The Hollomon-Jaffe parameter, H, is used to compare the response of a steel to a tempering treatment, e.g. postweld heat treatment (PWHT), by describing an equivalence of time and temperature for thermally activated processes. Therefore, the necessary time required to use an alternative PWHT temperature can be calculated in order to see if it is a viable alternative to a current treatment.
The parameter has the general form:
where H is the Hollomon-Jaffe parameter, T is the temperature in Kelvin (°C + 273.15), C is a constant (see below), and t is the time in hours for an isothermal condition.
Hollomon and Jaffe determined that C varies linearly for carbon contents between 0.30% and 1.10%. C = 15 is suitable for steels containing 0.90-1.20%C, and C = 19.5 is suitable for steels containing 0.15-0.45% carbon. In practice, C = 20 is commonly used for C-Mn and low alloy steels. Sometimes a higher factor of 30 has been employed for more highly alloyed steels, e.g. 9%Cr steels.
Heating and cooling cycles may be included in the parameter, in which case, the expression is:
where t is the holding time in hours,
K1 = heating rate in K/hour,
K2 = cooling rate in K/hour.
The Hollomon-Jaffe parameter is similar to the Larson-Miller parameter, which was originally used to describe creep properties, but used degrees Rankine, rather than Kelvin.
What is the effect on C-Mn and low alloy steels of multiple tempering or stress-relieving heat treatment cycles carried out at the same or different temperatures?
More information on PWHT:
What is meant by postweld heat treatment/stress relief heat treatment?
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Hollomon J H, Jaffe L D, 'Ferrous Metallurgical Design', John Wiley and Sons Inc., 1947, pp.245.