Effect of cold work and heat treatment on the magnetic permeability of austenitic stainless steels

Introduction

Austenitic stainless steels are generally non-magnetic with magnetic permeabilities of around 1.0. Permeabilities above 1.0 are associated with the amount of either ferrite or martensite phases present in the 'austenitic' steel and so depend on:

  • cold working and heat treatment conditions
  • composition effects

This article discusses the effects of cold working and heat treatment.

Cold working and heat treatment

Cold working of austenitic stainless steels can partially transform austenite to martensite. As martensite is ferromagnetic, cold worked austenitic stainless steels can show a degree of 'pull' towards a magnet. This usually occurs at sharp corners, sheared edges or machined surfaces 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 austenite 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 / 1120 C with rapid cooling). This transforms any cold-formed martensite back to austenite, the non-magnetic phase, which is then retained on cooling.

The best austenitic stainless steel types for low permeability applications are those with high austenite stability as these have low permeability in both annealed or cold worked conditions. These include the nitrogen bearing types, 304LN (1.4311) and 316LN (1.4406) or the high nickel types such as 310 (1.4845).

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