Many people on the internet believe that non-rusting stainless steel is not magnetic. Based on this misconception, they then recommend using a magnet for purposes of identification. With simple steel, they claim, the magnet will cling to the metal, but not with high-quality stainless steel.
This statement is essentially false. Using a magnet for the identification of non-rusting stainless steel is not recommended by experts!
Non-rusting steels include ferrites and martensites (chromium steel), duplex or austenites (chromium-nickel steel). These may be magnetic or non-magnetic, depending on their composition. Technically, one speaks here of magnetisable or non-magnetisable.
When is a metal “magnetic”?
With regard to magnetic properties, we are talking about ferromagnetism. This property is present when a metal is attracted by a magnet. The metal can itself become magnetised, and the magnetisation continues for a time even after the external magnetic field is gone.
The force of attraction between a magnet and a ferromagnetic material is the basis of how we perceive and recognise magnetism in everyday life.
The most common examples of ferromagnetic metals are nickel, cobalt, iron and steel. Non-rusting steel groups such as ferrite, martensite and duplex are also magnetic.
When is a metal considered “non-magnetic”?
Paramagnetism is a form of magnetism in which a material exhibits no measurable magnetism when no external magnetic field is present. In the presence of an external magnetic field, however, the magnetisation can be measured. The external magnetic field is also intensified.
For everyday purposes, paramagnetic metals do not react noticeably to a magnet. They are not attracted by a magnet and are therefore commonly referred to as non-magnetic.
Typical paramagnetic metals are, for example, aluminium, chromium, platinum and austenitic non-rusting steel.
How is the magnetisability measured?
One physical property to be determined is µr. This value describes how easily a material can be magnetised. The higher the number, the greater the magnetisability.
Paramagnetic materials have a µr value not much greater than one. Ferromagnetic materials typically have a µr value in the range of 100 up to several thousand.
Why do people think that non-rusting stainless steel is “non-magnetic”?
Roughly 70% of all non-rusting steels used throughout the world are chromium-nickel steels. These stainless steels are austenites and are therefore not magnetisable. In other words, the materials that are commonly known as V2A or V4A are very often used when non-rusting stainless steel is called for. The chances are therefore good that a sink, the insides of a washing machine and other everyday objects were produced of precisely this material.
Strictly speaking, most chromium-nickel steels are not entirely austenitic. In practice, it is the case that the V2A and V4A grades always contain certain portions of ferrite (called delta ferrite). Delta ferrite shares of up to 10% are quite typical. From an everyday perspective, however, this is not generally sufficient to produce noticeable magnetisation.
How do “non-magnetic” non-rusting stainless steels become “magnetic”?
If the material is subjected to a certain degree of plastic cold deformation, then its structure changes in the reshaped areas. This occurs already with a slight alignment or bending of the material.
The austenitic structure undergoes lattice shearing to form what is called deformation induced martensite. This is then ferromagnetic. Depending on the material and the degree of deformation, this can result in technically “non-magnetic” chromium-nickel steels becoming noticeably magnetic. In the deformed area, the metal then reacts noticeably to typically available magnets with perceptible attraction. Examples include bends in pipes, screws or the cut edge of a stainless steel plate.
Is non-rusting stainless steel of lower quality when it is magnetic?
The question of whether magnetic stainless steel is of lesser quality can be clearly answered with “No”. The material is selected based on its intended use. For instance, the bottom of an induction pot is particularly good when it is highly magnetisable. It can then be heated up especially quickly. The pot itself, however, is frequently made of V2A because it must withstand corrosive liquids and acids.
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