316 stainless steel is slightly magnetic -- will this affect its corrosion resistance?

A. If the valve body is a casting, the most likely reason for the slight ferromagnetism is that the alloy is CF8M, which is the casting alloy that corresponds to the wrought alloy 316. It has slightly different chemical specification than 316. 316 is usually made with its chemistry balanced to give a fully austenitic (and hence incidentally nonmagnetic) microstructure. CF8M is usually made with its chemistry balanced to give 3 to 8 percent ferrite (which is magnetic) in the microstructure. This is done primarily because the presence of the small amount of ferrite very greatly reduces any risk of cracks or tears during solidification in the casting process. Benefits that occur as side effects are (1) increased yield strength relative to 316, (2) increased corrosion resistance because of higher chromium content than 316, (3) usually increased resistance to pitting because of slightly higher nitrogen content than 316.

Machining of nonmagnetic 316 does not normally cause the surface layer to become slightly magnetic. There are several stainless steels in the 300 series where heavy machining cuts and hence greater deformation of the layer immediately below the cutting tool, can cause a small amount of (magnetic) martensite to form from the austenite, but 316 does not usually show that effect.

A. 316/316L can be made magnetic by cold-working. Were not talking about a lot of magnetic permeability, but enough to move a needle on a compass.

According to the manufacturers of this alloy it is not magnetic and will not become magnetic after cold working. However, after years of working with this alloy, I notice that it does indeed become magnetic when cold worked, especially when the tools used are showing magnetism.

If you require it to be non-magnetic, then the parts can be de-magnetized with commercially available de-magnetizing equipment. Simply pass the unit over your parts(or parts through the unit), and it will be ferromagnetic free!

Try 316 Lvm as it is the most resistant to cold induced magnetism, due to it's extra low interstitial quality, and increased nickel content.

A. Hi, Amrit. Based on the previous answers it sounds unlikely that CF8M or 316 stainless steel could be "highly attracted" by a magnet -- but slightly attracted, yes.

Regards,

A. I have found that although the parts being welded are non-magnetic 316 stainless, the consumables used (and recommended for the purpose) often contain enough chromium to result in a level of ferrite high enough to be noticeably magnetic. This is common practice (as far as I know); but for areas where ferrite must be avoided at all costs, such as nitric acid plants, then specially alloyed consumables that give a purely austenitic (non-magnetic) structure are available.

There are numerous manufacturers of loop demagnetizing equipment. I'm sure there will be a manufacturer in India. One common application is demagnetizing after magnetic particle inspection, so anyone who offers equipment for MPI will also offer demagnetizers.

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Hi Jeff, Hi Jon,

Your answers are always generous and helpful, and usually fully accurate. But I think maybe you made a mental slip on this one? Degaussing equipment will certainly remove magnetism, i.e., the degaussed item will no longer attract steel. But to my knowledge, degaussing equipment does not remove ferromagnetism, i.e., it does not make a material which is attracted to magnets (like iron) no longer attracted to magnets.

Thus I question that degaussing is an answer to Mihir's problem of his stainless bolts being attracted to a magnet. I think annealing, as described by Lee Gearhart in letter 45435 may be a more promising approach. Then again, I may be in over my head :-)

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey

A. I believe Bill from Australia provided the best answer.
For magnetic issues, cast 316 SS does turn into a magnetic material, not because of 8-9% of ferrite content in casting operation, but due to moving rotational parts and since magnetism changes. Some areas turned into fully magnetic since magnetism increases due to rotation. We are working with pumps.

A. Hello Sankerdas,
With magnetism and non-magnetism of stainless steels depends on the centered structure of the steel. In a face centered cubic structure (fcc), of the austenite phase is non magnetic. A delta ferrite phase with a body centered cubic structure (bbc) has ferromagnetic properties which is magnetic. Your steel may have inclusions of ferrite stringers. Have you looked into nil ferrite steel? I don't know any way to de-magnetize 316L steel. That does not mean there is no way to do it. I would contact your supplier and tell them you need the face centered cubic structured steel which is non - magnetic. Hope this helps.

A. Hi. I don't know the answers, but I do see a semantic issue between magnetism and ferromagnetism that might be confusing people...

- Items like refrigerator magnets, that attract iron and steel, are "magnets" or "magnetic materials".
- Materials such as iron and steel that are attracted to a magnet but do not act as magnets themselves are called "ferromagnetic materials", not magnetic materials.

Plain steel is ferromagnetic. As a result of processing, it may become magnetic, so steel hardware is sometimes passed through a "degausser" to remove any magnetism it has acquired during processing. The degaussing renders it non-magnetic but NOT non-ferromagnetic -- the steel will still be attracted to a magnet.

Some "non ferromagnetic" stainless steels can become somewhat ferromagnetic through cold working, etc. I suppose it is even possible that they can become slightly "magnetic". If so, I would expect that degaussing would render them non-magnetic, but I don't think degaussing will solve the ferromagnetism problem. Then again, I'm just guessing :-)

Regards,