Stress-relieving is a commonly applied technique, with two major variables; soaking time and temperature. The Hollomon parameter is a method of characterising the combined effect of these two variables, to allow comparisons of treatments applied with different temperatures or times. Heat treatments which have the same Hollomon parameter can be considered to have the same effect.

The Hollomon parameter for any given heat treatment is calculated with the following equation:

Hollomon Parameter = Temperature * (Hollomon-Jaffe Constant + log (Soak Time))

Where the Hollomon-Jaffe constant is generally approximated to either 20, or calculated using the equation:

Hollomon-Jaffe Constant = 21.3 - (5.8 * Wt% Carbon)

A calculator that will calculate the Hollomon parameter for a given heat treatment is given here:

Temperature (Kelvin):
Soak time (Hours):
Hollomon-Jaffe Constant:
Hollomon Parameter:

The calculator above is basic, and doesn't take account of the possible stress-relieving that can occur during heating or cooling, especially if the rate is slow. In that scenario, the Hollomon parameter can be calculated using:

Hollomon Parameter = Temperature * (Hollomon-Jaffe Constant + log (A))

Where "A" is calculated by:

A = Soak Time + Temperature + Temperature
2.3 * Heating Rate * (Hollomon-Jaffe Constant - log (Heating Rate)) 2.3 * Cooling Rate * (Hollomon-Jaffe Constant - log (Cooling Rate))

Unfortunately, that calculation is a little tricky for a web form to recreate, so see the excel sheet below instead. (The excel sheet can also do the basic calculation, and a minimum and maximum Hollomon parameter calculation to use to determine whether or not a post-weld heat treatment is acceptable if it is performed with the wrong time or temperature. Obviously, this spreadsheet can't overrule any code rules. Or a materials engineer.)

Collie Welding Hollomon-Jaffe Calculator

Information about the Hollomon-Jaffe calculation was taken primarily from the Steel Heat Treatment Handbook, edited by George E. Totten, available from:

Amazon UK

Amazon US