Duplex, UNS 31803 (F51) vs. UNS 32205 (F60)
In a previous blog I wrote about the generic properties, types and use of duplex. We regularly receive more specific questions about Duplex, among others about the different designations and PREN. Consider this as a continuation of the previous mentioned blog in which we will deal with the difference between UNS 31803 and UNS 32205.
In the early 1980s modern duplex emerged and the initial Duplex-specification was captured as UNS31803. This is still the most widely used Duplex grade. The attractive combination of properties, (as described in this blog) made it applicable for various demanding applications.
The most used equivalent designations for this grade are:
F51: According to ASTM A182(‘Standard Specification for Forged or Rolled Alloy and Stainless Steel Pipe Flanges, Forged Fittings, and Valves and Parts for High-Temperature Service’), the ‘F’ prefix stands for Forged product. Duplex steels are numbered between F50 and F71.
1.4462: is the EN numeric designation for this material (werkstoffnummer). X2CrNiMoN22-5-3 is the EN chemical designation.
To learn more about the different designations in stainless steels, read this blog.
The chemical composition of UNS 31803, %:
An important property of this material is its excellent resistance to pitting corrosion. This is a form of corrosion were cavities are created on the surface, often in chloride-containing environments.
A methodology to compare the sensitivity to this kind of corrosion is the PREN formula, which stands for Pitting Resistance Equivalent Number. The content of Chromium, Molybdenum and nitrogen determine the resistance: PREN = %Cr + 3.3 x %Mo + 16 x %N
The PREN difference between the bottom and the top of the chemical specification is:
21 + (3,3 x 2,5) + (16 x 0,08) = 28,08
23 + (3,3 x 3,5) + (16 x 0,2) = 37,75
To maximize the Pitting Corrosion of UNS S31803 manufacturers were producing this grade to the upper end of the allowable specification. Following this, a new specification was introduced called UNS 32205 or F60 according to ASTM A182, intended to distinguish the duplex produced at the top of the composition.
The chemical composition of UNS 32205, %:
The minimum PREN for this grade is 34,14. Duplex made according to UNS 32205 is therefore interchangeable with UNS 31803, but not the other way around.
For more demanding applications, in particular for the offshore oil and gas production, one has developed superduplex (PREN range 38 – 45) and even hyperduplex (PREN range >45). For less demanding applications Lean Duplex is applied (PREN range 22-27).
Arcus provides a wide range of pipes, flanges and fittings in various types of duplex and other grades. Please consult our experienced sales team for any enquiries or questions you might have.