Ferritic stainless steels are essentially chromium containing alloys with body-centered cubic (bcc) crystal structures. Chromium content is usually in the range of 10.5 to 30%. Some grades may contain molybdenum, silicon, aluminum, titanium, and niobium to confer particular characteristics. Sulfur or selenium may be added, as in the case of austenitic grades, to improve machinability. The ferritic alloys are ferromagnetic. They can have good ductility and formability, but high-temperature strengths are relatively poor compared to the austenitic grades. Toughness may be somewhat limited at low temperatures and in heavy sections.
Ferritic steels are not hardenable by heat treatment, due to their low carbon content. These alloys posses good resistance to stress corrosion cracking, pitting corrosion, and crevice corrosion. They are used in a variety of applications where corrosion resistance, rather than mechanical properties (strength, toughness and ductility) is the primary service requirement, such as:
High chromium superferritic grades, have been developed over the years for use demanding environments. These alloys possess superior corrosion resistance relative to the austenitic and martensitic grades. However, they are relatively expensive and difficult to fabricate.
Ferritic stainless steels are generally limited to service temperatures below 750°F (400°C) due to the formation of embrittling phases, which may also precipitate during welding. Primary concerns with welding ferritic grades are maintaining adequate toughness and ductility in the as-welded condition.