Duplex steels have a mixed ferrite and austenite microstructure, to which various other metals such a molybdenum are added to increase mechanical strength and corrosion resistance. Duplex and super duplex pipe fittings don’t conform to one exact composition. There are several grades, which vary in corrosion resistance.

While it may seem odd to select a duplex steel with higher corrodibility, it must be remembered that selecting alloys for pipe flanges is often a juggling act. Slightly higher corrodibility may be offset by superior mechanical strength, so a steel of this grade may be the choice if you are working at high pressures, but in a fairly neutral environment.

Duplex steels are graded according to their alloy content. Super Duplex steels have the highest level of alloy, and therefore the lowest resistance to crevice corrosion and pitting. Pitting resistance of steel is measured by its PRE (pitting resistance equivalent).

Ordinary austenite and ferrite stainless has a PRE of 17 – 18%. s1803 duplex has a PRE of 34% – the same as N08904 austenite steel, although the latter has poorer resistance to acids, and is more sensitive to stress-corrosion cracking. The highest PRE is achieved with S32750 and S32760 super duplex – both of which we supply at Chemipetro. They differ in that S32750 has added tungsten to increase pitting resistance, but both have a PRE of at least 43%.

The high chromium content of super duplex steels makes them the ideal choice for acidic and caustic environments. Duplex and super duplex pipe flanges are widely used in chemical, petrochemical, pulp and paper and marine environments, often replacing high nickel super-austenitic steels such as S31254.