In metallurgy, stainless steel, also known as inox steel or inox from
French inoxydable (inoxidizable), is a steel alloy with a minimum of 10.5% chromium content by
mass.
Stainless steels are notable for their corrosion resistance, which increases with increasing chromium
content. Molybdenum additions increase corrosion resistance in reducing acids and against pitting
attack in chloride solutions. Thus, there are numerous grades of stainless steel with varying
chromium and molybdenum contents to suit the environment the alloy must endure. Thus stainless
steels are used where both the strength of steel and corrosion resistance are required.
Stainless steel’s resistance to corrosion and staining, low maintenance, and familiar lustre make it
an ideal material for many applications. Stainless steels are rolled into sheets, plates, bars, wire, and
tubing to be used in cookware, cutlery, surgical instruments, major appliances and as construction
material in large buildings, such as the Chrysler Building. As well as, industrial equipment (for
example, in paper mills, chemical plants, water treatment), and storage tanks and tankers for
chemicals and food products (for example, chemical tankers and road tankers). Stainless steel's
corrosion resistance, the ease with which it can be steam cleaned and sterilized and no need for
other surface coatings has also influenced its use in commercial kitchens and food processing
plants.
Austenitic stainless steels, also called 200 and 300 series, stainless steels have an austenitic
crystalline structure, which is a face-centered cubic crystal structure. Austenite steels make up over
70% of total stainless steel production. They contain a maximum of 0.15% carbon, a minimum of
16% chromium, and sufficient nickel and/or manganese to retain an austenitic structure at all
temperatures from the cryogenic region to the melting point of the alloy.
• 200 Series—austenitic chromium-nickel- manganese alloys. Type 201 is hardenable
through cold working; Type 202 is a general purpose stainless steel. Decreasing nickel
content and increasing manganese results in weak corrosion resistance.
• 300 Series. The most widely used austenite steel is the 304, also known as 18/8 for its
composition of 18% chromium and 8% nickel. 304 may be referred to as A2 stainless
The second most common austenite steel is the 316 grade, also referred to as A4
stainless and called marine grade stainless, used primarily for its increased resistance to
corrosion. A typical composition of 18% chromium and 10% nickel, commonly known
as 18/10 stainless, is often used in cutlery and high-quality cookware.
Ferritic stainless steels generally have better engineering properties than austenitic grades, but
have reduced corrosion resistance, because of the lower chromium and nickel content. They are
also usually less expensive. Ferritic stainless steels have a body-centered cubic crystal system and
contain between 10.5% and 27% chromium with very little nickel, if any, but some types can contain
lead. Most compositions include molybdenum; some, aluminium or titanium. Common ferritic grades
include 18Cr-2Mo, 26Cr-1Mo, 29Cr-4Mo, and 29Cr-4Mo- 2Ni. These alloys can be degraded by the
presence of sigma chromium, an intermetallic phase which can precipitate upon welding.
Martensitic stainless steels are usually not as corrosion-resistant as the other two classes but are
extremely strong and tough, as well as highly machinable, and can be hardened by heat treatment.
Martensitic stainless steel contains chromium (12–14%), molybdenum (0.2–1%), nickel (less than
2%), and carbon (about 0.1–1%) (giving it more hardness but making the material a bit more brittle).
It is quenched and magnetic.
Duplex stainless steels have a mixed microstructure of austenite and ferrite, the aim usually being to
produce a 50/50 mix, although in commercial alloys the ratio may be 40/60. Duplex stainless steels
have roughly twice the strength compared to austenitic stainless steels and also improved resistance
to localized corrosion, particularly pitting, crevice corrosion and stress corrosion cracking. They are
characterized by high chromium (19–32%) and molybdenum (up to 5%) and lower nickel contents
than austenitic stainless steels.
The properties of duplex stainless steels are achieved with an overall lower alloy content
than similar-performing super-austenitic grades, making their use cost-effective for many
applications. Duplex grades are characterized into groups based on their alloy content and
corrosion resistance.