Carburizing is a technique to get (more) carbon in the iron or steel. That is obtained by glowing it in a carbon -rich environment above 700°C, without oxygen (O2) can reach it.

steelcarbonPut the work piece or steel in carbon (charcoal powder, bone meal..), and around those you cover with loam. This you let dry slowly next to a wood fire. Then you slowly work it into the glow, the clay must not break because too much oxygen can enter then.

The heat penetrates every hour 0.1mm carbon in the iron.
Want a rod of 5 mm carburized to the core, then he must therefore be annealed 25 hours.
When carburizing a kind of compound material with a hard top layer is created with an unchanged, tough core. Is there carbon in the steel then it becomes hard if you quickly cool it in water: carbon gets not the time to escape or change in structure. You let it cool slowly (in the air) it is too soft. This is called annealing. When quenching the steel becomes hard and brittle. Iron continues even after quenching, soft and chewy.
If you cool a rod of 5 mm thickness, which is only 10 hours carburized, then only the outer layer of 1 mm will be hard and brittle, the core of 3 mm soft and chewy. As a result, the bar will break less. By combining the iron and steel with each other, they cancel each others bad properties: tough to brittle, hard against soft (flexible).