50 grades of stainless steel
BSSA

The BSSA has selected 50 Grades of Stainless Steel as its theme for 2014. Each week throughout the year, we will showcase one grade and illustrate the variety of corrosion resistant, mechanical and physical properties which determine its suitability for a wide range of applications.

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17

304LN 1.4311 S30453

MRI Scanner

 

MRI Scanner

Courtesy of Siemens

Cryogenic Portable Tank Container

 

Cryogenic Portable Tank Container

Courtesy UBH International

304LN is used where an extremely low level of magnetic permeability is required. This derives from its high nitrogen content.

Approximate Composition – 18% Cr, 9% Ni, 0.15% N (Exact composition ranges vary between EN and ASTM standards).

This grade combines the following characteristics:

  • Moderate pitting and crevice corrosion resistance
  • Excellent impact toughness at sub-zero temperatures
  • Higher 0.2% PS than 304L
  • Non-magnetic. Very low ferrite level and very resistant to martensitic transformation. Relative magnetic permeability < 1.005
  • Good weldability. Care needed to avoid solidification cracking

304LN was originally developed as a higher strength version of 304L using nitrogen. However, the increase in strength is only modest and duplex grades are now mostly used for this purpose. The grade is now used in specialised applications where a very low magnetic response is needed. In standard 304L, a typical level of relative magnetic permeability is around 1.05-1.1 due to small amounts of ferrite in the austenitic structure. A completely non-magnetic material would have a value of 1.0. 304LN can achieve a maximum value of around 1.005, which is the lowest practical level of any steel grade. It is also very resistant to transformation to the magnetic martensitic phase which can occur on forming or cooling to very low temperatures. Therefore, its main uses are in specialised equipment where magnetic “interference” needs to be avoided and/or for use in cryogenic equipment.

Applications which illustrate these features include:

Magnetic resonance imaging (MRI) scanners, reinforcing bar for structures surrounding MRI scanners, pressure vessels, cryogenic applications.

304LN is the archetypal non-magnetic grade. 

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