Magnet Test for Stainless Steel and Nickel Alloys: What It Sorts and What It Cannot

Is Inconel Magnetic?

No. The Inconel grades, including 600, 601, 617, 625, 690 and 718, are non-magnetic at room temperature, and a magnet will not grip them. The same holds for Hastelloy, Incoloy, Nimonic and Haynes alloys and for the cast superalloys used in turbine blades.

The reason lies in the structure. Pure nickel is magnetic, but these alloys keep nickel in a face-centred cubic (austenitic) structure diluted with large amounts of chromium, iron, molybdenum and other elements, which suppresses the magnetism at room temperature. The model below shows that structure for Inconel 625.

Two cautions follow. First, "non-magnetic" is not proof of Inconel, because austenitic stainless steels such as 304 and 316 behave the same way. Second, nickel alloys low in alloying additions, above all Monel 400, sit close to the point where magnetism switches on, so they can give a faint pull. For the full set of checks before calling something Inconel, see how to tell if scrap is Inconel.

Which Scrap Metals Are Magnetic, and Which Are Not?

Carbon steel, ferritic and martensitic stainless steels and duplex grades are magnetic; nickel alloys, austenitic and super-austenitic stainless steels and titanium are not; a few materials sit in between. The table gives the response you should expect from clean, annealed metal at room temperature, and what to do next.

How Do You Run a Magnet Test Properly?

Run it the same way on every piece: one magnet, clean metal, away from welds and bends, and a simple three-level scale for the result. Consistency matters more than the magnet you choose, because the test is only useful when one piece can be compared with the next.

A strong pull means the magnet holds firmly and you have to lever it off. A weak drag means it slides but resists, or lifts a small piece only just. "None" means no difference from wood. Write the result on the piece with a paint marker if the lot will be handled again before testing.

A very strong rare-earth magnet will tug on materials that a normal sorting magnet ignores, which makes weak responses harder to read. A moderate magnet gives cleaner separation for this job.

How Should You Sort a Mixed Pile Using the Magnet Result?

Sort by the strength of the pull into three groups, then let weight and an analyser finish the job. The decision chain below is a common order to follow, because it removes the cheapest metal first and saves instrument time for the pieces that need it.

Take a lot from an offshore topside refit as a worked example: super duplex seawater piping, 316L instrument tubing, a few Monel 400 valve bodies and some carbon steel spools clad inside with Inconel 625. The magnet sends the super duplex and the clad spools to the strong-pull pile, the 316L to the no-pull pile and the Monel, unpredictably, to either. Read naively, that result puts the high-nickel 625 cladding in with the steel. Read with the decision chain in mind, it simply tells you which three piles to hand to the analyser, and that every strong-pull pipe needs its bore checked before it is written off as steel.

Weight is a surprisingly useful second check. Titanium has a density of about 4.5 g/cm³, against roughly 7.9–8 g/cm³ for 304 and 316 and 8.2–8.9 g/cm³ for most nickel alloys. A titanium offcut feels clearly light in the hand next to a stainless one of the same size. The difference between stainless and nickel alloy is too small to feel reliably, but on a full pallet of known dimensions the scrap weight calculator can show whether the weight is plausible for the grade claimed.

Why Does a Magnet Sometimes Give the Wrong Answer?

A magnet gives the wrong answer when something other than the base alloy decides the response: cold work, temperature, a different metal joined or clad to the piece, or contamination. Each of these is common in real scrap.

Cold Work

Heavy bending, cold drawing, spinning and machining can turn part of an austenitic stainless steel’s structure magnetic. Bends, cut edges, threaded ends and turnings of 304 can all show a slight pull. That does not make them ferritic stainless, so test the flat, undisturbed metal before deciding.

Temperature and Monel 400

Monel 400’s magnetic transition sits close to room temperature, so the same bar can drag a little on a cool morning and barely at all after a day in the sun. Never sort Monel by magnet. Its 31% copper makes it easy to spot on XRF instead, and the Monel 400 vs K-500 comparison covers how the two Monel grades are told apart.

Clad, Overlaid and Welded Parts

Oilfield and process pipe is often carbon steel clad or weld-overlaid on the inside with Inconel 625 or Incoloy 825. A magnet on the outside grips firmly, and the piece goes to the steel pile with valuable alloy inside it. Valve bodies with overlaid seats and steel flanges welded to alloy pipe do the same. Test both surfaces of any pipe and look at cut ends for a visible layer. The oil and gas scrap page covers where clad material comes from.

Contamination

Steel bolts left in a flange, a carbon steel backing ring or a coat of rust and filings on the surface can all make a non-magnetic alloy appear to pull. Strip attachments and test clean metal.

What Can a Non-Magnetic Result Not Tell You?

A non-magnetic result cannot tell you which alloy you are holding, and that is where the money is. The chart shows five metals that all leave a magnet cold, yet range from an iron-based stainless with about 11% nickel to nickel alloys carrying close to 60%, with very different molybdenum, niobium and tungsten.

That is why a magnet result should never be written on a lot description as evidence of grade. "Non-magnetic, stainless-looking" covers everything from 316 stainless to Hastelloy C-276. The nickel alloy vs stainless steel comparison explains the chemistry that separates the two groups, and how handheld XRF sorts nickel alloys covers the instrument that names the grade in seconds.

Magnetic duplex has the opposite problem. Every duplex grade pulls, so the magnet separates duplex from 304 and 316 but cannot separate 2205 from 2507 or S32760. Those are told apart by chromium, molybdenum, copper and tungsten, as explained in selling duplex and super duplex scrap.

How Do You Use a Magnet on Turnings and Mixed Bins?

On turnings, use the magnet to find and pull out contamination rather than to identify the alloy. A drum of nickel alloy turnings should give almost no response; if a magnet drawn through a handful comes out furred with chips, carbon steel or 400-series stainless swarf has got into the drum.

That matters because iron chips dilute the nickel and alloy content of the lot, and a buyer’s sample will find them. Keep separate drums at each machine, and check a handful from the top, middle and bottom before the drum is closed. Two exceptions: duplex turnings are magnetic in their own right, and stainless turnings may pull slightly from cold work, so on those the magnet tells you little. The turnings and mixed and unsorted scrap pages cover preparation in more detail, and solids vs turnings explains why clean, single-grade chips are worth the effort.

What Should Come After the Magnet Test?

After the magnet test comes an analyser: handheld XRF for the grade, and OES or laboratory analysis where light elements or carbon decide it. The magnet simply decides which pieces are worth that instrument time.

For the choice between methods, see XRF, OES or laboratory analysis. If you have a sorted pile and want an offer, send photographs, the magnet groupings and any readings you have. We ask sellers for that kind of grade evidence, and we’ll review your enquiry.

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