Composition of Stainless Steel

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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