The parameters used in heat treatments to reduce the hardness and increase the durability of tempered steels are traditionally determined by equations like the Holloman-Jaffe parameter, directly inspired by the Larson-Miller relation. These equations measure the effect of the different metallurgical transformation stages, such as tempering and stress relieving. However, these older formulas pertain to isothermal (Constant Temperature) treatments that are practically non-existent in industrial-level facilities because of the heating temp offset which precedes the minimum maintenance time at the ideal temp to obtain the desired physical, mechanical, and metallurgical specifications.
Although these equations are still widely used in the heat treatment industry, a certain number of procedural challenges have been identified related to temperature optimization and time estimates for tempering or relief treatments; today, they have attained the limits of metallurgical predictability and are generally considered too high. That said, recent studies report using sound scientific data to optimize these equations to make them more reliable in predicting post-treatment outcomes and to update changeover times and oven heat parameters.
In an article by Lauralice Canale1, different methods are presented to address the problems related to these basic equations, including the findings of Gingras2, Guo3, Inoue4, and Wan5. Undoubtedly, the Larson-Miller and Holloman-Jaffe parameters, the gold standard in the realm of heat treatments, must be optimized for industrial applications.
1 Canale, L. et al. A historical overview of steel tempering parameters. International Journal of Microstructure and Materials Propreties. 2008.
2 Gingras, R. & Grenier, M. Software assists in optimizing tempering process. Industrial Heating. 2005.
3 Guo, C. Mathematical model for tempering time effect on quenched steel based on Holloman parameter. Acta Metall. Sin. 1999.
4 Inoue, T. A new tempering parameter and its use for practical heat treatment. Journal of the Iron and Steel Institute of Japan. 1982.
5 Wan, N. et al. Mathematical model for tempering time effect on quenched steel based on Holloman parameter. J. Mater Sci. Technol. 2005.