eli5, Why does metal become harder when you heat treat it?
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Steel is a mixture of iron and carbon atoms. At working temperatures, the iron can “dissolve” more carbon than it can at lower temperatures. When you heat-treat steel, typically by quenching in water or oil, you are very rapidly cooling the steel. This prevents the “dissolved” carbon from exiting the structure of the iron atoms; they get trapped in there. This forms a different crystal structure in the steel than untreated steel. This alternative structure, because there is additional carbon stuck in it, is harder than untreated steel.
Heat treating (annealing) involves cooling the metal slowly. Slower cooling rates reduce the internal stresses in the metal by recrystallizing the atomic structure of the metal.
You are making it dirty by adding carbon. The metal is now weaker which makes it far less likely to shatter when hit against another hard object.
Most metals are actually made of something similar to crystals! One of the things that make crystals notable is that the slower they are formed, the bigger they form.
For metal, that means if it is slowly cooled the ‘crystals’ are big and the entire thing is ordered. Depending on the properties of the metal, this means that it might be harder and less likely to move, or it might be more flexible and less likely to break. On the other hand, quickly cooling (quenching) the metal can make many small crystals form and interlock better. Once again depending on the specific metal properties, this can make it more flexible or more hard.
“Heat treating” can be heating and a mix of quick and slow cooling in an attempt to cause the ‘crystals’ to form in a way best for the final goal of the metal.
On a basic level.
A metal is like a brick wall of atoms. The electrons if these atoms get so close together that they repel each other out of their places and begin feely flowing around the metal piece.
If you touch a metal rock in the wire you won’t get electricuted because there across the entire metal there is an equal number of protons and electrons. So there is no net electric charge.
The free flowing nature of electrons makes it like glue. (or cement)
It can be between 2 nuclei both of which are attracting it at the same time like tug of war. So they’re tied together.
Now, that happens, but billions of times. As depending on the metal, each nucleus can hold as many as 4 electrons (tugs of war) at the same time. So they’re all tied together.
And electrons work like cement.
Now. Will a layered brick wall be sturdier, or a brickwar where each brick is in a different orientation?
The layered one will.
That’s exactly what is done when a metal is treated.
Off sized bricks (impurities, mostly carbon) get removed (depending on the product needed they may leave traces)
And then the correct bricks are manipulated into a lattice in some form. (often just hammering)
just like ice. when you heat ice to boiling and the cool it very fast, it becomes denser and harder. Metals do the same thing, except that it should be cooled much slower as metal is denser than ice already.
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