Stainless steels are a large product group with varying properties, with the common denominator being a high content of chromium (Cr). The chromium causes the steel to transition from being a normal, active alloy to being an effective passivatable alloy with significantly better corrosion properties.
Stainless steels are steels with a chromium addition of 10.5% or more. In the presence of oxygen, the chromium forms a very thin layer—a passive layer—of chromium oxide that protects the steel against corrosion. If the passive layer is damaged, for example by mechanical action, it regenerates quickly in environments where oxygen is present. Even very low levels of oxygen are sufficient for the passive layer to regenerate in the event of damage.
The strength of the passive layer, i.e., the corrosion resistance, increases rapidly with increased chromium content up to about 20 percent. Further improvement of the steel's corrosion properties is obtained with additions of alloying elements such as nickel, molybdenum, copper, and nitrogen. In addition to different properties regarding corrosion resistance, different compositions of alloying elements result in steels with different crystal structures. The crystal structure, in turn, affects properties such as strength, toughness, mechanical processing, forming, and weldability. Based on the crystal structure of the steels, a distinction is made between ferritic, austenitic, austenitic-ferritic, and martensitic stainless steels.
Austenitic steels are the most common type of stainless steel. The steel's structure provides a very wide range of operating temperatures while maintaining toughness down to low temperatures, good weldability, and good ductility, which results in good formability. The strength is typically between 200 and 300 MPa. However, austenitic stainless steels have a pronounced work-hardening property, which is why they can be cold-worked to obtain very high strengths. Common applications include storage tanks, household goods, pipes, pressure vessels, and load-bearing structures. The primary use in construction is for architectural purposes.