Why is some stainless steel magnetic and other stainless steel is not magnetic?


Why is some stainless steel magnetic and other stainless steel is not magnetic?

In: Chemistry

They have separate crystalline structures. If the atoms are too widely spaced the magnetic moments cannot align and thus it cannot exhibit ferromagnetism.

Iron has two different “shapes” you could say. The atoms within it are aligned in a special pattern, which strangely changes at 911°C from body-centered-cubic (bcc, also called ferrite) to face-centered-cubic (fcc, also called austenite). Interestingly, fcc is not magnetic.

Now for stainless steel. Steel is iron with less than 2% carbon, but that’s not necessary to know. To make steel stainless, you need to add at least 12% chrome (or chrome equivalents) that will react with the oxygen before the iron can. This steel has a bcc pattern and is magnetic, because the chrome has a bcc pattern, too.

If you add more than 10% nickel(or nickel equivalents), which is fcc, to the 12% chrome, the pattern in the iron changes from bcc to fcc, even at room temperature. The stainless steel isn’t magnetic anymore.

You can look up the “Schaeffler diagram” for some more details(martensite is what you get when you harden steel)

Why would you add that much nickel, when the steel is already stainless? The bcc pattern becomes “brittle” at lower temperatures, meaning it won’t stretch before breaking, which is important for safety. Depending on the kind, this can already happen at 0°C. The fcc pattern though doesn’t have this effect, it doesn’t even care about -200°C, and you can build things like containers for liquid gas out of it.

Stainless steel is a low iron alloy mostly. Its full of Chromium and stuff to make it corrosion resistant. This also makes it incredibly hard so it is harder for the iron atoms to align.

Higher iron/softer steel means more magnetism.

Not an engineer, but a shade tree motorcycle mechanic and DIY homeowner. Have done some work with stuff made from really cheap stainless, and some that was really expensive.

“Stainless” is more of a suggestion than a hard characteristic. More accurate to say “won’t visibly corrode with certain limits of PH, temperature, salinity, etc”

“Stainless” steel is a wide collection of alloys that exhibit a bunch of different properties. Some are magnetic, some are pretty strong, some have incredible corrosion resistance. Some are biocompatible. Stainless that is both corrosion resistant AND really strong is usually really expensive.

Stainless steels can have different „ingredients“ (phases). These phases develop if you add chromium and/or nickel to the molten mass and let it cool to room temperature. There are other additives that behave in a similar way.
Some ingredients are non magnetic (Austenite), some are. So it depends on how much of an ingredient is in the steel.

I browsed some of the responses and most were overly complicated or only partially right. True stainless steel is not magnetic. Appliance makers want their fridge to be magnetic, but to do that, trade offs have to be made. Some have lowered the “stainless” content low enough to be magnetic, but they rust and tarnish more easily. Other methods have been to use different material altogether that mimic stainless. The manufacturers are constantly developing different finishes to improve problems. Another problem with stainless is it attracts and shows fingerprints. Manufacturers are now working on metallic doors that are both magnetic and also don’t show fingerprints so easily.

If induction is so great, why don’t top restaurants use them?

Think of stainless steel like pasta. You have many different types of pasta and they serve different purposes. Now for stainless steel (an alloy), you have ‘family’ subsets and within those different families you have further ‘grade’ subsets. The four families of stainless steel are:

**Austenitic** Stainless Steel

Ferritic Stainless Steel

Duplex Stainless Steel

Martensitic & Precipitation Hardening Stainless Steel

The bolded family is easily the most popularly used family of stainless steel. Grades within that family that you might hear thrown around are **316, 304, 316L, and 304L.** If you have something that is stainless steel it’s like a 95% chance it’s one of these.

The austenitic family has *almost* no magnetic response, the other families do. Austenitic steel has a “face centered” structure. It’s the geometry of this face-centered structure that makes electron alignment difficult. The reason why this structure happens during cooling is probably beyond an ELI5.

[Steel Crystal Structure](https://pipingtech.com/wp-content/uploads/2020/02/3-major-types-of-Stainless-Steel-Austenitic-Ferritic-and-Martensitic.jpg)

It’s a marketing gimmick, Timmy. “There are over 150 grades of stainless steel, of which 15 are most commonly used.” It’s all dependent upon how much of the element nickel is in the mix. Nickel is what determines if the grade that you are looking at is magnetic or not.(per the internet. When I was working on a environmental spill response crew, I was told it was the amount of chromium in the mix).

I work with aircraft superalloys & most have a high level of nickel. Inconel, L605 & A286 are nonmagnetic, but 17-4 & 15-5 (both 400-series) SS is very magnetic. 400-series is a breeze to machine & can be heat-treated for hardness.