Stainless Steel - Magnetic Properties

Topics Covered

Introduction
Austenitic Grades
Stress Relieving
Cold-Working
Magnetically Soft Stainless Steels

Introduction

Magnetic permeability is a property of a material that responds to magnetism. It is usually represented based on the extent to which a magnet attracts the material. Except austenitic grades, all types of stainless steels strongly respond to a magnetic field.

Austenitic Grades

Austenitic stainless steel grades exhibit low magnetic permeability, because of which they do not show any response to magnets in the annealed condition. However, they show some amount of magnetic response following cold-working process, such as heavy polishing, shot blasting, centerless grinding, rolling or wire drawing. Grade 304 steels attain quite strong magnetic response following substantial cold working, while grades 310 and 316 will remain non-responsive to a magnet in most cases.

Variations in magnetic permeability are mainly due to factors such as martensitic formation and atomic lattice straining. Generally, magnetic response due to cold-work will be less in the conditions of higher nickel to chromium ratio. Therefore, magnetic response is only used for classifying different grades of stainless steels.

Stress Relieving

Stress relieving of any austenitic stainless steel can reverse the magnetic response generated from cold-working. This conversion of magnetic steels to a non-magnetic condition can be performed by heating them to temperatures of about 700 to 800°C using an oxy-acetylene torch. However, the steels may become sensitive to carbide precipitation if they are not stabilized.

When the steels are solution treated at temperatures of 1000 to 1150°C, their magnetic response can be removed without affecting the corrosion resistance. While considering magnetic permeability as a key factor of the designing process, or incorporating it into a specification, it is essential to mention the permeability during the time of purchase of stainless steels.

Cold-Working

Most of the polished and cold-worked steel bar products, such as grades 303 and 304, will have a considerable quantity of magnetism gained through cold-working. However, grades 310 and 316, having high nickel content, tend to have much less magnetism. The rate of stimulation of magnetic response due to cold-working can be altered with respect to the chemical control of single standard analysis range.

Magnetically Soft Stainless Steels

Stainless steels need to be magnetically soft for certain applications, such as solenoid shafts. These shafts often require plungers that are sensitive to the magnetic field caused by the surrounding coil in the presence of electric current. The plungers will regain their original position when the supply of current is cut, allowing the magnetic field to collapse. Steels showing this kind of behaviour are referred to as “magnetically soft”.

Magnetically soft Ferritic stainless steels, variants of grade 18/2 (18% chromium and 2% molybdenum) steels are usually employed for corrosion resisting applications. Controlled additions of silicon and sulphur avoid the need for machining these steels. However, magnetic properties of the steels can be enhanced using special mill processing.

Date Added: Jan 2, 2002 | Updated: May 1, 2014
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