302 Stainless Spring Wire - Cold Worked Magnetism Control

A 302 stainless spring wire is austenitic and should not be magnetic, yet a finished spring made from it often is. Cold drawing and cold coiling transform part of the austenite into deformation martensite, and that phase is ferromagnetic. The effect is negligible in a lightly coiled spring and clearly measurable in a tightly coiled one made from heavily drawn wire. For most applications it can be ignored. For springs that sit beside a reed switch, a Hall sensor or inside an MRI examination room, it decides whether the part is acceptable at all. This page explains where the magnetism comes from, how much is normal, and what to write on the order when a non-magnetic spring is required.

Product Specifications

Why Cold Forming Makes Austenitic Wire Magnetic

Deformation Martensite

The austenitic structure of 302 is non-magnetic because its crystal lattice is face centred cubic, which does not hold a permanent magnetic moment. When the metal is deformed at room temperature, part of that austenite transforms into body centred martensite along the shear bands created by the strain. Martensite is ferromagnetic, so the wire acquires a magnetic response proportional to how much of the phase has formed. The amount depends on three things: the total strain, the temperature during deformation, and the stability of the austenite in that particular heat.

Why 302 Reacts More Than 316

Austenite stability is set mainly by nickel, carbon, manganese and nitrogen. 302 contains 8-10% nickel, while 316 carries 10-14% plus molybdenum, so 316 resists the transformation far better under the same deformation. 301, with lower nickel and higher carbon, transforms most readily and is deliberately used where high strength from cold work is wanted. The practical ordering is therefore straightforward: if a spring must stay non-magnetic after forming, 316L or 316 is a safer starting point than 302, and 302 is a safer starting point than 301.

What the Magnet Does Not Tell You

A magnetic spring is not a defective spring. The transformation does not reduce corrosion resistance in a general sense, does not indicate the wrong grade, and does not by itself change the tensile strength, the fatigue limit or the dimensional accuracy. A customer who reports that the wire is magnetic is describing a phase condition, not a quality failure. The correct response is to establish whether the application actually needs low permeability, and only then to change the grade or add an annealing step.

Measuring and Specifying the Magnetic Response

The Practical Threshold

Magnetic response is expressed as relative permeability, the ratio of the material permeability to that of vacuum. Annealed austenitic wire sits around 1.002-1.005, which is close enough to non-magnetic for almost any purpose. After cold drawing and coiling the figure typically moves into the 1.05-1.5 range, and heavily drawn fine wire can go higher. Where the requirement is critical, the acceptance limit is usually written as a number rather than described with a word: some instrument applications accept 1.05, magnetic resonance environments often ask for 1.01 or below, and a few sensor designs need the annealed condition specifically.

How to Check a Coil or a Finished Spring

Three methods cover most situations. A permanent magnet gives a quick go-no-go answer and is enough to separate a non-magnetic delivery from an obviously magnetic one. A gauss meter or permeability meter gives a number that can be compared with the purchase specification, and ASTM A342 describes the permeability measurement for magnetic materials in this range. For finished springs the most meaningful test is the one that mimics the application: bring the spring to its working position next to the sensor and confirm that the switching point has not moved.

Applications

Sensor and Reed Switch Springs

A reed switch operates on a small magnetic field, and a spring placed within a few millimetres of it can shift the actuation point or hold a contact closed. Springs in this position are normally specified in 316L with a stated permeability limit, and the coil is checked after forming rather than only the wire before forming.

Medical and Analytical Equipment

Magnetic resonance imaging rooms, electron microscopes and mass spectrometers all impose limits on the magnetic moment of components brought near the working volume. Springs and wire forms for these instruments are usually specified with an upper permeability figure and a test method, with the sample taken from the finished part because the coiling operation is what creates the martensite.

Fasteners and Clips near Magnets

Clips that retain a magnet, a sensor or a small motor are often made from 302 for its strength, and the resulting magnetic response can distort the field the assembly depends on. Where the geometry cannot be changed, a soft annealed or a 316 grade wire keeps the permeability close to the annealed value.

Product Advantages

• Relative permeability stated for the delivery condition, so the magnetic requirement can be checked against a number rather than assumed.
• 316L and 316 grades available for applications where the formed spring must stay close to non-magnetic.
• Bright annealing at 1050-1120 °C with fast cooling, restoring the austenitic structure and returning permeability toward the annealed value.
• Diameter tolerance of ±0.01 mm on fine sizes, keeping the deformation during coiling consistent from part to part.
• Grade advice based on the actual magnetic limit, since a higher nickel grade is often cheaper than sorting finished springs.

Quality Control

Diameter, tensile strength and elongation are tested on every lot, and the surface is inspected over the full coil length. Where a permeability limit is specified, the measurement method and the sample position are agreed in writing, and the result is reported against the heat number. Because the transformation depends on the strain history, the coiling operation is treated as part of the magnetic specification rather than as a separate downstream matter.

Selection and Ordering Notes

1. State the permeability limit and the test method if the application is magnetically sensitive, instead of asking for a non-magnetic grade in general terms.
2. Say whether the measurement is to be made on the wire or on the finished spring, since coiling adds strain and raises the value.
3. Choose the grade against the requirement: 302 where magnetism is irrelevant, 316L where a low figure must be held after forming.
4. Consider a soft annealed or a bright annealed delivery where the part is formed with little reduction and must stay close to the annealed permeability.
5. For sensor assemblies, describe the distance to the sensor and the switching point allowed, as that is what the permeability limit ultimately protects.

Magnetism in austenitic spring wire is a phase effect created by forming, and it is controlled by choosing the grade and the delivery condition before the spring is made. Send the application, the working distance to the sensor and the acceptable permeability, and the wire specification can be matched to the magnetic requirement as well as to the mechanical one.