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Types of Stainless Steel

Types of stainless steel is the large group.The large group of stainless steels can be divided into two groups Austenitic and Ferritic, the Ferritic group being split again into two groups,Martensitic and Ferritic. We know Stainless steel is a name given to a group of steel alloys that contain more than 12% Chromium. Chromium has a high affinity for oxygen and forms a stable oxide film on the surface of the stainless steel. This film is called the passive oxide layer and forms instantaneously in ordinary atmospheres.The film is self healing and rebuilds when it has been removed. It is this film that gives Stainless Steel its corrosion resistance.

AUSTENITIC GRADES
This group of stainless steels contains 17 – 25% Chromium and 8 – 20% Nickel with various additional elements to achieve the desired properties. In the fully annealed condition Austenitic stainless steels exhibit a useful range of mechanical and physical properties.Mechanical properties can be increased with cold working. Welding of this group must be caried out with correct methods but the low Carbon content results in fewer problems than with the Ferritic and Martensitic grades. Normally these stainless steels are non-magnetic but will become slightly magnetic when cold worked. Basic grades of Austenitic stainless steels are listed below.
Type AUSTENITIC GRADES : 303,304,304L,310S,316,316L,321.


MARTENSITIC GRADES
This group contains a 12% - 14% Chromium and 0.08% - 2.00% Carbon. The high Carbon content of the martensitic stainless steels allows them to respond well to heat treatment to give various mechanical strengths such as hardness 500004574. However, the Carbon is detrimental when welding and care must be taken. In the heat treated condition, this group of stainless steels show a useful combination of corrosion resistance and mechanical properties that qualify them for a wide range of application.
Type MARTENSITIC GRADES : 409,410,420,431

FERRITIC GRADES
This group contains a minimum of 17% Chromium and 0.08 – 2.00% Carbon. The increase in Chromium imparts increased resistance to corrosion at elevated temperature, however the lack of mechanical properties due to the fact that it cannot be heat treated, limits its applications. Like Martensitics they are magnetic and the welding of the group should be carried out with care.
Type FERRITIC GRADES : 430

DUPLEX / SUPER DUPLEX GRADES
This group of stainless steel has an annealed structure which typically consists of equal parts of austenite and ferrite. These steels have 18 – 29% chromium, 3 – 8% nickel and various other elements, particularly nitrogen and molybdenum. This group of steels has several advantages over austenitic steels. The duplex grades are highly resistant to chloride stress corrosion cracking, they have excellent pitting and crevice corrosion resistance and have about twice the yield strength of the common austenitic grades.
Type SUPER DUPLEX GRADE : 2205,S32750.