Magnetism of Austenitic Stainless Steel Fasteners

[phamENG]

Before we start, yes, this is for a personal project, but I design with stainless a few times a year and also wonder for the sake of my specification writing with regard to fastener selection, etc.

Austenitic (such as 316) stainless is not magnetic in its "raw" form. However, cold working can realign the grain structure and form a martensite or ferrite within the steel and create magnetism without affecting corrosion potential as it's the chemistry that prevents corrosion and not the magnetism. This I know.

However...I've noticed recently a search for some good stainless steel hardware that all of the flat washers I can find are magnetic. I carry a magnet in my pocket and they snap to it like a block of iron or steel. I get cold working of bolts and maybe nuts...but a flat washer? Is there really that much cold working? Anyone have any thoughts on this? I'm concerned about the quality as I'm replacing a pair of heavily corroded washers on my boat. They sit under stainless fasteners and against a stainless fitting, and the quality of work form the previous owner makes me think they would have at least used "stainless steel" washers there.

Yes, this is for a personal project, but I design with stainless a few times a year and also wonder for the sake of my specification writing with regard to fastener selection, etc.

Thanks!

 

[EdStainless]

There could be a couple of reasons.
If they are 304/316 they likely have been stamped from cold rolled sheet.
The other possibility is that they have cheated and supplied a 400 alloy.

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P.E. Metallurgy, consulting work welcomed

 

[phamENG]

Would stamping really cause enough cold working to magnetize it?

The swap for 400 is what I'm concerned with.

 

[EdStainless]

If the sheet was as-cold-rolled then it would be magnetic before the stamping.
If the difference between 430/304/316 matters then none of them will work.
They are all 16-18Cr so they have the same pitting resistance.
316 just takes a little longer.
Now if they used 409/410 that could be an issue.

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P.E. Metallurgy, consulting work welcomed

 

[dvd]

Blanking of washers definitely would create cold-worked zones on the i.d. and o.d. of the material, in addition to prior cold-working of strip steel fed into the dies.

It might be interesting to cut sections from some washers that include and exclude the edges to see if the magnetism changes.

 

[phamENG]

Thanks, Ed and dvd. I went to a marine supply shop and picked up a few from them. The same magnet gets slight purchase at the edges (as one would expect from stamping them), but it's not enough to actually hold the piece. Unlike the more prevalent stainless washers that don't have an alloy listed that snap to it as well as any carbon steel washer.

This is all, of course, inconclusive as there are reasonable explanations for it...but it sure makes me wonder what you get when you buy 'stainless steel' hardware from a big box store - a common supply source for a lot of smaller contractors in my area, even those who build docks in brackish and salt water parts of my region.