Rolls are heat treated for several reasons:

1. To relieve tension after forging – called normalization
2. To obtain good mechanical properties in the core
3. To reach a high hardness level
4. To stress relieve and reach the specified hardness on the outer surface – called tempering  

Heat treatment is usually carried out in large furnaces that can provide fine temperature control, or using the induction heating method.
Cooling is either done using oil or water quenching. Certain applications require natural convection or forced-air blowing depending on the grade.
Heat treatment cycles are normally very long lasting for days using finely controlled programs to ensure accurate duration and temperature.
The hardening phase is followed by tempering which is done at a lower temperature, between 100 and 500 degrees Celcius to obtain the required hardness and to reduce internal stresses.
Special requirements, such as for the roll necks, are obtained using flame or induction hardening. 

Induction hardening
Induction hardening means that the surface of a roll can be hardened without needing to heat the roll all the way through. In this way, the cooling process can be carried out much faster, leaving a thicker outer layer with consistent hardness throughout. It also assures a high homogenety of hardness and permits good reproducibility as many parameters can be combined and monitored in each individual roll. We mainly use the double frequency progressive system for our induction hardening of rolls.

 

The process begins by slowly rotating the roll while it is lowered between two induction rings. This raises the surface temperature. When it has reached the required level, the roll is moved into the water spray zone where it is rapidly cooled. Finally, it is progressively submerged into a water pit where it remains until it reaches the ambient temperature.