Stainless steel’s magnetic behavior depends entirely on which grade — and the answer is not a simple yes or no. Approximately 70% of commercially produced stainless steel is non-magnetic or weakly magnetic (austenitic grades: 304, 316, 317, 321). Approximately 25% is magnetic (ferritic grades: 430, 409, 439; and martensitic grades: 410, 420, 440). The remaining ~5% of duplex stainless is partially magnetic. The same “stainless steel” label covers metals with completely opposite magnetic behaviors.
For engineers, fabricators, and quality inspectors, understanding which grades are magnetic and why matters for: fixture design (magnetic workholding works for ferritic stainless; not for austenitic), inspection procedures (magnetic particle testing works for ferritic; eddy current testing is needed for austenitic), material identification (a magnet test quickly identifies ferritic from austenitic, though not conclusively), and end-use applications (MRI environments require non-magnetic austenitic; transformer cores require the specific magnetic properties of electrical steels).
The Crystal Structure Explanation
The magnetic behavior of stainless steel is determined by its crystal structure — not its chemical composition alone. The same alloying elements (iron, chromium, nickel) arrange into different crystal structures that have fundamentally different magnetic behaviors:
Face-centered cubic (FCC) — austenite
The austenite phase has iron and alloying atoms arranged in a face-centered cubic (FCC) crystal structure: atoms at the corners and center of each face of a cube. FCC austenitic iron is paramagnetic — the electronic structure of FCC iron does not support the exchange coupling between neighboring atoms that produces ferromagnetism. Austenite is essentially non-magnetic.
High nickel content (8–12% in 304; 10–14% in 316) stabilizes the FCC austenite phase at room temperature. This is why nickel additions are the key to making non-magnetic stainless steel.
Body-centered cubic (BCC) — ferrite and martensite
The ferrite phase has iron atoms arranged in a body-centered cubic (BCC) structure: atoms at the corners and center of a cube. BCC iron is ferromagnetic — the nearest-neighbor spacing in the BCC structure places iron atoms at the correct distance for exchange coupling, producing organized magnetic domains. Ferrite is strongly magnetic.
Martensite is a distorted BCC structure produced by rapid cooling (quenching) of austenite — it is also ferromagnetic. Martensitic stainless grades (410, 420, 440) are magnetic because their crystal structure is BCC/BCT.
The Stoner criterion (I × D(E_F) > 1 for ferromagnetism) — covered in the aluminum magnetic article — is satisfied for BCC iron but not for FCC iron. The crystal structure determines whether the exchange coupling condition is met; the alloying determines which structure is present.
Grade-by-Grade Magnetic Behavior
Austenitic stainless steel: non-magnetic in most conditions
304 (18Cr-8Ni) and 316 (16Cr-10Ni-2Mo): The most widely produced stainless steel grades. Both are austenitic in the annealed condition and non-magnetic — a hand-held magnet does not stick to annealed 304 or 316. The magnetic susceptibility of annealed austenitic stainless is approximately +3 × 10⁻⁴ to −3 × 10⁻⁴ (weakly paramagnetic to weakly diamagnetic depending on exact composition and heat treatment) — too weak to produce any measurable force with a permanent magnet.
But cold-worked austenitic stainless IS magnetic: When austenitic stainless steel is cold-worked (drawn, stamped, bent, rolled, or formed), the deformation partially transforms the FCC austenite into BCC martensite (strain-induced martensite). The transformed martensite is ferromagnetic. The amount of transformation — and therefore the magnetic response — increases with:
- Deformation amount (more cold work = more martensite)
- Composition (lower nickel content = more susceptible to transformation; 304 transforms more readily than 316)
- Temperature (cryogenic temperatures increase transformation susceptibility)
Practical implication: A laser-cut 304 plate in the annealed condition is non-magnetic. The same plate after press brake bending may show visible magnetic attraction at the bend zone where martensite has formed. A 304 stainless sheet that has been rolled 50% in thickness may be substantially magnetic throughout the worked area. This is why a magnet test does not conclusively identify 304 as austenitic — a cold-worked 304 will test as magnetic.
The nickel equivalence rule: Higher nickel content suppresses martensite transformation. 316 (10–14% Ni vs. 304’s 8–10% Ni) is substantially more resistant to strain-induced martensite formation. 317L and 321 are even more resistant. For applications requiring austenitic stainless to remain non-magnetic through fabrication, specify higher-nickel grades.
Ferritic stainless steel: always magnetic
430 (17Cr, no nickel): The most common ferritic stainless. Always magnetic — the BCC ferrite structure is maintained at all temperatures from room temperature up to the Curie temperature (approximately 750°C for 430 stainless). A magnet sticks strongly to 430 stainless in any condition (annealed, cold-worked, or formed).
409 (11Cr, no nickel): Ferritic, automotive exhaust system standard. Always magnetic.
439 (18Cr, Ti-stabilized, no nickel): Ferritic, higher corrosion resistance than 430. Always magnetic.
Magnetic permeability of ferritic stainless: Ferritic stainless grades have relative magnetic permeability (μ_r) of approximately 600–1,200 — strongly magnetic, though less permeable than carbon steel (μ_r ≈ 1,000–10,000) or silicon electrical steel (μ_r up to 100,000).
Why ferritic grades exist despite being magnetic: Ferritic stainless offers significant advantages: lower cost (no nickel), better thermal conductivity, lower thermal expansion coefficient, and no sensitization susceptibility. For applications where magnetism is not a disqualifying property — automotive exhaust, kitchen appliances, architectural trim — ferritic is cost-effective and adequate.
Martensitic stainless steel: magnetic, hardenable
410 (12Cr, no nickel): Martensitic in the hardened condition. Used for cutlery, surgical instruments, turbine blades. Always magnetic in the hardened and tempered condition; can be weakly paramagnetic in the fully annealed (ferritic) condition.
420 (12–14Cr, 0.15–0.4%C, no nickel): Higher carbon than 410; hardens to HRC 50–58 after heat treatment. Knife blades, scissors, bearing races. Magnetic in both annealed and hardened conditions.
440C (16–18Cr, 0.95–1.2%C, no nickel): The hardest standard stainless; HRC 58–60 maximum. Used for ball bearings, cutting instruments. Strongly magnetic.
The hardening mechanism: Martensitic grades are hardened by austenitizing (heating to 980–1,010°C to dissolve carbon) and quenching. The quench transforms austenite to martensite (BCC/BCT structure), which is both hard and magnetic. Subsequent tempering does not remove the magnetism but restores some toughness.
Duplex stainless steel: partially magnetic
2205 (22Cr-5Ni-3Mo): Duplex stainless has a microstructure that is approximately 50% austenite (FCC, non-magnetic) and 50% ferrite (BCC, magnetic). The result is partial magnetism — a magnet adheres to 2205, but less strongly than to ferritic stainless. Magnetic susceptibility approximately 1/3 to 1/2 of ferritic grades.
Duplex stainless is used in chemical processing, marine, and oil/gas applications for its combination of high strength (yield 450–550 MPa vs. 205–240 MPa for austenitic) and good corrosion resistance. Its partial magnetism is usually not a concern in these applications.
Precipitation-hardening (PH) stainless: variable
17-4 PH (S17400) in H900 condition: Semi-austenitic in the solution-annealed condition (weakly magnetic); martensitic and strongly magnetic after precipitation hardening. The condition must be specified to know the magnetic behavior.
15-5 PH: Similar to 17-4 PH — condition-dependent magnetism.
Magnetic Behavior Summary Table
| Grade family | Common grades | Crystal structure | Magnetic? | Permeability (μ_r) |
|---|---|---|---|---|
| Austenitic | 304, 316, 317, 321 | FCC (austenite) | No (annealed); partially (cold-worked) | 1.001–1.05 |
| Ferritic | 430, 409, 439 | BCC (ferrite) | Yes, always | 600–1,200 |
| Martensitic | 410, 420, 440C | BCT (martensite) | Yes, always | 700–1,000 |
| Duplex | 2205, 2304 | FCC + BCC (mixed) | Partially | 30–100 |
| PH (precipitation hardening) | 17-4 PH, 15-5 PH | Condition-dependent | Depends on condition | 50–1,000 |
Practical Implications: When Magnetism Matters
Magnetic workholding on laser cutting machines
A magnetic chuck or electromagnetic workholding fixture holds ferromagnetic workpieces by direct magnetic attraction. This works for:
- Ferritic stainless (430, 409, 439) — strong attraction, reliable holding
- Martensitic stainless (410, 420, 440C) — strong attraction
- Cold-worked austenitic stainless — may hold if sufficient martensite is present, but unreliable
Does NOT work for:
- Annealed austenitic stainless (304, 316) — no magnetic attraction; must use vacuum holding or mechanical fixturing
- Duplex stainless — partial holding; not recommended for precision workholding
Non-destructive testing (NDT) method selection
Magnetic particle testing (MT): Applicable only to ferromagnetic (ferritic/martensitic) stainless. The particles are attracted to flux leakage at surface and subsurface discontinuities. Cannot be used on austenitic stainless — it is not ferromagnetic and will not retain the particles.
Eddy current testing (ET): Applicable to austenitic stainless and all conductive materials. Used for weld inspection, surface crack detection, and wall thickness measurement on non-ferromagnetic stainless. For austenitic stainless laser cut or welded parts, eddy current testing is the correct NDT choice.
Material identification by magnet test
The magnet test can quickly separate ferritic/martensitic stainless from austenitic stainless in the annealed condition. However:
- Cold-worked 304 stainless will test positive (magnetic) even though it is nominally austenitic — the test is not definitive for worked material
- 316 and 304 both appear non-magnetic in the annealed condition — the test cannot distinguish between them
- XRF (X-ray fluorescence) analysis is the definitive method for stainless grade identification when the magnet test is ambiguous
MRI and magnetic field-sensitive environments
In MRI scanner rooms (field strengths 1.5–7 Tesla), ferromagnetic objects experience strong attraction force that can become a projectile hazard. Only austenitic stainless (304, 316, 317L, 321) in the fully annealed condition is used for MRI-compatible fixtures, instruments, and equipment — and even then, cold-worked austenitic stainless that has developed strain-induced martensite must be tested before use in high-field environments.
Learn more:
Engineer’s Guide: Is Stainless Steel Magnetic? Explaining Different Grades
Frequently Asked Questions About Stainless Steel Magnetism
Q: Is 304 stainless steel magnetic?
Annealed 304 stainless is non-magnetic — it does not stick to a permanent magnet. However, cold-worked 304 (bent, formed, drawn, or significantly machined) may be partially to strongly magnetic because mechanical deformation transforms some of the non-magnetic FCC austenite into ferromagnetic BCC/BCT martensite. The magnet test result for 304 stainless depends on the temper and the amount of cold work the specific piece has experienced.
Q: Is 316 stainless steel magnetic?
Like 304, annealed 316 stainless is non-magnetic. 316 is more resistant to martensite formation than 304 because its higher nickel content (10–14% vs. 8–10% for 304) more effectively stabilizes the FCC austenite phase. Cold-worked 316 is less likely to show magnetic response than equivalent cold-worked 304, but heavily deformed 316 can show some magnetism.
Q: Why does some stainless steel stick to a magnet?
Stainless steel grades with BCC or BCT crystal structures (ferrite, martensite) are ferromagnetic and stick to magnets. These include ferritic grades (430, 409, 439) and martensitic grades (410, 420, 440C). Even austenitic grades (304, 316) can become partially magnetic after cold working because deformation converts some FCC austenite to ferromagnetic BCC/BCT martensite.
Q: How can you tell 304 from 316 stainless steel?
A magnet test cannot distinguish 304 from 316 — both are non-magnetic in the annealed condition. Reliable identification methods: XRF (X-ray fluorescence) analysis measures the molybdenum content (316 contains 2–3% Mo; 304 contains ≤0.1% Mo), providing definitive grade identification in seconds; chemical spot testing; or certified mill certificate with heat number traceability.
Q: Can stainless steel be magnetized permanently?
Ferritic and martensitic stainless steels can be magnetized (by placing them in a strong magnetic field) and will retain some magnetization afterward — they are magnetically soft but not hard. They are not used as permanent magnet materials because their coercivity (resistance to demagnetization) is low. Austenitic stainless steel in the annealed condition cannot be meaningfully magnetized — its susceptibility is too low.
References
- ASTM A240/A240M, Standard Specification for Chromium and Chromium-Nickel Stainless Steel Plate, Sheet, and Strip for Pressure Vessels and for General Applications, ASTM International, current edition. (Composition requirements for 304 (18Cr-8Ni), 316 (16Cr-10Ni-2Mo), 430 (17Cr), and other stainless steel grades — the composition ranges that determine crystal structure and therefore magnetic behavior; the material standard that establishes what each grade designation means chemically.)
- ASM Handbook, Volume 1: Properties and Selection — Irons, Steels, and High-Performance Alloys, 10th Edition. ASM International, Materials Park, OH, 1990. (Crystal structure of austenite (FCC), ferrite (BCC), and martensite (BCT) in stainless steel; strain-induced martensite transformation in austenitic stainless during cold work; magnetic permeability of ferritic and martensitic stainless grades; Curie temperature data.)
- Kittel, C., Introduction to Solid State Physics, 8th Edition. Wiley, New York, 2005. (Ferromagnetism in BCC iron: exchange coupling mechanism; the Stoner criterion for ferromagnetic instability (I × D(E_F) > 1); why FCC iron is paramagnetic and BCC iron is ferromagnetic — the solid-state physics basis for the crystal structure-magnetism relationship in stainless steel.)
- Haynes, W.M., ed., CRC Handbook of Chemistry and Physics, 104th Edition. CRC Press, Boca Raton, FL, 2024. (Magnetic susceptibility and permeability data for austenitic, ferritic, and martensitic stainless steel grades; Curie temperatures for iron-chromium alloys; the quantitative magnetic property data in the grade comparison table.)
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