Many types of steel are too soft for certain applications. Very often it is necessary to harden a particular piece of steel. For example, if you wish to make your own chisels or a sword or a knife blade, it is necessary to harden most types of steel so they will hold an edge. In order to do this, you will need to infuse carbon into the outer layer of your steel. Carbon has been used to add strength to steel for centuries. For purposes of this article we will assume that the item you wish to harden will fit into a Weber Bar-B-Q.
Open the air vents on your Weber Bar-B-Q and remove the cooking grill. Lay down a layer of charcoal on the charcoal grill and light it.
Using a torch, heat your steel item until it glows red-hot. Do this very close to the Webber grill. When the metal is glowing red hot use your tongs to immediately transfer it to the burning coals of the Bar-B-Q. Lay the item on top of the glowing coals and quickly pour additional coals on top of the item. Then put the lid on the Bar-B-Q and close the top air vent halfway.
Allow your steel item to "cook" in the charcoal for two hours with the lid on the Bar-B-Q at all times. At the end of two hours, remove the lid and, using your tongs, remove your steel item from the coals. At this point, the item will be too hot to touch but will no longer be glowing red hot.
Re-heat your steel item with your torch until it glows a brilliant red in color. Pick it up with your tongs and immediately quench it in a vat of room-temperature water. After 30 seconds, remove it from the water, but be careful not to drop it or strike it, as it will now be brittle.
Re-heat your steel once again, but this time only heat it until it turns a brilliant shade of blue. Pick it up with your tongs, and immerse it once again into a vat of room-temperature water. This is the annealing process--it leaves the steel hardened but no longer brittle. Your steel is ready to be worked.
TL;DR (Too Long; Didn't Read)
Always wear heavy protective clothing when working with hot metal. Always wear eye protection and heavy gloves. Have access to water or a fire extinguisher at all times in case of an accident.
Do not plunge your hot steel into cold water. Always use room-temperature water.