitic structure (BCC) as a result of quenching .
Martensitic steels have poor weldability and ductility. Corrosion resistance of these steels is moderate (slightly better than in ferritic steels).
Applications of martensitic stainless steels: turbine blades, knife blades, surgical instruments, shafts, pins, springs.
Austenitic-ferritic (Duplex) stainless steels contain increased amount of chromium (18% -28%) and decreased (as compared to austenitic steels) amount of nickel (4.5% - 8%) as major alloying elements. As additional alloying element molybdenum is used in some of Duplex steels.
Since the quantity of nickel is insufficient for formation of fully austenitic structure, the structure of Duplex steels is mixed: austenitic-ferritic.
The properties of Duplex steels are somewhere between the properties of austenitic and ferritic steels. Duplex steels have high resistance to the stress corrosion cracking and to chloride ions attack. These steels are weldable and formable and possess high strength.
Applications of austenitic-ferritic stainless steels: desalination equipment, marine equipment, petrochemical plants, heat exchangers.
Precipitation hardening stainless steels contain chromium, nickel as major alloying elements.
Precipitation hardening steels are supplied in solution treated condition. These steels may be either austenitic or martensitic and they are hardened by heat treatment (aging). The heat treatment is conducted after machining, however low temperature of the treatment does not cause distortions.
Precipitation hardening steels have very high strength, good weldability and fair corrosion resistance. They are magnetic.
Applications of precipitation hardening stainless steels: pump shafts and valves, turbine blades, paper industry equipment, aerospace equipment.
|No.||Grade||C max, %||Mn max,%||Cr,%||Ni,%||Mo,%||N, %||Cu,%||Cb+Ta,%|
|AISI 201||Austenite||0.15||6.0||17.0||4.5||-||0.25 max||-||-|