Oak Insights

This was a school project.
Case hardening is a process used to harden the outer layer of a steel object, while maintaining the strength and toughness of the core, and without altering the chemical composition of the entire object.

The Heating Cycle
In the first step the steel, which is a combination of iron and no more than 2.08% carbon, is heated to around 930 degrees C in an environment filled with carbon-containing material. The material most commonly used is a gas that contains methane. Solids and molten materials are also used, but for most industrial purposes a gas is the most cost-effective method. When heated, the gas releases the CO that hardens the steel.

See the heating cycle

Crystallization
When the steel reaches 930 degrees C, its crystalline structure expands and allows the carbon atoms to enter the crystal lattice. This part takes a few hours.

See crystallization 1

When the steel is quenched, the structure returns to its original shape, but leaves the carbon trapped inside (the blue dot in the diagram).

See crystallization 2

The cooling cycle
The steel is kept in the carbon-rich environment at a constant 930 degrees C for a few hours, then quenched (cooled rapidly) in a solution of oil or water.
When the steel is quenched, the structure returns to its original shape, but leaves the carbon trapped inside.

See the cooling cycle

Sources:
The Encyclopedia Britannica 1
The Encyclopedia Britannica 2
The Ducati Website