Spheroidizing

Spheroidizing occurs when carbon steel is heated to approximately 1290°F for 30 hours. The layers of cementite in the pearlite microstructure are transformed into spheroid, resulting in the softest and most ductile form of steel.

Full annealing

Carbon steel is annealed by first heating slightly above the upper critical temperature—maintaining that temperature for an hour—then cooling at a rate of approximately 36°F per hour. This process produces a coarse pearlitic structure that is ductile with no internal stresses.

Process annealing

Process annealing relieves stress in cold-worked, low-carbon steel (> 0.3% C). The steel is heated to 1025–1292°F for one hour. Dislocations in the microstructure are repaired by the reforming of the crystal before cooling.

Isothermal annealing

High carbon steel is first heated above the upper critical temperature. Then it is maintained, cooled to the lower critical temperature, and maintained once again. It is then gradually cooled to room temperature. This process ensures that the material reaches a uniform temperature and microstructure before the next cooling step.

Normalizing

Carbon steel is heated to the normalizing temperature for one hour. At this point, the steel completely enters the austenite phase. The steel is then air cooled. Normalizing creates a fine pearlitic microstructure with high strength and hardness.

Quenching

Medium or high carbon steel is heated to the normalizing temperature, then quenched (rapid cooling by submersion in water, brine, or oil) to the upper critical temperature. The quenching process produces a martensitic structure—extremely hard, but brittle.

Tempering quenched steel

The most common heat treatment because its outcome can be accurately predicted. Quenched steel is reheated to a temperature below the lower critical point, then cooled. Temperatures vary according to intended outcome—with the 298–401°F range being the most common. This process restores some toughness to the brittle quenched steel by allowing some spheroidite to form.