What is Duplex Stainless Steel

What is Duplex Stainless Steel

The super duplex stainless steels are a more highly alloyed, higher-performance subgroup of the duplex stainless steel family. The higher alloy content gives super duplex steels corrosion resistance better than that of standard austenitic stainless steels such as 316L. By looking at the exact levels of alloying elements, it is possible to calculate a "pitting resistance equivalence number (PREN), which predicts an alloy's pitting corrosion resistance capabilities. The PREN number is calculated as the following:

PREN = %Cr + 3.3(%Mo + 0.5%W) + 16(%N)

The PREN of super duplex stainless steels is generally in the range of 38-45. It is much higher than the typical values of 25-36 for lean and duplex stainless steels and 23-32 for standard austenitic steels.

The super duplex steels are as corrosion resistant as the more expensive high-nickel/high-molybdenum "super austenitic" grades, particularly for resisting pitting corrosion, crevice corrosion, and stress corrosion cracking in acids, caustics, and chloride-rich environments like seawater. As a result, they may be used in industries such as the chemical, petrochemical, pulp, and paper sectors. They may replace both conventional and super-austenitic steels, nickel-based alloys.

Duplex stainless steel grades must be cooled as quickly as possible to room temperature after hot forming to avoid the precipitation of intermetallic phases (Sigma phase in particular) which drastically reduce the impact resistance at room temperature as well as the corrosion resistance.

Alloying elements Cr, Mo, W, Si increase the stability and the formation of intermetallic phases. Therefore super duplex grades have a higher hot working temperature range and require faster cooling rates than the lean duplex grades.