What is a super austenitic stainless steel?

A super austenitic stainless steel is considered to be one with a Pitting Resistance Equivalent Number (PREN) greater than 40. This is usually attributed to alloys with high additions of molybdenum, typically 6% or greater.

Austenitic stainless steels are the most commonly used family of stainless steels in general engineering applications. The 3xx series are relatively straightforward to fabricate and weld, possess excellent toughness across a wide range of temperatures, and reasonable levels of corrosion resistance. By increasing the amount of chromium and molybdenum present, it is possible to significantly increase the resistance to pitting corrosion. However, in order to retain the favourable mechanical and physical properties of austenitic stainless steel, the nickel content must also be increased. This will increase their prices too, making alternative grades of more interest.

Alloy 254 (6Mo, UNS S31254, F44, 1.4547, 254SMO) is the most common so-called super austenitic stainless steel. The high molybdenum content helps it achieve a PREN of 43, compared with just 25 of Alloy 316L. Alloy 654 (UNS S32654, 654SMO, 1.4652) contains nearly 8% molybdenum which pushes its PREN to 54. Some people consider Alloy 904L (UNS N08904, 1.4539) a super austenitic stainless steel too, although it lies somewhere between a nickel-based alloy and stainless steel, with 25% nickel and 20% chromium. Such is the raised alloy content of these grades that many of their historical applications have been substituted by duplex and super duplex stainless steels.