Are stainless steels magnetisable?

Traditionally, it is assumed that stainless steels are non-magnetic. In fact, one of the most popular tests to detect whether a steel is stainless or not is to put a magnet next to it.

However, the magnetic characteristics of stainless steels actually depend on their chemical composition and their metallurgical structures. For example, ferritic or martensitic-type stainless steels (which include ferrite or martensite-type crystalline structures) are magnetisable and are therefore attracted to a magnet. These are the F1 and C1, C3 and C4-grade steels according to the EN 3506-1 standard: "Mechanical characteristics of corrosion-resistant stainless steels. Part 1: Bolts, screws and pins".

Meanwhile, austenitic-type steels (those that include austenite-type crystalline structures) are not magnetisable in the annealed state and therefore should not be affected by the presence of magnetic fields; these are the cases of A2 and A4-grade steels according to the EN 3506-1 standard, or its equivalent designation AISI 304 and 316, respectively. However, when they are cold worked (for example: drawn, rolled or cold formed), these steels can present a weak magnetisation, meaning that they could be weakly attracted by a magnet. This effect does not alter their corrosion resistance properties, which are in any case based on their chemical composition; therefore, it should not be concluded that the possible attraction of an austenitic stainless steel to a magnet (grades A2 and A4) may be detrimental to its quality in any way.

The magnetic sensitivity of a steel is measured by "relative magnetic permeability", represented by the Greek letter µ_r.

• Elements with values µ_r >> 1: are called “ferromagnetic”. They will be strongly attracted to a magnet.
• Elements with µ_r < 1: are called “diamagnetic”. They will not be attracted to a magnet.
• Elements with µ_r ≈ 1: are called "paramagnetic". They will be weakly attracted to a magnet the further away they are from the unit.

Typical values of relative magnetic sensitivity are:

• Iron µ_r > 200
• Cold rolled carbon steels µ_r > 180
• Ferritic stainless steel F1: µ_r 5
• A2-grade austenitic steel: µ_r 1.8
• A4-grade austenitic steel: µ_r 1.015

The CH-A2, MTH-A2, MTH-A4, MTP-A4, TNUX-A4 stainless steel anchors are made of austenitic steel, and are therefore susceptible to slight attraction by a magnet as long as the base material has been subjected to cold working (drawing, cold deformation or cold rolling) during the manufacturing process.