Stainless steel is an iron-based alloy with chromium content anywhere from 10.5% to 30%. The alloying elements are why there is a classification of stainless steel since a slight change can affect the performance and appearance of stainless steel. The different classifications are ferritic, martensitic, austenitic, duplex, and precipitation-hardening.
Ferritic stainless steel has chromium content between 11 to 17% although there are specialized ferritic at 29%. It is less expensive than other stainless steel types because of the low carbon and zero nickel content. Its limitations are poor welding, not suitable for sub-zero temperatures, and lower ductility which means it is not as stretchable.
Martensitic stainless steel is low carbon steel with 12% chromium and a very minimal amount of iron. They are usually heat tempered and hardened which means tough, not brittle. Austenitic stainless steel, you get high chrome and nickel which means it has high ductility and tensile strength. It does not require pre-heating because it is resistant to hydrogen cracking. It is generally tough on corrosion but under certain high heat conditions can become vulnerable to rusting.
Duplex stainless steel undergoes a 2-phase microstructure consisting of the use of ferritic and austenitic stainless steel. It has the properties of both types of stainless steel making it stronger, ductile, tough, and corrosion and stress resistant. It is also cheaper because of the lower alloying content.
Precipitation-hardening stainless steel has chromium and nickel and is likened to a combination of austenitic and martensitic stainless steel grades. It can handle high heat while being resistant to corrosion. It is hardened using elements like aluminum, copper, titanium, molybdenum, or niobium.
Stainless steel can also come in different forms like sheets, pipes, tubes, plates, strips, bars, wire, and foil. When selecting stainless steel, your main concern would be the degree of resistance to rust and corrosion, availability, cost, properties at specific temperatures, and fabrication characteristics. In the U.S. the usual types are the 304, 316, 409, 410, and 430.