Superaustenitic stainless steels exhibit great resistance to corrosion and high strength thanks to their high molybdenum, nitrogen and nickel contents. These steels approach the performance of highly corrosion resistant nickel based alloys at a significant lower cost.
These stainless steels are designed for specific applications :
low ferrite content grades for amagnetic properties and/or high corrosion resistance
borated stainless steels for neutronic radiation shielding
optimized chemical composition with low residual elements, addition of specific alloying elements, and homogeneous microstructure to enhance the corrosion resistance in nitric, sulfuric or urea-carbamate media
These are the most popular of the stainless steels because of their ductility, ease of working and good corrosion resistance; they are derived from the 18Cr-8Ni stainless steel Type 304. The other grades are developed to provide special corrosion resistance properties by adding Molybdenum, Chromium and/or decreasing Carbon content (L grades).
Addition of stabilizing elements like Titanium or Niobium (Columbium) provides better weldability. Addition of Nitrogen increases mechanical strength as for grades 304N or 316N.
They are also available in the heat resistant version (H grades) with a higher Carbon content and coarser grain size to improve creep resistance. They can be used up to 870°C /1600°F.
They also exhibit good ductility and toughness even at cryogenic temperatures.
All are essentially nonmagnetic in the annealed condition, although some may become slightly magnetic by cold working.
Ferritic Martensitics and Precipitation Hardening stainless
Ferritic stainless steels: These grades are characterized by a very low content of nickel (less than 1%). Their low carbon content prevents hardening by heat treatment, their pitting corrosion reistance is equivalent to 300 series.
Martensitic stainless steels: Basic corrosion resistant grades (Cr~13%), the higher carbon content compare to ferritic allow them to be hardened by heat treatment while still being fairly weldable.
Precipitation hardening stainless steels: The addition of aluminum, copper or aluminium in these chromium-nickel grades allow them to be hardened by a solution and aging heat treatment.