HEAT TREATMENT

The purpose of heat treating plain-carbon steel is to change the mechanical properties of steel, usually ductility, hardness, yield strength, and impact resistance.

Types of heat treatments

Spheroidizing: Spheroidite forms when carbon steel is heated to approximately 700 °C for over 30 hours.The purpose is to soften higher carbon steels and allow more formability. This is the softest and most ductile form of steel.

Full annealing: Plain-carbon steel is heated to approximately 40 °C above Ac3 or Ac1 for 1 hour.The steel must then be cooled slowly, in the realm of 38 °C (100 °F) per hour. Usually it is just furnace cooled, where the furnace is turned off with the steel still inside. Fully annealed steel is soft and ductile, with no internal stresses, which is often necessary for cost-effective forming. Only spheroidized steel is softer and more ductile.

Process annealing: A process used to relieve stress in a cold-worked plain-carbon steel with less than 0.3 wt% C. The steel is usually heated up to 550–650 °C for 1 hour, but sometimes temperatures as high as 700 °C.

Normalizing: Plain-carbon steel is heated to approximately 55 °C above Ac3 or Acm for 1 hour; this assures the steel completely transforms to austenite. The steel is then air cooled, which is a cooling rate of approximately 38 °C (100 °F) per minute. This results in a fine pearlitic structure, and a more uniform structure. Normalized steel has a higher strength than annealed steel; it has a relatively high strength and ductility.

Quenching: Plain-carbon steel with at least 0.4 wt% C is heated to normalizing temperatures and then rapidly cooled (quenched) in water, brine, or oil to the critical temperature.This crystalline structure has a very high amount of internal stress. Due to these internal stress quenched steel is extremely hard but brittle, usually too brittle for practical purposes. These internal stresses cause stress cracks on the surface. Quenched steel is approximately three (lower carbon content) to four (high carbon content) times harder than normalized steel.