When you take a solid piece and look closely into the pattern, you will find that the molecules of the solid arrange themselves in a certain lattice structure. This is due to the inter-molecular bonds. Whenever you try to change the shape of the structure in order to create your own component, you are indirectly trying to change this crystal structure. The molecules tend to restore their original positions causing internal stresses. In order to get rid of these stresses, we use heat treatment.
Consider the crystal structure of a metallic bar as shown in the figure. Of course, the real structures are more complicated, but just for understanding purpose let us consider they have this simple structure with circles representing the molecules and lines connecting them as the inter-molecular bonds.
Now, let’s bend the metallic bar. As you can see from the second figure, the molecules present in the inner side of the bend (concave side), come close to each other, while the molecules on the outer side (convex side) of the bend move away from each other. As a result, the molecules on the inner side push each other away while those on the outer side try to pull each other towards them. Think of the bonds like springs, if you stretch it, it will try to contract and if you compress it, it will try to expand. The lattice also undergoes permanent deformation, but let’s not talk about that. This push and pull action tries to bring this bar back to its initial shape thus giving rise to internal stresses.
Now, let’s heat the bar. What happens is that the old molecular bonds weaken and break; and molecules also re-position themselves into new lattice pattern, kind of like people trying to be comfortable in a crowded train. On cooling, new bonds are formed to suit new lattice arrangement. Now the structure acts as if the metallic bar originally was supposed to be bent.
And now you get the stronger finished component.
Based on the way and how long it is heated and cooled, different types of properties can be achieved.
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