An easy experiment with a gas range and a box of paper clips can give you a good understanding of how to strengthen, and soften, steel. Should you ever be hurled into a medieval world of elves and sorcerers, you can make your living as a smithee.
Metallurgy isn't just a matter of melting things down and pouring them into a mold (or onto Viserys Targaren's face). There are some surprising techniques that need to be followed if you want to make a good sword. And you can demonstrate a few of them with a gas stove and a box of paperclips. Grab them, and a glass of water, and it's time to experiment. (Io9 is not responsible for ouchie fingers or a burnt-down apartment complex.)
Fire up your gas stove and hold one paper clip over the burner, preferably while protecting your hands with oven mitts. Wait until it gets good and hot, and then put it aside on a plate, and let it cool off slowly.
Grab another paper clip and put it over the flame. Have the cup of water handy. If possible, make it ice water. When the paper clip has gotten hot, throw the clip in the water to cool it quickly.
Grab another clip. Heat it up. When it gets hot, take it out of the flame, but don't leave it alone. Instead, bend it back and forward again and again until it cools down.
Take a fresh, un-heated paper clip and bend it back and forth, to get a feel for its strength. Then test each of the other paper clips. The second two should feel a lot stiffer than the untreated paper clip. But the first one, that was allowed to cool slowly, should be weaker and easier to bend than it was before.
And so, you see that you wouldn't make a good living as a blacksmith if you only heated swords to repair them. Each heating would leave them softer than before. But why? Metals are made of atoms that have formed repeating patterns. The patterns are regular, but they are not perfect throught the metal. There are dislocations. These dislocations, when they're on their own, make it easier to bend the steel. When they criss-cross eachother, though, they jam up and make the metal harder to bend.
Heating the metal gives the atoms in the metal extra energy, and lets them flow more easily than they did before. The dislocations that usually jam together can flow past each other during the long, slow cooling process. The result is a softer metal, with fewer crisscrossing dislocations. The slowly-cooled paperclip is softer after it was melted.
Bending the metal back and forth while it is in this state creates more dislocations. These jam against each other more, and cause the cooled metal to become harder to bend than it was before. There is a trade-off, though. The sword is strong, but it's brittle. If one of those crisscrossing dislocations does give, the dislocations don't flow past each other. They break. Steel treated this way can shatter.
But hitting the metal with cold water really changes things. The atoms in a metal at high temperature are organized differently than metal at regular temperature. A sudden change in temperature doesn't given the atoms time to go back to their normal low-temperature state before the matter solidifies. Regular steel has a cubic structure, which is malleable. Steel that has been heated forms many layers of overlapping plates. These plates are very resistant to bending, but they're brittle and can shatter if hit hard enough.
And how to make a sword? The three processes are combined. To begin with, the sword is shaped from a hunk of steel into something vaguely sword-shaped. During this process, it's heated many times. Sometimes it is hammered - a process that both shapes it and pounds in those dislocations so it hardens. Sometimes it is cooled without hammering, a process called annealing. This softens it up again and keeps it from being too brittle. When it comes time to do the fine work, the sword is annealed many times, making it soft enough to be sharpened, usually by grinding. After the sword is sharp enough, it is hardened using the cool water process, known as quenching. The sword is quenched over and over, at gradually decreasing temperatures, until it gets the right mixture of hardness and shatter-resistance.
Then you can go out and slay something. (Io9 is not responsible for what you slay, either. Unless it's a werewolf. Then we want credit.)