By heating and subsequent cooling down to room temperature, the workpiece is hardened through to its core. This method can produce in form of changes (warping) and cracks (brittle).
Annealing means heating a workpiece up to and maintaining a temperature of 200 to 600 °C over a prolonged period of time with the purpose of increasing the toughness of the hardened material after chilling to maintain the partial texture change. Annealing reduces the hardness of the material.
Also known as temper hardening. Means annealing in the upper temperature range.
Here the carbon content of the outer layer of the workpiece is increased (carbonisation). If nitrogen is added to this process, this is referred to as carbonitration.
While formerly this process involved the use of molten salt, today it is accomplished with gas for environmental reasons. With case hardening you can obtain very hard outer layers on the workpiece surface while the core maintains its original toughness. This process can be controlled to increase the thickness and hardness of the outer layer.
This hardening process involves the use of boron to increase the resistance to wear in the outer layer.