Heat treatments consist of a large variety of processes, such as hardening of steels, carburizing, nitriding and other processes. The main goal is to achieve a specific property or combination of properties that are required and expected for the end product. By changing and adjusting different process parameters we can change the outcome and properties of the end product. Typical parameters that can be tweaked include the time and temperature of the quenching, carburizing and nitriding processes, the chosen time and temperature of tempering, the cooling medium and the cooling configuration, the choice of furnace, the atmosphere and its composition as well as other parameters.
Two of the key parameters for achieving the desired mechanical properties are the temperature and time of tempering. By changing these two parameters we can achieve different hardness and mechanical properties of the end product. The correlation between the time and temperature of tempering and steel hardness was describe in 1945 by John Herbert Hollomon and Leonard David Jaffe. The mathematical expression is called the Hollomon-Jaffe parameter. Some sources refer to it as the Larson-Miller parameter. Some sources simply call it the tempering parameter.
In this master thesis we quenched specimens of 1.2343 and 1.2379 steel in a vacuum furnace and in oil. We determined a specific hardness that we wished to achieve by combining different times and temperatures during tempering. We used the Hollomon-Jaffe parameter to calculate the needed tempering times and temperatures. We analysed the specimens with different techniques to figure how the different parameters affected the microstructure and to figure the validity of the tempering parameter for those steels. With the gained knowledge it would be possible to optimise the process in heat treating facilities.
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