A common misconception is that stainless steel is non-magnetic. There are five classes of stainless steel, and only one is non-magnetic. However, it just happens to be that the most widely used type (austenitic) is the one that is not magnetic.
| Class | Magnetic? | Common Types |
|---|---|---|
| Ferritic | Yes | 430,442 |
| Austenitic | No | 304,316 |
| Martensitic | Yes | 410,416 |
| Duplex | Yes | 2205, 2507 |
| Precipitation Hardening | Yes | 17-4, 17-7 |
Magnetism has more to do with the steel’s grain microstructure than with its chemical make-up, so whether or not your stainless will be magnetic will depend on not only its microstructure, but also how it was manufactured or processed. The most common stainless steels, 304 and 316, have an austenitic microstructure and start out life non-magnetic. However as they are manufactured from raw steel to fasteners, they go through several cold forming processes (drawing, roll threading, heading) which changes the microstructure from austenitic to martensitic, thereby making the material more magnetic. The more cold forming, the more its microstructure is changed to martensite, and subsequently the more magnetic the fastener is likely to be.
If magnetism in austenitic stainless is not wanted, steps can be taken to limit it. Manufacturing processes can be chosen that do not cold form, i.e. cut threading vs roll threading. Alternatively, the finished fastener can go through an annealing treatment which will restore the microstructure to its original austenitic state.
Excellent explanation. If I understand correctly, you are saying the cold-working is changing the stainless from non-magnetic austenitic to magnetic martensitic, and thus less corrosion resistant?
@Steven- We do not doubt that the changing of the microstructure could have some corrosion ramifications, but we cannot speak to the severity or extent. Many cold formed fasteners perform quite well in corrosive environments. You would need to contact a metallurgist to discuss how the link between cold forming, magnetism and corrosion.
What about 18-8?
@Brian- 18-8 is a stainless family that encompasses many of the 300 series austenitic steels, among them 303, 304, 302 and a few others. For the purposes of this FAQ, we could lump 18-8 in with 304 stainless.
FOR Duplex 2205 which ASTM Grade to be used for stud and Nut
@Tauseef- We are not aware of any fastener specific ASTM standards that include Duplex 2205, but the raw material is covered in ASTM A276 type S32205.
Dear Dan, and what about in case of a U stamp pressure vessel including duplex 2205 bolting set ? It seems to me that ASME SA276 is not included into ASME section II-D, where the allowable stress are defined. Therefore in case of U stamp calculation, in our company we are considering ASME SA479 for pressure parts calculations, because SA479 is included in ASME II-D. Also do you confirm that fabrication of duplex 2205 bolting is possible from SA479 rod bars, instead of SA276.
Regards
David S.
@David- Most of the 304/316 bar we use is manufactured to both A276 and A479. We have very limited experience with the duplex grades, so we cannot be certain that the same will hold true for those grades, although my hunch is that it will.
Good Afternoon Dane,
Wonder if you could help me, we purchase NON- MAG Stainless Hardware
lately when it comes in it is magnetic, so inspection rejects the parts. I
see it noted in your information you talk about Annealing of parts.
Where would I have this done ? Why do you think this is happening more
and more when my orders come it.
Any information will be helpful.
Kind Regards,
Carol
@Carol- Annealing can be done at any heat treat facility, and is a relatively simple procedure. As for why you are just now experiencing this issue, we cannot be sure. Perhaps your supplier changed manufacturers, or the manufacturer changed something about their process, it is hard to say.
I bought the rawl bolt type fasteners that are attracted by magnets. I plan to use it underwater in my pool. will they remain rust free?
@Martin- We don’t know anything about Rawl products, so are not able to answer that question properly.