I work in a steel factory where we stamp out internal parts for motors and generators and the material is anywhere from .018-.028 of an inch thick. And then we run it through a furnace for 4 hours at around 1400 degrees.
Afterwards I test he material through a magnetic tester and we record the core loss and the permeability. What does all this mean?
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Steel is a combination of iron, carbon, and a few other odds and ends. The exact balance between iron and carbon likes to stick to a few ratios.
So if you have an overall 4:1 ratio (made up numbers), it will form a mix of crystals that are 2:1 and 8:1 so the average is 4:1.
The space between these crystals will have a more chaotic mix. The more time the crystals have to form, the larger they get and the more the chaotic mix sorts itself out into crystals. Heating steel allows the crystals to move freely and reform.
Depending on the temperature a metal is at how long it is kept at that temperature, cooled, etc different kinds of microstructures get formed in the metal.
The elements in the alloy like iron, silicon, carbon, vanadium, etc, are the same after a basic heat treatment, they are just end up arranged differently.
To be more specific the atoms tend to form clusters of atoms in a regular pattern and particular orientation. Basically metal crystals. Sometimes the solubility of alloying elements changes and the atoms precipitate out and change the structure at different temps. The whole metal overall is made of a bunch of crystals with different orientations and compositions all next to each other. And the overall pattern is called the grain structure of the metal.
Here’s a photo of some metal grains to help make more sense of that.
https://www.researchgate.net/profile/Ad-Rollett/publication/279554094/figure/fig2/AS:391421854011394@1470333650244/Grain-structure-of-pre-annealed-material-annealed-a-900-o-C-for-hr.png
Anyway this structure affects the properties of alloys such as it’s mechanical strength, and electrical properties like permeability.
Since you mentioned permeability and core loss with thin sheet metal I suspect you are working with electrical/silicon steel. The stuff used in transformers and motor core laminations.
Permeability is basically how well it works as part an electromagnet. The point of using metal cores in transformers and such is that you can make a stronger magnetic field in a smaller space.
Core loss has to do with changing current and energy losses. As your run a changing current through a transformer or motor the magnetic field causes the little grains in the steel to shift a bit trying to align with the field. That ends up absorbing energy and turning it into heat. For a transformer or motor you don’t want that because any energy lost to heat doesn’t end up powering your devices. And if sever melts your wire insulation. Core loss also depends on frequency too. Higher frequency means more losses which makes sense as you are basically wiggling those steel grains more times.
Another important thing for transformer laminations is that they be nonconductive as possible. Basically if the steel is conductive, they also act as the secondary of a transformer. Just the current generated (called Eddy currents) just goes in circles. The reason we use laminations at all instead of a solid block is that by coating them with varnish it insulates all the laminations from each other reducing the available paths Eddy currents can flow. And being just current flowing in circles eventually it gets absorbed by the resistance eof the steel and turns into heat. Also not good for transformers when you would rather have that power do something useful and not cook your wires.
Nonconductive is kindof difficult for metals. But the silicon in silicon steel somehow changes the structure to make the steel very non conductive at least compared to regular steel.
As others have mentioned, it has to do with the molecules forming crystals at certain temperatures.
If you take a hard piece of carbon steel (like a file), you can heat it up until it glows at a pale yellow color(*straw?). If you then wrap it with insulation and let it cool down slowly, the crystals come apart. This is called annealing, and it makes the steel softer (and easier to cut and shape).
If you use a file to make a knife, annealing the first step. Once you have shaped the file into a knife, you will want to harden it again. This is “heat treating”
You heat the knife blade up until a magnet no longer sticks to it (straw colored). And then you want to cool it rapidly and freeze the crystals in the orientation that makes the steel harder.
Water would boil, so you dip the glowing hot blade in hot oil.
The molecules are like loose Lego pieces. If you align them and snap them together, they form shapes that are stronger than a simple pile of loose pieces.
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