magnetic stainless steel

Q:I work in an operating theatre. Some of our microsurgical instruments which are made of stainless steel are becoming magnetized, then they repel the needle they are supposed to hold. This is a recent problem and never occurred over the last 20 years. Could there be a magnet in the microscope that they are used under? Would this cause the magnetism? How can I prevent this? I have tried using a demagnitizer but they don’t seem to work? Please help.

-carol
hutt hospital, new zealand
A: This is a very good question, and obviously finding the answer is important. So we’ll try to help a little bit.

It’s hard to see how the steel could be magnetized much unless there were some magnetic field present, and the Earth’s field is very weak. So maybe your guess about the microscope is right- perhaps it has a coil for some purpose, or holds something in place with a magnet. Why not take a little compass and put it through the usual routine of the instruments to see where it’s in a strong field? Or (you probably have a big budget) you could be fancy and buy a Gaussmeter with a probe to stick in the places your instruments go to actually measure the magnetic field.

Are your instruments in a different environment than they used to be? Or have the instruments changed? Good stainless steel is very hard to magnetize.

Are they new instruments or old? If they’re new ones, maybe you should sample something from another manufacturer. But I suspect they’re old ones. Probably they’re sterilized in an autoclave. The iron-chromium alloy may be subtly degrading under the high temperature, forming little pockets of easily magnetizable purer iron. If so, you may find that you just have to occasionally replace the instruments, since once this happens they will easily remagnetize even if you succeed in demagnetizing them.

So maybe the magnetism is a useful first indicator of metallurgical degradation. Even so it’s odd that they’re seeing a significant field.

Please let us know how this one turns out.

Mike W.

There might be some stray field from an MRI unit close by. The limit is 5 Gauss in publicly accessible areas, but that perhaps should be checked with a Gaussmeter. I agree with Mike that it’s more plausible that it’s a smaller magnet close by that’s magnetizing your instruments.

Tom

(republished on 07/13/06)

Follow-Up #1

Q:Is it that a magnetic stainless steel is always of not good quality? Is it that they get corroded soon? or may be the self-repair oxide layer that is formed is not strong enough?

-Devesh (age 23)
A: My understanding is that the magnetism indicates that there are regions of nearly pure iron, without enough chromium. Those regions would be more susceptible to corroding than would good high-chromium stainless.

Mike W.

(published on 08/14/06)

Follow-Up #2

Q:I know this is an old answer, but I stumbled across it and I have disagree with the previouse responses, the level of magnetism that stainless steel exhibts has nothing to do with the "quality" of the stainless. It is not a question of the "quality" of the stainles, but rather what type of stainless. Many types of stainless are highly magnetic, it is only the "Austenitic" stainless groups that are not magnetic. The austenitic stainless group is 18-8 steel, AKA "300" series like 301, 302, 304, 316 etc... Most stainless steel average folks run into (appliances, store bought bolts, etc..) is a 300 series steel, hence the myth that "ALL" stainless steel is not magnetic. The 400 series of stainless is magnetic, so an object made with 430 stainless will be attracted to a magnet. It is by no means a "lesser" grade, it is just a different grade. Interstingly, if you coldwork a piece of 304 stainless (like a medical instrument), it will become magnetic. So if you are constantly bending or flexing the instrument it will gradually become magnetic and hold a magnetic charge. If you stress relieve the material, by heating it (over 700 degrees,) it will once again lose its magnetic properties. If you need to avoid the cold-worked magnetism quality, use instruments made from 316 stainless not 304 stainless.

-Dave (age 32)
Hunstville, AL, USA
A:Thanks.

Mike W.

(published on 10/04/09)

Follow-Up #3

Q:Hello I have seen the question concerning magnetized microsurgical equipment.

It may not be a problem of equipment degradation, some microsurgical equipment are designed to posess paramagnetic properties so that OT assistants can find them with a magnet if dropped onto the floor (they are so small to be found with naked eye, hence we sweep the floor with a magnet to pick them up).

In any case, I suggest Carol to contact the equipment company salesperson, who is usually over-enthusiastic to provide more than enough advice.

-Surgeon
A: Thanks for the helpful comment. I suspected material degradation because the problem had just arisen. The source, however, could have been some new equipment exposing the instruments to a field.

Mike W.

(republished on 07/13/06)

 

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