What is the PREN of super duplex stainless steels?

PREN stands for the pitting resistance equivalent number, which is a value calculated from the alloy composition that indicates its typical resistance to pitting corrosion. The calculation for PREN is based upon composition values for chromium, molybdenum and nitrogen.

The industry specifications NACE MR1075 and Norsok M650 require the three main super duplex stainless steels (Ferralium 255, UNS S32750 and UNS S32760) to have a PREN >40.

There can be differences from production heat to production heat, with slight variability in content of the elements that are included in the PREN calculation, namely chromium, molybdenum and nitrogen. In addition, the specification of the super duplex grades is different too, with slightly different elements included in their respective recipes. Therefore, it is possible that the PREN value will naturally vary from heat to heat, from 40 up to 42. The PREN formula can sometimes include the influence of other elements, namely tungsten additions (present in S32760 only). However, the PREN formula does not consider elements such as copper, which is proven to improve the corrosion resistance of metals against acids, sulphuric acid in particular. Therefore, defining PREN>40 for super duplex stainless steels is a sensible approach.

Ferralium 255 (S32550) is available only from Langley Alloys, as round bar from ½”-14” (12.7-355.6mm), plate up to 3” (76.2mm), pipe, flanges and fittings. Langley Alloys stocks S32750 and S32760 solid bars in sizes from 5/8” to 20” (15.87-508mm) diameter, plate up to 3” (76.2mm) thick, plus pipes in scheduled sizes, fittings and flanges.

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