Four Main Categories of Stainless Steel
Many types of stainless steels have been developed to resist different corrosion environments and working conditions. There are more than 60 grades of stainless steel. However, the entire group can be divided into four classes – Austenitic, Ferritic, Martensitic, and Duplex. Each is identified by the alloying elements which affect their microstructure.

Austenitic
Austenitic grades are non-magnetic. In addition to chromium, typically at 18% level, they contain nickel which increases corrosion resistance. They are the easiest to fabricate and the most widely used category of stainless steel.

Ferritic
Ferritic grades are magnetic. They have low carbon content and contain chromium as the main alloying elements, typically at 13% and 17% levels.

Martensitic
Martensitic grades are magnetic, containing typically 12% chromium and a moderate level of carbon. Their applications are mainly in cutlery manufacture, cutting tools and general engineering, where abrasion resistance and high strength are required.

Duplex
Duplex grades have a mixed ferrite/austenite structure and are magnetic, containing 18% to 28% chromium and 4.5% to 8% nickel. They are used in applications where high chloride content is present, to avoid pitting corrosion and improve stress corrosion cracking resistance.

The most common and popular stainless steel categories are Austenitic and Ferritic, which account for 95% of stainless steel applications.