Spheroidization
Spheroidizing occurs when carbon steel is heated to about 1290°F for 30 hours. The cementite layer in the pearlite microstructure is transformed into a spheroid, resulting in the softest and most ductile form of steel.
Full annealing
Carbon steel is annealed by preheating slightly above the top critical temperature—maintaining that temperature for an hour—then cooling at a rate of about 36°F per hour. This process results in a ductile coarse pearlite structure without internal stress.
Annealing process
The annealing process reduces stress in cold-machined low-carbon steels (> 0.3% C). Steel is heated to 1025–1292°F for one hour. Dislocations in the microstructure are fixed by the re-formation of crystals before cooling.
Isothermal annealing
High carbon steel is first heated above the upper critical temperature. It is then retained, cooled to a lower critical temperature, and maintained once again. Then it is gradually cooled to room temperature. This process ensures that the material reaches a uniform temperature and microstructure before the next cooling step.
Normalization
Carbon steel is heated to normalization temperature for an hour. At this point, the steel fully enters the austenite phase. The steel is then cooled with air. Normalization creates a microstructure of fine perlite with high strength and hardness.
Cooling
Medium or high carbon steel is heated to normalized temperatures, then cooled (rapid cooling by immersion in water, brine, or oil) to upper critical temperatures. The cooling process produces a martensite structure—very hard, but brittle.
Steel tempering extinguished
Heat treatment is most common because the results can be predicted accurately. The quenched steel is reheated to a temperature below the lower critical point, then cooled. Temperatures vary according to desired results—with the 298–401°F range being the most common. This process restores some toughness to the quenched brittle steel by allowing some spheroidite to form.
