Courtesy of Siemens
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.