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.
|Alloy||Common Name||Related Specifications||Tensile Strength||Proof Test||Elongation|
|British||European||United States||N/mm2 (ksi)||N/mm2 (ksi)||(%)|
|Alloy 316L||UNS S31603||1.4404
|515 (75)||205 (30)||35|
|Fermonic 50 – Annealed||UNS S20910 XM-19||1.3964
|690 (100)||415 (60)||35|
|Fermonic 50 – High Strength / Extra High Strength||UNS S20910 XM-19||1.3964
|862 (125)||724 (105)||20|
|Fermonic 60||UNS S21800||ASTM A479/A276
|655 (95)||345 (50)||35|
|Alloy 254||UNS S31254||1.4547
|650 (94)||300 (44)||35|
Alloy 316L is an austenitic stainless steel supplied in the hot worked and annealed condition. The addition of Mo provides the product with much improved corrosion resistance compared with Alloy 304, particularly with respect to pitting and crevice corrosion in chloride environments. Alloy 316L resists atmospheric corrosion, as well as, moderately oxidizing and reducing environments. It also resists corrosion in polluted marine atmospheres. The alloy has excellent resistance to intergranular corrosion in the as-welded condition.
Fermonic 50 – Annealed:
Fermonic® 50 – Annealed is a nitrogen-strengthened austenitic stainless steel which is supplied in the annealed condition. It provides almost double the yield strength of common austenitic stainless steel grades such as Alloy 304 and Alloy 316L, offering the potential to reduce section size and therefore weight and cost. Higher strength can be achieved through cold working. Importantly, it remains fully austenitic – and therefore, fully non-magnetic – even after severe cold-working.
Fermonic 50 – High Strength / Extra High Strength:
Fermonic® 50 – HS/EHS is a nitrogen-strengthened austenitic stainless steel that can be supplied in the cold worked or warm worked condition. Although very small diameter bars of Fermonic® 50 (<2.25”) can be cold worked to achieve higher strength levels, larger section sizes can only achieve dramatically increased mechanical properties by deforming at raised temperatures. This results in a deformed and partly recrystallised micro structure that is ‘stress free’, achieves high strengths with good ductility and consistent properties through the bar (unlike cold worked material). The superior properties of Fermonic® 50 HS/EHS are available up to 9” diameter bars.
Fermonic® 60 is a fully austenitic alloy supplied in the hot worked and annealed condition. The general corrosion resistance is between that of Alloy 304 and Alloy 316 stainless steel, whereby is it generally better than Alloy 304 in most environments but superior to Alloy 316 with respect to chloride pitting resistance. More significantly, additions of Silicon and Manganese provide superior wear, galling and fretting resistance than many other corrosion resistant alloys making it preferable for applications where there is repeated movement/sliding or frequent assembly/disassembly.
Alloy 254 is a high alloy austenitic stainless steel supplied in the hot worked and annealed condition. Significant additions of Mo, Ni and Cu gives this alloy a good resistance to pitting and crevice corrosion, especially in environments containing halide ions e.g. chloride, bromide and fluoride solutions. It is therefore widely used in applications in brackish water, seawater, pulp mill bleach plants and other chloride process streams, where it can be a cost-effective alternative to more expensive nickel and titanium alloys in such environments.