If iron cobalt and nickel are magnetic (which I think is the right term, but it feels wrong since magnets stick but it doesn’t magnet to other steel) then why does using nickel to make stainless steel render steel non-magnetic?
Or is my metallurgical understanding just completely off?
In: Physics
I think you are misunderstanding a few things. I don’t think Iron and Nickel are magnetic by default. They are however ferrous, meaning that a magnet will stick to them and it is possible to magnetise them.
*Some* stainless steel is magnetic! Of the five most common types of stainless steel, *austenitic* stainless steel is generally not magnetic, and this lack of magnetism is due to its crystal structure more than the elements that make it up — how the atoms are arranged in the metal.
Not all iron is magnetic. Austenite, which is a type of iron with a specific arrangement of atoms (specifically, a face-centered cubic unit cell) is non-magnetic.
Ferrite iron, which is magnetic, can transition to austenite at high temperatures – such as those used to make steel.
For the same reason that cast iron is non-magnetic (well, it’s still technically magnetic, but much less so): The high temperatures they are subject to make their inner structure change, and that inner structure is what makes them magnetic in the first place.
As for what’s so special about this inner structure, think of each individual atom as its own tiny magnet (so with a north and south pole). A ferromagnetic material (such as Iron) has all of the atoms aligned with eachother (as in, all of the north poles are pointing in the same direction).
Stainless steel, such as AISI type 316 in the austenitic phase, has a magnetic permeability of 1.008 or so (i.e. almost negligible). Where this can change is when the metal is cold-worked during forming, hammering or drawing operations. The act of working the steel causes a phase transition from austenite to martensite within the microstructure of the steel in the strained regions, and this causes the steel to become ferromagmetic in those regions. The magnetic response can be subsequently eliminated with heat treatment. A full solution anneal will drive a phase change back to the fully austenitic condition.
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> Or is my metallurgical understanding just completely off?
This is more of a physics question than a metallurgical question.
Metallurgically, stainless steel with high nickel has a different crystal structure than “regular” steel. This crystal structure (austenite) is actually the same one as high temperature steel, which is also not magnetic.
With large simplification (partly because I don’t fully understand the quantum mechanics involved), the spacing of the atoms is different between the 2 crystal structures. This difference is enough that the metal loses ferromagnetism.
Ferromagnetism (aka regular magnetism) basically happens because the spin of atoms’ electrons are aligned with each other. Depending on the distance between atoms and some quantum mechanical properties, atoms will be more-able or less-able to influence the spin if their neighbors.
In “magnetic materials” such as iron and nickel, each atom links up spin to its neighbors, self-reinforcing the magnetic field. In in stainless steel the atoms are spaced enough that the atoms don’t manage to self-reinforce each other.
I think you are confusing magnetism with ferromagnetism.
Iron cobalt and nickel are ferromagnetic, not magnetic.
>If iron cobalt and nickel are magnetic
The specific term you’re looking for is “ferrous”, meaning “like ferrum” (ferrum is the classical name for iron, hence Fe).
Ferrous materials are affected by magnetic fields, but do not emit their own.
One thing to keep in mind is that materials are magnetic due to how the atoms (and their electrons) are organized relative to each other, ie, their crystalline structure.
Iron is magnetic because iron atoms organize themselves in a way that makes them so. And they organize this eys because of the “shape” of the atoms.
It’s like a puzzle, and if you add some nickel, you’ll have another distribution, just like if you add added fee triangles in a square tile pattern. If you add more, the structure changes again to make everything fit.
So, magnetism in the end is not intrinsic to the element, it’s that the element has characteristics that create these magnetic materials. If you add them together, they may or may not become some magnetic material as well.
Very smart 5yo you are.
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