Composition effects 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:

  • chemical composition
  • cold work and heat treatment conditions

This article discusses composition effects.

 

Composition effects

Grades 304 (1.4301), 321 (1.4541) and 316 (1.4401) have 'balanced' compositions to enable them to be readily weldable. This is achieved by ensuring that in their normal annealed (softened) condition, they contain a few percent of delta ferrite. This results in permeabilities slightly over 1.0.

Additions of nickel and nitrogen promote and stabilise the austenite phase, whereas molybdenum, titanium and niobium stabilise ferrite.

The lowest permeability austenitic stainless steels are therefore the nitrogen bearing 304LN (1.4311) and 316LN (1.4406) types or the high nickel 310 (1.4845) and 305 (1.4303) types.

In contrast, higher permeabilities can be expected in grades such as 301 (1.4310), 321 (1.4541) and 347 (1.4550), with either lower nickel contents or additions of titanium or niobium, which are powerful ferrite stabilising elements.

During the welding of these steels, structural changes occur. Some of the austenite in the parent material can transform to delta ferrite at high temperatures and on cooling this is partly retained at room temperature. Welding filler rods and wires are usually 'over-alloyed' to prevent dilution in the fusion zone but more importantly are balanced to have deliberately high ferrite levels of 5% or sometimes 10%, to minimise the risk of hot cracking during welding.

Consequently the permeability of the metal in the weld and the surrounding heat affected zone can be significantly higher than in the original parent material. Similar effects can occur following plasma or flame cutting of austenitic stainless steels.

In general, castings have compositions with a bias towards ferrite compared to wrought grades and consequently will be more magnetic.

Effect of Cold Work and Temperature on Martensite Formation

The transformation of austenite to martensite can be triggered either by cold work or by the effect of low temperatures. The stability of an austenitic steel to such transformation is measured by using the Md30 temperature. This is defined as the temperature at which 50% of the austenite originally present will be transformed to martensite when subjected to a cold true strain of 0.30. This is about 35% engineering strain. The formula to calculate this temperature was first proposed by Angel and subsequently modified to take account of the grain size.

Md30 = 551 - 462(C+N) - 9.2Si - 8.1Mn - 13.7Cr - 29(Ni + Cu) – 18.5Mo - 68Nb – 1.42 (ASTM grain size - 8)

It will be noted that all elements contribute to the stabilisation of austenite to the martensite transformation. The following table gives an approximate value for some common austenitic steels:

Steel Type Md30 (deg C)
 1.4310 (301)  +20
 1.4372(201)  +20
 1.4301(304)  -20
 1.4307(304L)  -30
 1.4311(304LN)  -80

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