Composition of Stainless Steel

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Intrduction

Stainless steel is an alloy of Iron with a minimum of 10.5% Chromium. … Stainless steel also contains varying amounts of Carbon, Silicon and Manganese. Other elements such as Nickel and Molybdenum may be added to impart other useful properties such as enhanced formability and increased corrosion resistance.

corrosionCorrosion is a natural process that converts a refined metal into a more chemically-stable form such as oxide, hydroxide, or sulfide. It is the gradual destruction of materials (usually a metal) by chemical and/or electrochemical reaction with their environment. Corrosion engineering is the field dedicated to controlling and preventing corrosion.

Stainless steel is rolled into sheets, plates, bars, wire, and tubing to be used in: cookware, cutlery, surgical instruments, major appliances; construction material in large buildings, such as the Chrysler Building; 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). Corrosion resistance, the ease with which it can be steam cleaned and sterilized, and lack of need for surface coatings has also influenced the use of stainless steel in commercial kitchens and food processing plants.

COMPOSISTION

The composition of stainless steel first starts with Chromium. When steel is combined with 10.5% Chromium the resultant alloy is classified as “stainless steel”. When Chromium is added to steel, its natural affinity for Oxygen creates a stable Oxide surface film. This protects the surface of the metal from further physical or chemical changes.

The composition of stainless steel falls into four groups: Martenistic, Ferritic, Austenitic and Duplex.

Martenistic Group

This group of stainless steel contains 12 to 14% Chrome and 0.08 to 2.0% Carbon. The addition of Carbon makes the steel respond well to heat treatment, resulting in different mechanical strengths, such as hardness and corrosion resistance. These mechanical properties make this magnetic steel perfect for a wide variety of applications.

Type 410 Stainless Steel contains 13% Chrome and .15% Carbon. This alloy exhibits good ductility and resistance to corrosion. This steel alloy is easily forged and machined, as well as showing good cold working properties.

Type 416 is usually supplied is bar form and is similar to Type 410, however machinability is improved by the addition of Sulphur.

Type 431 stainless steel is another steel alloy more often seen in bar form. This composition of stainless steel includes 17% Chrome, 2.5% Nickel, 0.15% Carbon, which gives it a better corrosion resistance than 410 and 416.

Ferritic Group

This group of stainless steel starts with a minimum of 17% Chrome and 0.08 to 0.2% Carbon. The higher percentage of Chromium increases the metal’s resistance to corrosion at high temperatures, but its uses are limited because it cannot be heat treated. This is another stainless steel alloy that is magnetic.

Type 430 is a typical Ferritic steel with good corrosion resistance in temperatures up to 800°C. However, because it does not machine well, it is usually seen as strip and sheet metal.

Austenitic Group

The composition of this non-magnetic stainless steel consists of 17 to 25% Chromium and 8 to 20%. Other elements are then added so the steel will have certain properties. Steels in this group have a useful range of physical and mechanical properties.

Type 304 is an economic alloy that displays good corrosion resistance in unpolluted and freshwater, but is not recommended for seawater.

Type 321 is similar to Type 304, but has Titanium added in direct proportion to the metal’s carbon content. The resultant stainless steel has improved high temperature properties.

Type 347 stainless steel uses Niobium instead of the Titanium used in Type 321.

Type 316 stainless steel has 2 to 3% Molybdenum added which increases the corrosion resistance. While used in offshore environments, this alloy still pits when fully immersed in seawater, although the 12% Nickel does ensure the metal maintains the austenitic structure.

Type 317 while similar to 316, is more resistant to pitting when in cold seawater due to the 3 to 4% Molybdenum.

6 Moly, aka UNS S31254, with its higher levels of Molybdenum and Chromium has a high resistance to seawater.

L Grades of Austenitic grades have a reduced tensile strength due to the lower Carbon content of 0.03 to 0.035%.

Duplex Stainless Steels

Often identified by UNS numbers, with a balance of Molybdenum, Chromium, Nickel and Nitrogen, this group of steels is a mix of ferrite and austenite. These are highly resistant to corrosion and have high strength. Due to dangers of embrittlement, temperature ranges for extended use range from -50 to +300°C.

UNS S31803 has a composition of stainless steel that includes 0.03% max Carbon, 0.15% N, 3% Mo, 22% Cr, and 5.5% Ni. It is typical of Duplex stainless steels and the most widely used.

UNS S32304 is a low alloy Duplex somewhat similar to 316’s corrosion properties, but with a tensile property that is approximately double that of 316. It is most often used in structures where mechanical strength is vital. The composition of stainless steel of this type is 0.1% N, 4% Ni, 0.03% max Carbon and 23% Cr.

UNS S32750 is a super Duplex with enhanced resistance to corrosion and good mechanical properties. The typical composition of stainless steel of this type of 0.28% N, 4% Mo, 7% Ni, 0.03% max Carbon and 25% Cr.

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Role of Alloying Elements

Chromium is the essential ingredient in stainless steels. As the chromium content is increased above 10.5%, the passive film becomes stronger and is able to resist more aggressive environments, particularly those containing chlorides. High chromium also helps the passive film to heal itself more rapidly if it is disrupted, for example, by scratching of the surface.

At elevated temperatures the chromium reacts with the oxygen in the air to form a thick, visible oxide layer. The color and thickness of the chromium oxide will depend on the temperature and time of exposure. The chromium oxide layer is visible and is thick enough to be scraped off and measured. This is quite unlike the thin, transparent, protective, passive film.

Various alloying elements are added to stainless steels to improve corrosion performance in specific environments, or to modify or improve the mechanical, or physical properties of the stainless steel. There are several hundred different stainless steels, all formulated to provide a specific combination of corrosion resistance, weldability and mechanical properties.

Nickel is the most common alloying element in stainless steels. It changes the crystal structure of the steel from ferritic to austenitic. The austenitic structure has improved ductility, formability and weldability. Nickel also improves corrosion resistance in reducing environments such as sulfuric acid. The most common stainless steel is Type 304 which has about 18% Cr and 9% Ni.

Molybdenum is added to stainless steels to improve their resistance to pitting and crevice corrosion in chloride-containing environments. Type 316 is the most common Mo stainless steel. Its nominal composition is 17%Cr-10%Ni-2%Mo.

Since about 1970, nitrogen has been an important alloying addition to stainless steels. The high performance austenitic stainless steels and the second-generation duplex stainless steels all contain a deliberate addition of 0.10 to 0.50% nitrogen. For these stainless steels, nitrogen improves pitting and crevice corrosion resistance, makes them stronger, and retards the formation of sigma phase during welding. In duplex stainless steels, nitrogen promotes the re-formation of austenite at higher temperatures and helps maintain an acceptable austenite-ferrite phase balance in the as-welded condition. Precipitation of intermetallic phases such as sigma reduce the toughness and corrosion resistance of stainless steels.

Additions of copper to a stainless steel increase the corrosion resistance in reducing environments such as sulfuric acid. Alloy 20 and 904L are examples of stainless steels with deliberate copper additions.

Other alloying additions are used to enhance specific properties. For example, sulfur is added to the free machining stainless steel, Type 303, for improved chip breaking during turning operations. Aluminum and silicon additions improve the oxidation resistance of other stainless steels.

Stainless Steel Corrosion

When stainless steels are selected properly, fabricated correctly and maintained adequately, they will perform without attack indefinitely. This is the case in the vast majority of applications. However, if the environment becomes overly aggressive, for example due to a process change or a process upset, the passive film may be overwhelmed, usually on a very localized basis, and no longer be a protective barrier for the stainless steel underneath. Corrosion will then occur. For stainless steels, there are several forms of possible attack including: pitting, crevice corrosion, stress corrosion cracking, galvanic corrosion, and intergranular corrosion.

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