Stainless Steel and Magnetism?

H

HoosierQ

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Feb 9, 2010
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When I had a boat, there was always a "rule of thumb" for testing the quality and rust resistance of rails, cleats, and other hardware exposed to salt water. That was to carry a magnet and if the magnet didn't stick, you had good stainless. To be honest, I never tried it...I was a lake boater.

That was 20 years ago. Today, having long since divested of any and all watercraft and moved on, I am here. Magnets stick to all my stainless knives. I just tested VG-10 and S30V and whatever Victorinox uses and of course the magnet sticks. I expected it to.

So I guess I wonder either a) was that "boat stuff" all a crock, b) are boats using H1 or something (does a magnet stick to H1?) else?

Just wondering in terms of knowing more about steel. Boat guys know a lot about chines and tumblehome and deadrise and freeboard but steel? Not so much...beyond never use a steel propeller in fresh water with lots of obstacles...use aluminum...better to break a prop than the drive shaft. Use bronze in saltwater if you can. That's about it for marine metalurgy.
 
There's a HUGE array of stainless steels, made for vastly different applications. The stuff that's made for extreme corrosion-resistance, such as the hardware used on/around boats, or stainless steel plumbing, or 'surgical stainless' made for medical implants (joint replacements, etc.) will have much higher chromium content (maybe a high percentage of nickel, too). Most of those are also what might be termed 'low-carbon' stainless steels, which means they won't have enough carbon to do what knife blades are designed to do, which is hold an edge. Carbon is what allows steels to be heat-treated to achieve the hardness necessary for knife blades and other hardened steel tools. A decent 'stainless' knife steel will always be a compromise between edge-holding and corrosion-resistance, with most of the emphasis on edge-holding; therefore, less chromium and more carbon.

Regarding magnetism, I think the combination of very high chromium/nickel, and the lower iron & carbon content of stainless made for products other than true cutlery, is at least part of what makes these stainless steels less magnetic.
 
Most non magnetic stainless of a good quality is the 300 series stainless. Used this all the time in the pharmaceutical industry.
302,304 and the higher 316 and 316L had less chance of tarnishing or rusting and were required by the FDA in many applications.
 
typical marine steel is 304 or 316. It is the cheapest marine grade.
Magnetism varies. I learned: "If it is non magnetic, you can be reasonably sure it is stainless. If it is magnetic, maybe it is, maybe it isn't." It is a low tech test for untrustworthy places. What other test can you do in an African port?

Better, more expensive SS is magnetic. Think of forged & tempered shackles. I suspect these are mostly 400 series. Way better quality than 300 series. All magnetic.

In Kenya, on a boat, the test has a little merit. With knifes, it has none.
 
Quote from http://www.physlink.com/education/askexperts/ae546.cfm
As for whether they are magnetic, the answer is that it depends. There are several families of stainless steels with different physical properties. A basic stainless steel has a 'ferritic' structure and is magnetic. These are formed from the addition of chromium and can be hardened through the addition of carbon (making them 'martensitic') and are often used in cutlery. However, the most common stainless steels are 'austenitic' - these have a higher chromium content and nickel is also added. It is the nickel which modifies the physical structure of the steel and makes it non-magnetic.

Fe, Co and Ni have 3 unpaired electrons, these elements magnetic because unpaired electron generates polaric magnetic field (unlike paired electrons spin cancelled each other magnetic field).

I am not a chemist, so +/- 2cents opinion:D
 
bluntcut said:
Quote from http://www.physlink.com/education/askexperts/ae546.cfm
As for whether they are magnetic, the answer is that it depends. There are several families of stainless steels with different physical properties. A basic stainless steel has a 'ferritic' structure and is magnetic. These are formed from the addition of chromium and can be hardened through the addition of carbon (making them 'martensitic') and are often used in cutlery. However, the most common stainless steels are 'austenitic' - these have a higher chromium content and nickel is also added. It is the nickel which modifies the physical structure of the steel and makes it non-magnetic.

Fe, Co and Ni have 3 unpaired electrons, these elements magnetic because unpaired electron generates polaric magnetic field (unlike paired electrons spin cancelled each other magnetic field).

I am not a chemist, so +/- 2cents opinion:D
Click to expand...

Thank you for that. :thumbup:

I had long been aware (or at least suspected) that the very high chromium and/or nickel steels were less magnetic, but this is the first I've seen narrowing it down to the higher concentration of the nickel (combined with lower amount of carbon, in part). Something new to think about.

Along those lines, here's an interesting article from Scientific American: Why don't magnets work on some stainless steels?

(quoted from the above linked article)
"The most popular stainless steel is Type 304, which contains approximately 18 percent chromium and 8 percent nickel. At room temperature, the thermodynamically stable crystal structure of 304 stainless steel is bcc; nevertheless, the alloy's nickel concentration, as well as the small amounts of manganese (about 1 percent), carbon (less than 0.08 percent) and nitrogen (about 0.06 percent), maintains an fcc structure and therefore the alloy is nonmagnetic. If the alloy is mechanically deformed, i.e. bent, at room temperature, it will partially transform to the ferritic phase and will be partly magnetic, or ferromagnetic, as it is more precisely termed."
Click to expand...
 
Fascinating. Like I said, I never tried it out as I was never in the market for a marine boat...I live in Indiana. I was frankly always skeptical then as I figured the iron would do the trick in and of itself. Now I know more. To be clear, I was not intending or hoping to use the magnet test on knives. Quite frankly I've learned so much from all of you that I know what's good and what's not...if I know what it is and how it's been treated.
 
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