Guide
Why Is My 316 Stainless Part Magnetic, and Is It Still Acceptable?
316 stainless is an austenitic grade that is generally regarded as non-magnetic when annealed, yet a magnet can still pull on machined, formed or cast 316 parts. Short answer: a magnet is a poor grade test, because cold work, castings and weld metal all give austenitic stainless some magnetic response; accept or reject on chemistry from the mill certificate and PMI, and write a permeability limit on the drawing only if the part has a magnetic function.
By Saola Manufacturing engineering team · Published · Updated
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Why does a 316 part pull on a magnet at all?
Annealed austenitic stainless is generally regarded as non-magnetic, but cold work can change austenite into martensite, and castings and weld metal carry some ferrite on purpose.
The Australian Stainless Steel Development Association (ASSDA) explains that deforming the metal can make the structure change from austenite to martensite, which is magnetic, and that heavily worked items such as wire or the dished end of a pressure vessel show this most (ASSDA technical FAQ, accessed 2026). Once formed, the martensite can be magnetized enough to pick up light objects such as paper clips. The effect is weaker in alloys with more nickel, nitrogen or carbon.
Two other routes do not involve cold work at all. Austenitic castings typically contain a few percent ferrite and are weakly attracted to a magnet, and weld metal carries intentional ferrite, typically a few percent, added to resist hot cracking; the same source says the magnetic effect of welds is usually insignificant because they are a small part of a structure. For comparison, it quotes relative permeability typically below 1.02 for highly alloyed austenitics and at least 200 for mild steel, so a weak pull on 316 is a very different thing from the pull of a carbon steel part.
Which 316 parts are most likely to be magnetic?
Cold-finished bar, formed features, class 70 and 80 fasteners, welds and cast fittings are the usual suspects, while annealed bar and plate should show almost no pull.
ASTM A276 describes bar in Condition A (annealed), Condition S (strain hardened or relatively lightly cold worked) and Condition B (relatively severely cold worked) (ASTM A276/A276M-25), so two bars of the same grade can behave differently depending on the condition ordered. A steel construction advisory note adds that property class 50 fasteners are annealed and non-magnetic, unlike classes 70 and 80, which may show some magnetic properties (SCI Advisory Desk AD 342). For castings, a stainless casting supplier's technical FAQ says CF-8M, the casting equivalent of 316, has an increased magnetic response because of its greater ferrite content, and that this should not adversely affect corrosion resistance (H&G Houston FAQ, accessed 2026).
The sources do not quantify how much magnetic response an ordinary machining cut creates, so do not assume that turning or milling alone makes a part magnetic. The mechanism they name is cold work from drawing, forming or heavy deformation.
| Product form | Why it can pull on a magnet | What to do |
|---|---|---|
| Annealed bar or plate (Condition A) | Fully austenitic; essentially no pull is expected | A pull here is worth a chemistry check |
| Cold-finished or strain-hardened bar (Conditions S and B) | Cold work can form strain-induced martensite | Accept if chemistry and mechanical properties conform; order Condition A if low permeability matters |
| Formed, bent or heavily worked features | Deformation forms martensite where the metal is worked most | Expect local pull at the worked area; do not reject on the magnet alone |
| Class 70 and 80 fasteners | Made from cold drawn bar; may show some magnetic response | Judge on the property class and chemistry |
| Cast fittings (CF-8M) | A few percent ferrite is normal and is part of the alloy balance | A weak pull is expected; do not treat it as a defect |
| Welds | Intentional ferrite to resist hot cracking | Usually insignificant for the whole part |
Can a magnet tell you the grade?
No: annealed 304 and 316 are both austenitic and respond alike, so a magnet cannot tell them apart, and a weak pull does not show that a 316 part is the wrong grade.
What a magnet can do is catch gross errors. The same ASSDA note lists ferritic grades such as 409, martensitic grades such as 420, and duplex grades as strongly attracted even when annealed, so a strong pull on a part sold as annealed 316 points to a different material, such as a ferritic or martensitic stainless or even carbon steel. A weak pull, by contrast, is consistent with a cold-worked or cast austenitic part and carries no information about molybdenum, which is the element that separates 316 from 304.
Molybdenum is what the grade callout actually promises. Atlas Steels grade data list 316 at 2 to 3 percent molybdenum with 10 to 14 percent nickel (AZoM, 2001), and only chemistry measures that. The guides on PMI with XRF and 304 versus 316 cover how to verify the grade and when the difference matters.
How should incoming inspection decide whether a magnetic 316 part is acceptable?
Accept or reject on documented chemistry and a PMI spot check, and treat the magnet as a screen for gross material errors only.
A workable sequence for incoming inspection has four steps.
- Read the mill certificate first. Check the heat number on the part or tag against the certificate and read the chromium, nickel, molybdenum and carbon lines; the guides on reading a mill test report and on the EN 10204 3.1 certificate explain what each type proves.
- Run PMI on a sample. XRF reads chromium, nickel and molybdenum without damaging the part, which answers the question the magnet cannot.
- Ask what the magnet result is for. If the part has no magnetic function, a weak pull on a cold-finished, formed, welded or cast part is not a nonconformance. If a strong pull appears on annealed material, quarantine the lot and test chemistry.
- Record the result and the method. A note such as "magnet screen only, grade verified by PMI" prevents the same debate on the next lot; see the guide on incoming inspection of machined parts.
When is solution annealing or a permeability limit worth specifying?
Only when the part has a magnetic function, such as a housing near a sensor or a magnetically sensitive assembly, because annealing removes the pull at the cost of strength and shape.
ASSDA states that solution annealing followed by water quenching can restore non-magnetic behavior, but that this also reduces the tensile properties and may give rise to distortion (ASSDA technical FAQ, accessed 2026). It also notes that strain-induced martensite is reasonably magnetically hard, so once magnetized it is difficult to remove the effect with an electrical demagnetizer. For a cosmetic or purely structural part, annealing finished parts is rarely worth that trade.
If the function does need low permeability, handle it before machining rather than after. Order annealed bar (Condition A under ASTM A276), minimize cold forming, and write a measurable requirement: a relative permeability limit agreed with the supplier, with the measuring method and the location on the part. The source gives below 1.02 as typical for highly alloyed austenitics, which is a reference point and not a number to copy onto a drawing without a supplier discussion. For parts that will be welded, remember the weld metal will always carry some ferrite.
What should the drawing and RFQ say about magnetic response on 316?
State the grade, the product specification and the bar condition, and say plainly whether magnetic response is an acceptance criterion or not.
A clear callout removes the argument before it starts. For turned or milled 316, write the UNS number, the bar specification, Condition A where low permeability matters, and a note such as "magnetic response is not an acceptance criterion unless a permeability limit is stated". For cast 316, call the casting grade as CF-8M to ASTM A743 and expect a weak pull; the A743 scope covers iron-chromium and iron-chromium-nickel castings for general corrosion-resistant application (ASTM A743/A743M-19). The guide on 316 versus 316L covers the carbon callout.
For turned or milled parts (see CNC turning and CNC milling), the bar or casting is the one named on the drawing. Ask for the material certificate with each lot and for the production site on the quote.
Frequently asked
Does a magnetic 316 part mean the supplier substituted another grade?
Not by itself. Cold work, castings and welds all give genuine austenitic stainless some magnetic response. A substituted or wrong grade is found by chemistry on the mill certificate and PMI, not by a magnet.
Will a magnetic 316 part rust faster?
For castings, the H&G Houston FAQ says the extra ferrite should not adversely affect corrosion resistance (H&G Houston FAQ, accessed 2026). For cold-worked parts the sources here give no figure, so if corrosion matters, specify the surface condition and a passivation requirement and verify it.
Can a magnetic 316 part be demagnetized?
Poorly. ASSDA says strain-induced martensite is reasonably magnetically hard, so an electrical demagnetizer does not remove the effect easily. Solution annealing does, but it lowers strength and can distort the part (ASSDA technical FAQ, accessed 2026).
Is a magnet test acceptable at receiving at all?
As a screen, yes. A strong pull on material sold as annealed 316 is a reason to test chemistry. A weak pull is not a reason to reject, and no pull does not confirm 316.
Does passivation change magnetic behavior?
No. Passivation cleans the surface and restores the passive film; it does not change the bulk structure that causes the pull. Passivation is done at partner plants Saola manages.
Sources
- Magnetic effects of stainless steels, ASSDA technical FAQ, Australian Stainless Steel Development Association, accessed 2026
- Stainless castings in salt spray environment, technical FAQ, H&G Houston, accessed 2026
- AD 342: Stainless steel fasteners, SCI Advisory Desk, The Steel Construction Institute, undated; refers to the 2009 revision of EN ISO 3506
- ASTM A276/A276M-25, Standard Specification for Stainless Steel Bars and Shapes (store page), ASTM International, 2025
- ASTM A743/A743M-19, Castings, Iron-Chromium, Iron-Chromium-Nickel, Corrosion Resistant, for General Application (store page), ASTM International, 2019
- Stainless Steel: Grade 316, supplied by Atlas Steels, AZoM, 2001