The Hollomon Parameter HP describes the effect of a heat treatment by isothermal holding at a temperature for a certain time. The effect of the heat treatment depends on its temperature and its time. You can achive the same effect with a low temperature and a long holding time or with a higher temperature and a short holding time. This exchangability of time and temperature can be described by the following formula:
HP = (273°C + T) · (20 + log_{10}(t/60)) 10^{-3}
This formula is not consistent concerning the units. That means that you have to enter the parameters in a certain manner. The argument of the logarithmic function has the unit minutes. The Hollomon parameter itself is unitless and realistic numeric values vary between 15 and 21.
The temperature T has to be entered in °C and can be calculated by resolving the upper equation to the temperature. The so calculated temperature has also the unit °C.
T = 10^{3} HP/(20 + log_{10}(t/60))-273°C
The time t has to be entered in minutes and can be calculated by resolving the Hollomon equation to the time.
t = 60 · 10^{(1000 HP/(273°C + T)-20)}
In the case of calculation mode tempering the thickness (to be entered in mm) is used to calculate the heating and cooling behaviour T(t) and its contribution to the Hollomon Parameter. In the case of calculation mode PWHT the heating and cooling rate can be entered manually and the thickness is not used.
If there are several consecutive heat treatments with individual temperatures T_{i} and times t_{i}, the total value for the Hollomon Parameter HP_{total} is calculated by
Calculating the single HP-values HP_{i}=HP_{i}(T_{i},t_{i}) and after that their corresponding times
t_{i} = 60 · 10^{(1000 HPi/(273°C + T')-20)}
where T' is an arbitrary temperature (for example: T'=T_{1}).
Calculating the total HP value
HP_{total} = (273°C + T') · (20 + log_{10}(St_{i}/60)) 10^{-3}
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