Heat treatment is usually a process which changes the properties of metal.
If you did metal work at school you might remember quenching you chisels or screwdrivers in water or oil.
Carbon steels react differently depending on the temperature you take it to and how you cool for. If you take it up to its upper critical point, red hot bright orange and quench instantly this will make it very hard, but very brittle. disks would shatter. so the temperature and the cooling rate will be decided to give you the best of both worlds harness and toughness.
Other heat treatment of metals include annealing = making the metal as soft as possible. Normalising = bringing the metal back to its original state after its been worked ie putting stress into the metal. Case hardening = putting a chemical compound or carbon into the outer surface of the metal which gives it a very hard outer surface but a tough inner. this is only for a few thousands of an inch. they nitride crankshafts etc. which is basically this but using chemicals.