Duplex and Super Duplex What the PREN Number Tells You

A Microstructure Made of Two Others

Stainless steels are separated into classes by their microstructural phase. Duplex grades have a mixed microstructure combining austenite and ferrite in an equivalent ratio, which means roughly 50 percent of each. Super duplex is one of three subcategories of duplex, so it carries the same dual microstructure.

What PREN Measures

Duplex grades have very high pitting resistance equivalent numbers, ranging from 22 to 45 across the family. Super duplex grades sit at the top of that band, between 38 and 45. The number comes from the chemistry, and the higher figure is why super duplex is chosen for the most aggressive chloride service.

Related product: 2205 Duplex Stainless Steel Sheet

Why Higher Alloying Cuts Both Ways

Super duplex chemistry is more highly alloyed than any conventional stainless such as 304 or even 316, which is where its corrosion resistance and high mechanical strength come from. The same high content of chromium, nickel, molybdenum, nitrogen and tungsten also makes intermetallic phases form more easily, and those phases cut impact resistance sharply.

Why the Manufacturer Matters More Here

Because the intermetallic risk is processing dependent, a super duplex product made by faulty methods performs very poorly in service. That is why buyers of these grades need to deal with manufacturers and suppliers who are experienced with them, rather than treating the grade as a commodity.

Choosing Between Them

2205 duplex gives pitting and crevice corrosion resistance superior to 316L or 317L in almost all corrosive media, with high strength, excellent impact toughness, high corrosion and erosion fatigue properties, plus lower thermal expansion and higher thermal conductivity than austenitic grades. Move to super duplex only when the chloride load genuinely demands the higher PREN.

Related product: Stainless Steel Equal and Reducing Tee