General help

Hardening Power

The HP value (hardening power) is one method to relate the quenching characteristic of an oil with the hardening result, in form of hardness of the workpiece. Based on measurements according to the standard ISO 9950 for oil, a formula has been derived which gives the HP.

The quenching characteristic is based on the cooling curves and cooling rate curves. In these curves, three points are the most critical and is shown in the figure below.

HP = k1+ k2·TVP + k3·CR - k4·TCP   (1)

 

where
TVP
=
 the transition temperature between the vapour phase and the boiling phase [°C]
CR
=
 the cooling rate over the temperature range 600°C —> 500°C
TCP
=
 the transition temperature between the boiling phase and the convection phase [°C]
k1, k2,k3, k4are constants
Through tests and regression analysis the constants were evaluated and the formula (1) got the form:

HP = 91,5 + 1,34·TVP+ 10,88·CR - 3,85·TCP

An oil with a higher HP value is more fast quenching and will give a higher hardness and thereby higher strength.