Annealing
Metals, despite their outward appearance, are crystalline in structure, and when they freeze from the molten state a structure will form based on the mixture of metals and non-metals in the solid, and the speed of freezing.
Working metals tends to put stesses and strains into the crystal lattice, and to make it harder and more brittle. (try twisting a paperclip)
Heating metals up to within a few points of melting (like cherry red) tends to allow the stresses and strains to relax back to a normal state. So after this they can be worked on again.
Quenching metals in air, water or oil has the effect of freezing a range of different crystal pattern into the metal, which can be useful if you want suspension springs for a car, or the hardness of a file.
Varying the chemistry of the composition .... this is getting too much like a lecture.....
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