Is duplex stainless steel magnetic? Yes, it is. The ferrite phase in its two-phase microstructure is ferromagnetic. Austenite isn’t.
Duplex 2205 runs roughly half ferrite and half austenite after solution annealing, so it lands in the middle. A magnet grabs 2205 the moment you touch it. Annealed 316 doesn’t respond at all.
That answer is easy to give and hard to use. Published relative permeability for 2205 ranges from about 1.3 to about 5. At least one distributor datasheet states an upper bound below 50. The number that should settle the question is the least stable part of the answer.
Ahmed runs receiving inspection at a fabrication yard in Jebel Ali. A shipment of 2205 flanges arrived with matching mill test certificates. He touched a magnet to the first flange out of the crate and felt the pull.
Stainless steel isn’t supposed to be magnetic, so he quarantined the batch. Three weeks of paperwork later, the material was confirmed correct and his assumption was confirmed wrong. The delay cost more than the flanges did.
This guide explains why the ferrite phase makes duplex magnetic and why the published values disagree. It covers how 2205 compares with 316 and 304. Then it covers what cold work and heat treatment do to the number. The duplex 2205 vs 316 comparison covers the full grade decision; this article covers the magnetic half of it.
Key Takeaways
- Duplex stainless steel is magnetic because its ferrite phase is ferromagnetic, while austenite is paramagnetic.
- The ferrite sits as isolated islands inside a paramagnetic austenite matrix, so duplex reads as weakly to moderately magnetic.
- Published relative permeability for 2205 spreads from about 1.3 to about 5, and one datasheet quotes an upper bound below 50.
- Cold working lowers the apparent magnetic reading in duplex, the opposite of what happens to cold-worked 316.
Is Duplex Stainless Steel Magnetic? The Direct Answer
Duplex stainless steel is magnetic. Its microstructure contains ferrite, which is ferromagnetic, alongside austenite, which is paramagnetic. Any grade built from both phases responds to a magnet in proportion to how much ferrite it carries.
The nuance is the word “magnetic.” It sounds binary. In materials it’s a spectrum.
Why “Magnetic” Isn’t a Yes or No Property
Materials fall into three practical groups.
- Ferromagnetic metals such as iron, nickel and cobalt pull hard toward a magnet and can hold magnetism afterward.
- Paramagnetic metals respond so weakly that you can’t feel it.
- Diamagnetic metals are faintly repelled.
Relative permeability (μr) puts a number on that spectrum. A value of 1.0 means the material behaves like empty space. Annealed 304 and 316 sit near 1.0. Ferritic grades like 430 sit in the hundreds.
Duplex lands between them.
What the Answer Depends On
Four variables move the number.
| Variable | Effect on magnetic response |
|---|---|
| Ferrite fraction | Sets the ceiling; more ferrite means a stronger pull |
| Ferrite chemistry | Chromium, molybdenum and nickel shift how the ferrite responds |
| Product form and cold-work state | Wrought, drawn and formed stock disagree with each other |
| Heat-treatment history | Solution annealing, welding and service exposure all count |
A wrought annealed plate, a cold-drawn bar and a casting from the same heat won’t agree.
Why Is Duplex Stainless Steel Magnetic? The Ferrite Phase
Duplex 2205 is a two-phase stainless steel. After solution annealing it holds roughly equal parts austenite and ferrite, with a specified ferrite content of 40 to 60%. Ferrite has a body-centered cubic structure and is ferromagnetic. Austenite is face-centered cubic and paramagnetic.
That single structural fact explains the magnetism.
Read the duplex 2205 grade guide for the full composition window, solution-anneal cycle and equivalent grades.
Why 50% Ferrite Doesn’t Mean “Half as Magnetic as 430”
Here’s the part most explanations skip. Duplex isn’t a simple blend.
Its ferrite exists as isolated islands scattered through a paramagnetic austenite matrix. Magnetic circuits need continuous paths. Once the ferromagnetic phase is broken into disconnected islands, those paths are interrupted.
The material responds to a field far less than a fully ferritic steel with the same ferrite fraction. That’s why duplex reads as weak to moderate rather than strong.
Saturation induction shows the same pattern:
| Material family | Saturation induction |
|---|---|
| Austenitic 304 and 316 | Below 0.4 T |
| Duplex 2205 | About 0.4 to 0.7 T |
| Ferritic stainless and carbon steel | Above 1.2 T |
Duplex sits between the two families. No amount of chemistry changes that.
The Number Moves: Published Permeability Values for 2205
Now the disagreement. Ask four sources for the relative permeability of 2205 and you get four answers.
| Source type | Reported value for 2205 | Stated condition |
|---|---|---|
| Supplier buyer guide | Relative permeability 2 to 5 | Unstated product form |
| Distributor datasheet | Upper bound below 50 at 20°C | Specification bound, not measured |
| Mill and fabricator references | μr ≈ 1.3 to 1.5 | Annealed wrought product |
| Austenitic comparison | μr ≈ 1.0 to 1.05 | Annealed 304 and 316 |
None of these is a lie. They answer different questions. A specification bound is the worst case a mill promises to stay under, not the value a test will read. A low-field measurement captures something different from a high-field one.
Ferrite content also varies heat to heat. The same grade can legitimately produce readings a factor of two apart. Treat any single permeability figure for duplex as a range for a stated condition. Anyone who hands you one number without the condition is quoting a datasheet, not a measurement.
Duplex Stainless Steel vs 316, 304 and Other Metals
The comparison table below is the one worth bookmarking. It puts every engineering metal family on the same scale.
| Material | Magnetic class | Relative permeability (typical) | Notes |
|---|---|---|---|
| Duplex 2205 | Weakly to moderately ferromagnetic | ~1.3 to 5 (sources vary) | 40 to 60% ferrite in islands |
| Super duplex 2507 | Ferromagnetic | Higher ferrite fraction | Same physics, more ferrite |
| 304 (annealed) | Essentially non-magnetic | ~1.0 to 1.05 | Becomes weakly magnetic after cold work |
| 316 / 316L (annealed) | Essentially non-magnetic | ~1.0 to 1.05 | The non-magnetic reference grade |
| 304 or 316 (heavily cold worked) | Weakly magnetic | Up to ~2 and above | Strain-induced martensite |
| Ferritic 430 | Strongly magnetic | In the hundreds | Continuous ferrite |
| Carbon steel | Strongly magnetic | Far higher again | Continuous ferrite and cementite |
| Titanium Grade 2 and Grade 5 | Paramagnetic | ~1.00002 | Practically non-magnetic |
Annealed 316 Is Essentially Non-Magnetic
With no ferrite to speak of, annealed 316 behaves the way buyers expect stainless steel to behave. Hold a magnet to it and nothing happens. That’s why 316L gets specified for magnetic-sensitive duty.
Cold-Worked 316 Becomes Weakly Magnetic
Austenitic grades have their own exception. Bending, deep drawing, shearing and thread rolling can transform some austenite into strain-induced martensite, which is ferromagnetic. Heavily formed 316 turns weakly magnetic. Grade 304 responds more strongly than 316 because of its lower nickel.
Lena, a procurement manager in Gothenburg, flagged a batch of cold-formed 316L hydraulic tube as suspected carbon steel contamination. The moly spot test came back positive for molybdenum. The mill certificate matched.
The weak magnet response traced to forming strain. Nothing was wrong with the tube.
Super Duplex 2507 and Lean Duplex 2304
The physics doesn’t change with the grade. Super duplex 2507 carries more ferrite, chromium, molybdenum and nitrogen. It resists pitting better and pulls a magnet a little harder. The duplex 2205 vs 2507 comparison covers the corrosion side of that grade step.
Lean duplex 2304 carries less of both. Magnetism tracks the phase balance, not the corrosion rating. That’s why the most corrosion-resistant duplex grades are also the magnetic ones.
Does Cold Working Make Duplex Stainless Steel Magnetic?
The reflex answer is yes. It holds for austenitic grades, so people carry it across. For duplex the documented behavior runs the other way. Cold rolling lowers the apparent ferrite and magnetic reading.
The mechanism isn’t phase change. Metallography and X-ray diffraction on cold-rolled 2205 show no martensite and no precipitates, so no ferrite decomposed. Three other effects do the work:
- Dislocation tangles form subgrain boundaries, refining the effective grain size and raising coercivity.
- Rolling texture shifts toward hard magnetization axes.
- Internal stresses resist magnetization.
So a cold-formed 2205 part that reads lower on a ferritescope than its parent plate is normal. It isn’t suspicious. Strain-induced martensite can appear in 2205 at the heaviest reductions, but far less than in lean duplex grades. Grade 2205 is the more stable composition.
Small reductions have even been reported to raise coercivity, maximum induction and remanence. Direction depends on how much deformation you apply.
Heat Treatment, Sigma Phase and Magnetism
Heat treatment moves the number the other way. It doubles as a defect signal.
At 800 to 900°C, Ferrite Decomposes and Magnetism Falls
Hold duplex in that window and ferrite breaks down: α → σ + γ₂, ferrite into sigma plus secondary austenite. Sigma is paramagnetic. As it grows, saturation induction, residual induction, coercive force and differential permeability all fall.
Around 4% sigma is already enough to compromise mechanical properties. Heat treatment at 850°C for as little as 15 to 120 minutes can form it. A falling magnetic reading after a thermal event isn’t good news. It usually means sigma.
The Blind Spot: Sigma Is Paramagnetic
Turn that around and you get the caveat worth carrying into any inspection plan. Because sigma is paramagnetic, a magnetic instrument can’t detect it. A weld or a heat-treated part can read a healthy ferrite number and still be embrittled. The instrument measures ferrite content, not phase health.
The duplex 2205 welding guide covers the ferrite-number acceptance bands and this measurement blind spot in full detail.
At 475°C, Magnetism Becomes a Monitoring Tool
There’s a useful case too. Aging around 475°C causes spinodal decomposition of ferrite into chromium-rich α′ and iron-rich phases. Coercivity rises only slightly even after 500 hours. The Curie temperature, though, climbs measurably.
Magnetic methods can therefore track 475°C embrittlement without cutting a specimen.
Measuring Magnetism and Ferrite Content
Relative Permeability Isn’t Ferrite Number
These two get used interchangeably. They shouldn’t be. Relative permeability describes how a material responds to a magnetic field. Ferrite number (FN) describes how much ferrite a weld or casting contains.
Different quantities, different instruments, different purposes. A permeability of 1.05 and a ferrite content of 0.5% describe related physics and share no numeric relationship.
Ferrite Measurement Instruments
| Instrument or method | Working range | Governing standard |
|---|---|---|
| Ferritescope (FMP30 / MP30 class) | ~0.1 to 80% ferrite, ~110 FN | Calibrated per AWS A4.2 / ISO 8249 |
| Magne-Gage | Up to ~140 FN | Magnetic-attraction instrument |
| Severn-type cantilever | Up to ~30% ferrite | ASTM A342, Method 3 |
| Metallographic point counting | Full range | ASTM E562 and ASTM E1245 |
One caution before you write a number into a report. A spread of up to about 12 points has been measured between different operators and different methods on the same material. Agree the method before the reading matters to anyone.
What Duplex Magnetism Means for Inspection
Duplex is ferromagnetic enough that magnetic particle inspection works in principle. Working inspectors report success on 2205 using continuous magnetization. Residual magnetism in duplex is low. A field in the range of 30 to 60 gauss does the job.
ASME Section V Article 7 is the technique reference supporters cite.
Then there’s the other side, stated honestly. The mixed microstructure gives the material uneven permeability. Ferrite content changes across the weld, the heat-affected zone and the base metal. Each change can produce a non-relevant indication that looks like a defect.
Some experienced practitioners argue that no international standard explicitly accepts MPI on duplex. They prefer liquid penetrant or eddy current testing instead. ASTM E1444 says nothing specific about the material.
Rather than pick a winner, do this:
- Qualify the technique on the actual material and joint.
- Verify it against test bars or known crack specimens.
- Where the specification or the customer demands it, confirm with penetrant.
The upstream lever is ferrite control during welding. Inspection difficulty scales with ferrite variation.
What Duplex Magnetism Means for Equipment and Instrumentation
This is where magnetism stops being trivia and starts changing design decisions.
Components near magnetic-sensitive instruments need μr ≤ 1.01 for MRI-critical service. General low-magnetic duty allows ≤ 1.05. Those thresholds rule duplex out and point straight to 316L. Three common cases:
- Magnetic flowmeter tubes. The meter assumes a non-magnetic bore and drifts when that assumption breaks.
- Magnetic-sensitive assemblies. Sensors mounted near the wetted part read the structure as part of the field.
- Magnetic separation equipment. The design relies on the housing not disturbing the field.
Diego, a project engineer in Antofagasta, swapped 316L for duplex 2205 on a spool carrying a magnetic flowmeter. The corrosion reasoning was sound. The meter reading drifted from commissioning anyway. The specification required a non-magnetic tube, and no duplex grade was ever going to deliver one.
Residual magnetism belongs on the list too. Machined duplex parts can hold a weak field, which matters in cleanroom and precision assembly work. The same property has an inverse use. Duplex can be deliberately magnetized for niche medical-device work, where being attracted to a magnet is the point.
Can You Use a Magnet to Identify Duplex Stainless Steel?
No, not on its own. A magnet tells you which family you’re holding. It can’t tell you the grade.
The trap is that a magnet attracts 2205, 430, 410, 17-4PH and every carbon steel. It also fails on the substitution buyers actually worry about. Neither 304 nor 316 is magnetic, so a 304-for-316 swap passes a magnet test unnoticed.
| Method | What it proves | What it cannot prove |
|---|---|---|
| Magnet | Magnetic family versus non-magnetic family | Grade; molybdenum content; phase balance |
| Moly spot test | Molybdenum present, so 316, 317, 904L, 2205 or 2507 | Which of those grades; ferrite balance |
| Ferrite meter | Phase balance within duplex | Sigma phase, which is paramagnetic |
| PMI (XRF or OES) | Chemistry, and therefore the grade | Phase balance; mechanical condition |
| Mill test certificate | The specified grade and heat as delivered | The condition of an individual part |
Run the sequence in order.
- A magnet screens the family. It separates duplex, ferritic, martensitic and carbon steel from austenitic.
- The moly spot test rules out 304, 430 and carbon steel. It can’t separate 2205 from 316, since both contain molybdenum.
- A ferrite meter or PMI breaks that tie. Only chemistry and phase data settle the grade.
- The mill test certificate carries the acceptance decision.
The guide to telling 304 from 316 covers the broader identification picture. The titanium magnetic article covers the opposite case, where a metal genuinely doesn’t respond.
When Duplex Magnetism Is a Reason to Choose 316 Instead
Magnetism rarely decides a material choice on its own. It decides it in four situations.
- Magnetic-sensitive service. MRI-critical components, assemblies next to magnetic instruments, and magnetic separation equipment need μr near 1.0. Austenitic 316L is the correct answer, not duplex.
- High-temperature service. The duplex ceiling sits near 300°C. Embrittlement rather than magnetism sets it.
- Cryogenic service. The ferritic phase limits duplex near −50°C, while austenitic 316 handles service down to −196°C.
- Budget and fabrication simplicity. Where the duplex premium buys nothing, 316 remains the economical grade. The 2205 vs 316 cost breakdown runs the numbers.
For ordinary chloride service in a piping run, a vessel or a heat exchanger, magnetism is usually a non-issue. Duplex ferrite drives both the corrosion resistance and the magnetic response. You take the second to get the first. The 2205 vs 316 corrosion comparison explains the corrosion side of that trade.
Frequently Asked Questions
Is duplex stainless steel magnetic?
Yes. Duplex contains a ferrite phase that’s ferromagnetic alongside austenite, which is paramagnetic. Duplex 2205 has a specified ferrite content of 40 to 60%, so it’s weakly to moderately magnetic. Annealed 316 is essentially non-magnetic.
Is 2205 stainless steel magnetic?
Yes. Grade 2205 is noticeably magnetic because of its ferrite phase. Published relative permeability values range from roughly 1.3 to 5, depending on source and condition. It’s less magnetic than carbon steel or ferritic 430.
Why is duplex stainless steel magnetic?
Because ferrite is ferromagnetic and austenite isn’t. The ferrite in duplex is broken into isolated islands rather than continuous paths. The material therefore responds less strongly than a fully ferritic steel with the same ferrite fraction.
Is duplex stainless steel ferromagnetic or paramagnetic?
It’s ferromagnetic overall, but weakly to moderately so. The austenite half is paramagnetic and the ferrite half is ferromagnetic. The measured response reflects both phases.
Is duplex more magnetic than 316?
Yes, clearly. Annealed 316 sits near μr 1.0 and usually shows no magnet attraction. Duplex 2205 sits between about 1.3 and 5 and pulls a magnet visibly. Cold-worked 316 narrows that gap.
Is super duplex stainless steel magnetic?
Yes. Super duplex 2507 is magnetic for the same reason. It carries a higher ferrite fraction than 2205, so the magnet response can be stronger. The physics doesn’t change with the grade.
Does cold working make duplex stainless steel magnetic?
It tends to make it less magnetically responsive, not more. Cold rolling of 2205 lowers the apparent ferrite and magnetic reading through dislocation subgrain formation, texture change and internal stress. Metallography confirms no martensite and no precipitates.
Can magnetic particle inspection be used on duplex stainless steel?
Physically yes, since duplex is ferromagnetic. In practice the decision is disputed. Non-relevant indications from permeability variation at welds and heat-affected zones lead some practitioners to prefer penetrant or eddy current testing.
Can a magnet identify duplex stainless steel?
No. A magnet confirms only that a material belongs to the magnetic family. That family includes duplex, ferritic, martensitic and all carbon steels. It can’t confirm a grade or detect a 304-for-316 substitution.
What is the relative permeability of duplex 2205?
Published values range from about 1.3 to about 5. One distributor datasheet states an upper bound below 50. The spread comes from different measurement conditions, product forms and ferrite fractions. Treat it as a range, not a constant.
Is duplex stainless steel suitable for MRI or magnetic-sensitive equipment?
No. MRI-critical components require relative permeability of 1.01 or lower. General low-magnetic duty requires 1.05 or lower. Duplex can’t meet either threshold, so austenitic 316L is the grade to specify.
Conclusion
Is duplex stainless steel magnetic? Yes, and the reason never changes: ferrite is ferromagnetic and austenite isn’t. That much is settled. What makes the question worth a full article is everything downstream of it.
Duplex 2205 runs 40 to 60% ferrite after solution annealing, and that ferrite sits in isolated islands inside a paramagnetic austenite matrix. The result is a weakly to moderately magnetic material, not a strong one. That’s why published relative permeability spreads from about 1.3 to about 5, and why one datasheet stops at 50. The number moves because it describes a microstructure, not a constant.
Three practical rules follow.
- Cold work lowers the apparent reading in duplex.
- Heat treatment lowers it further when sigma forms, and a magnetic instrument can’t see sigma at all.
- A magnet can’t identify a grade, only a family.
For most chloride service the magnetism is a non-issue. It becomes a specification for magnetic-sensitive equipment, a technique question for magnetic particle inspection, and a trap for anyone trying to use a magnet as a grade test.
Need duplex 2205 or 316L with documentation you can verify? We supply duplex and austenitic stainless steel in sheet, plate, pipe, tube and bar with mill test certificates reporting chemistry and mechanical results. We also cut and process to your dimensions. Request a quote or book a technical consultation on material selection for inspection-critical or magnetic-sensitive service.
