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The Hollomon–Jaffe parameter, or HP, also generally known as the Larson-Miller parameter, describes the effect of a heat treatment at a temperature for a certain time. This parameter is especially used to describe the tempering of steels, so that it is also called tempering paramenter.
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. T is in degrees Celsius. The argument of the logarithmic function has the unit hours. C is a parameter unique to the material used. 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. This work was based on six different heats of plain carbon steels with carbon contents varying from 0.35%–1.15%. The value of C was found to vary somewhat for different steels and decrease linearly with the carbon content of a steel grade. Holloman and Jaffe proposed that C = 19.5 for carbon and alloy steels with carbon contents of 0.25%–0.4%; and C = 15 for tool steels with carbon contents of 0.9%–1.2%.
- Hollomon, J.H.; Jaffe, L.D. (1945). "Time-temperatures relations in tempering steel". Transactions of the American Institute of Mining and Metallurgical Engineers. 162: 223–249.
- Brooks, Charlie (1996). Principles of the heat treatment of plain carbon and low alloy steels. ASM International. p. 158. ISBN 0871705389.
- Canale, L.C.F; Yao, X.; Gu, J.; Totten, G.E. (2008). "A historical overview of steel tempering parameters". 3 (4-5): 474–525. doi:10.1504/IJMMP.2008.022033.