From Oleg Shalaev
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Some secrets of Saruman's molding business

Quenching swords for Saruman.

Why Saruman's employees cool down swords in the water?

Wikipedia tells us that this is called "Quenching" and that it makes the metal harder.

But why metal becomes harder when you heat it and then rapidly cool down?

Hardness and dislocations

Liquid metals solidify into crystals at lower temperatures. Even if we try really hard to grow a perfect crystal, certain types of defects in its lattice are almost inevitable. One of such defect types is dislocation: an extra "atomic sheet" (atomic plane) ending inside the crystal.[1] Dislocations can move inside the crystal; their movements are accompanied with plastic or permanent deformation.

A dislocation: an atomic plane ending inside the crystal.
Dislocations move easily when there are no defects nearby.

To deform an ideal crystal without dislocations we would have to distort macroscopically large number of atomic bonds simultaneously. This costs a lot of energy.

But a real-life crystal will always have dislocations. One has to apply less energy to deform such an imperfect crystal, because deformation would just result in dislocation movement which is relatively inexpensive from the energy point of view.

In a good crystal which has only few defects, dislocations can move easily (see the animation). If there are no defects in the vicinity of the dislocation, its elementary movement act (step) is just bond switching between adjacent atoms.

If we want to make crystal harder, we have two options:

  • grow a crystal without dislocations (which is almost impossible) OR
  • hinder dislocation movement.

That is, we have to spoil the crystal (distort its beautiful ordered structure). Then the energy cost of dislocation movement will increase.

Formation of a crystal: haste affects quality

Spectators taking the seats in the theater are similar to atoms occupying lattice sites.

Imagine a big theater with equidistant rows of seats for spectators. The number of people matches the number of seats. People like comfort: everyone wants to take a seat and no one wants to sit on the floor.

Let us play a game with the spectators: we will give them some time to find a vacant seat, then the bell rings, and after that people can not walk any more: they have to sit no matter where they are. (No reserved seating: anyone can take any vacant seat.)

What happens if we give 20 minutes for people to find their seats? Then everyone will find a seat, and no vacant seats will remain. In this case we will have perfect order in our theater, just like in a perfect crystal.

But what if the bell rings after two minutes? Then some seats will remain vacant, and some spectators will have to sit on the floor between the rows or on the knees of other people.

Similarly, atoms can move inside the metal at high temperatures.[2] If we give them enough time (cool down slowly), they will be able to form (almost) perfect crystal finding their "vacant seats", that is, unoccupied sites of the periodic crystal lattice.

The faster we cool down the metal, the less perfect lattice it will have.[3]

Further technological improvements

Quenching is probably the most ancient way people used to improve mechanic properties of a metal. Addition of certain ingredients (e.g., carbon) is another way to improve its mechanic and chemical properties.

  1. There are other types of dislocations; here we consider only the simplest one.
  2. At low temperatures the movement probability decreases rapidly (exponentially).
  3. Nowadays physicists are able to cool down thin metallic sheets so fast that the resulting cold metal becomes amorphous (does not have a lattice).