Austenitic stainless steel is generally non magnetic with magnetic permeabilities of around 1.0. Permeabilities above 1.0 are associated with the amount of either ferritic or martensitic phases present in the austenitic stainless steel and so depend on:
- cold working and heat treatment conditions
- chemical composition effects
Cold Working and Heat Treatment
Cold working of austenitic stainless steel can partially transform austenitic to martensitic. As martensitic stainless steel is ferromagnetic, cold working austenitic stainless steel can show a degree of ‘pull’ towards a magnet. This usually occurs at sharp corners, sheared edges or machined surface but can be detected on wrought products such as rods or bars which may have been cold straightened, following the final hot rolling or annealing in the mill.
The degree to which this occurs depends on the compositional effects of austenitic stabilising elements. High nickel or nitrogen bearing grades tolerate more cold working before localised increases in permeability are noticed. These increases in permeability can be reversed by full solution annealing at temperatures around 1050 / 1120C with rapid cooling. This transforms any cold-formed martensitic to austenitic, the non-magnetic phase, which is then retained on cooling.
The best austenitic stainless steel types for low permeability applications are those with high austenitic stability as these have low permeability in both annealing or cold working conditions. These include the nitrogen bearing types, 304LN (1.4311) and 316LN 1.4406 or the high nickel types such as 310 1.4845.
Source: Zhejiang Yaang Pipe Industry Co., Limited (www.yaang.com)