Why is austenitic stainless steel mesh magnetic?

Why is the stainless steel mesh woven from austenitic stainless steel wire 304 grade magnetic? Austenitic stainless steel wire is non-magnetic, so why can it be stuck to a magnet when it is woven into a mesh by a mesh weaving machine? Recently, our customers have questioned our stainless steel mesh.

It is very common for austenitic stainless steel wire to become magnetic after being woven into a mesh, which seems to contradict the common sense that austenitic stainless steel is "non-magnetic". The root cause lies in the local microstructural changes that occur during the processing, mainly strain-induced martensitic phase transformation.

1. Austenitic stainless steel is essentially non-magnetic: Austenitic stainless steel (such as 304, 316, etc.) has a face-centered cubic austenite phase in the fully annealed state. This crystal structure makes it usually paramagnetic at room temperature and macroscopically non-magnetic (cannot be significantly attracted by a magnet).

2. Severe cold working introduced by the weaving process: The process of weaving stainless steel wire into a mesh involves a lot of plastic deformation:

Bending: The steel wire is repeatedly bent under the guidance of the heald, reed and needles of the mesh weaving machine.

Stretching: The steel wire is subjected to tension in the warp and weft directions.

Friction: Friction occurs between the steel wires and between the machine parts.

These deformations are all carried out at room temperature and are typical cold working.

3. Strain-induced martensitic phase transformation: This is the most critical reason.

Austenitic stainless steel (especially metastable austenitic stainless steel, such as the most commonly used 304) becomes unstable in its crystal structure when it is subjected to cold working deformation.

Part of the austenite will transform into a new phase called martensite. This martensite is a body-centered cubic or body-centered tetragonal structure. Martensite is ferromagnetic! It contains magnetic domains and can be attracted by magnets.

4. Why does the phase transformation occur locally?

The deformation during the weaving process is not uniform. The deformation is mainly concentrated in:

The bending part of the steel wire (stress concentration point).

The area where the steel wires cross, rub and press each other.

The surface of the steel wire (subject to the greatest friction and shear force).

In these local areas, the degree of cold working (strain) exceeds the critical value for inducing martensitic phase transformation, resulting in the transformation of austenite to martensite.

5. Although the stainless steel wire body may still be austenite (non-magnetic) in the undeformed area, tiny martensitic areas are formed at those local points that have undergone severe deformation (especially intersections and bending points). When a magnet is brought close to the mesh, especially close to these nodes (intersections), the magnet will attract these local ferromagnetic martensite, making the entire mesh weakly magnetic.

The strength of the magnetism depends on:

Stainless steel grade: Metastable austenitic steel (such as 304) is more prone to strain-induced martensitic phase transformation than stable types (such as 310 and 316, which may also occur under high deformation, but to a lesser extent). The lower the nickel content and the higher the carbon content, the greater the tendency of phase transformation.

Amount of cold working deformation: The more severe the deformation (greater weaving tension, smaller bending radius, more severe friction), the more martensite is produced and the stronger the magnetism.

Detection location: Detecting magnetism near the node is usually stronger than detecting in the middle of the mesh.

6. Even if a complete martensitic phase transformation does not occur, a large number of crystal defects (dislocations) introduced by cold working may slightly increase the magnetic permeability of the material, making it more susceptible to being weakly attracted by strong magnets. But this is usually a secondary factor, and the main magnetism comes from martensite. In addition, during processing or subsequent treatment, the mesh may come into contact with a steel mesh loom (especially magnetic accessories), which absorbs tiny ferromagnetic particles. But this can usually be removed by thorough cleaning and is not an intrinsic cause.

The main reason why austenitic stainless steel wire exhibits magnetism after being woven into a mesh is that the cold working deformation (bending, stretching, friction) during the weaving process induces a phase transformation from austenite to ferromagnetic martensite in local areas of the steel wire (especially intersections and bends). These tiny martensite areas scattered on the non-magnetic austenite matrix make the entire mesh attractive to magnets. This is a typical example of the effect of work hardening on the microstructure and properties of the material, not that the material itself is magnetic in the annealed state.

How to eliminate or weaken this magnetism?

1. Choose a more stable austenitic grade: Use grades with higher nickel content or added stabilizing elements (such as nitrogen), such as 316L, 317L, 904L, or high-nickel alloys (such as Alloy 20), which are less sensitive to strain-induced martensitic phase transformation.

2. Annealing: The woven mesh is subjected to high-temperature solution treatment (usually above 1000°C and then rapidly cooled). This can dissolve the martensite formed during cold working and restore the full austenitic structure, thereby eliminating magnetism. However, annealing will soften the mesh, which may not meet strength requirements, and may change the mesh size and surface condition.

3. Optimize the weaving process: Try to minimize the degree of deformation during the weaving process (such as reducing tension and increasing the bending radius), but it is usually difficult to completely avoid local high strain areas.

Therefore, for commonly used austenitic stainless steel wire meshes such as 304, slight magnetism is a normal phenomenon after cold processing (weaving), mainly caused by strain-induced martensitic phase transformation. Why is austenitic stainless steel mesh magnetic?

Why is the stainless steel mesh woven from austenitic stainless steel wire 304 magnetic? Austenitic stainless steel wire is non-magnetic, so why can it be stuck to a magnet when it is woven into a mesh by a mesh weaving machine? Recently, our customers questioned our stainless steel mesh.

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