What is Nitronic 50?

29th August 2019

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Nitronic 50 is a higher-performance austenitic stainless steel that provides approximately twice the yield strength of regular 3xx series stainless steels and improved corrosion performance. It is widely used in a variety of marine and industrial applications.

Compared with Alloy 316L, it contains increased chromium and molybdenum additions which support the improved corrosion resistance. Whilst Alloy 316L would have a Pitting Resistance Equivalent Number (PREN) of 25, the PREN of Nitronic 50 is 34. Super duplex stainless steels attain a PREN >40, but with different mechanical properties. This is the main reason why Nitronic 50 would be specified instead of super duplex alloys. It is listed in NACE MR 0175 for sour service applications given its ability to resist sulphide stress cracking.

In order to maintain the favourable mechanical properties of an austenitic stainless steel, such as ductility and toughness at both elevated and cryogenic temperatures, plus ease of welding and fabrication, a significant increase in the nickel content is required. Without this nickel addition, the microstructure would tend towards a duplex product, with even higher strengths but lower ductility and toughness. However, this does mean that Nitronic 50 is a more expensive alloy than duplex and super duplex stainless steels.

Like some super duplex stainless steels, it also uses an increased nitrogen content to improve the strength, so is sometimes referred to as a nitrogen-strengthened alloy. Additions of manganese also push up the strength through solid solution strengthening.

Nitronic 50 is a trademark of AK Steel Corporation. This alloy is also widely referred to as XM19, which is the designation of the generic alloy caption in ASTM A276 and ASTM A479, as well as UNS S20910 or DIN 1.3964.

This alloy is available in two conditions. The first condition is simply solution annealed, which is easier the easier to machine form. However, most of Langley Alloys customers specify the higher-strength version, which is offered in the hot rolled or strain hardened condition dependent upon the diameter. The larger diameters can attain the higher strength whilst still falling below the maximum hardness level (HRC<35) specified in NACE through hot rolling – using lower than normal rolling temperatures to influence the microstructure. For smaller diameters, we have found that the strain hardened condition achieves more consistent properties and lower through-bar hardness.

Langley Alloys’ Fermonic 50 is not just an alternative trademark. For the smaller diameters, our manufacturing route is unique to us, and ensures we can achieve the desired ‘High Strength’ properties across the widest possible range of bar diameters.

Common applications for Fermonic 50 include:

  • Valves fittings, particular stems due to the excellent alloy toughness
  • Marine applications, include rod rigging using cold drawn lengths, tie-downs and fittings
  • Pump shafts
  • Sucker rods for oil rigs
  • Fasteners