The effect of the heat treatment depends on its temperature and its time.
The same effect can be achieved with a low temperature and a long holding time, or with a higher temperature and a short holding time.
In the Hollomon–Jaffe parameter, this exchangeability of time and temperature can be described by the following formula: This formula is not consistent concerning the units; the parameters must be entered in a certain manner.
The Hollomon parameter itself is unitless and realistic numeric values vary between 15 and 21. where T is in kilokelvins, t is in hours, and C is the same as above.
Holloman and Jaffe determined the value of C experimentally by plotting hardness versus tempering time for a series of tempering temperatures of interest and interpolating the data to obtain the time necessary to yield a number of different hardness values.