A super austenitic stainless steel is an austenitic grade with a pitting resistance equivalent number (PREN) greater than 40. This is usually achieved through high additions of molybdenum, typically 6% or more.
Definition: a super austenitic stainless steel is any austenitic grade engineered to a PREN above 40, almost always through molybdenum additions of 6% or more.
Austenitic stainless steels are the most commonly used family in general engineering. The 3xx series are relatively straightforward to fabricate and weld, tough across a wide temperature range, and reasonably corrosion resistant. Increasing chromium and molybdenum significantly raises pitting resistance, but to keep the favourable mechanical and physical properties of austenitic stainless steel the nickel content must also rise. That increases price, which makes alternative grades more attractive.
Alloy 254 (6Mo, UNS S31254, F44, 1.4547, 254SMO) is the most common super austenitic stainless steel. Its high molybdenum content gives a PREN of 43, against just 25 for Alloy 316L. Alloy 654 (UNS S32654, 654SMO, 1.4652) contains nearly 8% molybdenum, pushing its PREN to 54. Some regard Alloy 904L (UNS N08904, 1.4539) as super austenitic too, although it sits between a nickel-based alloy and a stainless steel, with 25% nickel and 20% chromium.
Because these grades are so highly alloyed, many of their historical applications have been substituted by duplex and super duplex stainless steels.
| Grade | UNS | PREN |
|---|---|---|
| Alloy 316L | S31603 | 25 |
| Alloy 254 (6Mo, 254SMO) | S31254 | 43 |
| Alloy 654 (654SMO) | S32654 | 54 |