Stainless Steels

Austenitic Stainless Steels

Austenitic stainless steels are the most commonly used grades of stainless steel as they can provide predictable levels of corrosion resistance and excellent mechanical properties. Easily recognised from their non-magnetic properties, the completely austenitic grain structure means that they are extremely formable, readily welded and retain their ductility down to cryogenic temperatures. They can also be used at raised temperatures, as they retain more of their strength in comparison with ferritic alloys. They can be significantly strengthened by cold working, but this work hardening effect also means that machining may need to be undertaken more carefully. Larger section sizes can be strengthened by warm working. As a wholly austenitic structure, heat treatment has no effect.

Good corrosion resistance is imparted by 16-25% Cr additions, but can be significantly improved by further additions. For instance, Alloy 316L benefits from the addition of molybdenum to improve resistance to pitting and crevice corrosion in chloride environments. Alloy 254 contains significant levels nickel, molybdenum and copper to further enhance corrosion resistance in aggressive environments, making it suited to brackish or polluted water contact, or in applications containing halide ions (chloride, bromide, fluoride) where it can be a cost-effective alternative to more expensive nickel and titanium alloys in such environments.

The range of Fermonic^® 50 grades (also known as Nitronic^® 50 – a trademark of AK Steel Corporation) provide more than double the strength of Alloy 316L for instance, benefiting from controlled nitrogen additions to strengthen the austenitic structure. Higher strength allows section sizes to be reduced, whilst retaining the favourable characteristics of austenitic grades in general. The Fermonic^® 60 product (also known as Nitronic^® 60 – a trademark of AK Steel Corporation) is more specialised – it has superior galling resistance resulting from additions of manganese and silicon, making it preferable for applications where there is repeated movement/sliding or frequent assembly/disassembly.