Effect of Joule Rapid Tempering Parameters on Microhardness in a Medium-Carbon and Low-Alloy Cr–Mo Steel

The effect of the initial microstructure and the heating mode on the tempering of a medium carbon, low alloy Cr–Mo steel was studied. Two types of cylindrical specimens were tested: (1) with microstructural gradient in the axial direction (martensite in the specimens’ middle and ferrite plus pearlite in the ends) and (2) without microstructural gradient (martensite throughout the specimen body). The heating mode consisted of rapid heating to the tempering temperature (using a Joule heating device), holding for a few seconds, and cooling rapidly. Tempering was carried out at temperatures of 525 and 575°C, where the fourth tempering stage occurs. The specimens were analyzed using optical and scanning electron microscopy and Vickers microhardness measurements. The results show that using the tempering parameters established with the Hollomon–Jaffe parameter promotes a decrease in microhardness compared to conventional tempering with the same tempering parameter. The degree of tempering using the Joule heating device promotes the decrease in microhardness to values of 200 HV with a dominant microstructure of recrystallized ferrite. Regarding martensite, the electric current increases the tempering degree of martensite; that is, even if the tempering is performed for only a few seconds, the microhardness decreases rapidly. Then, tempering can be carried out at shorter times or lower temperatures, and the Hollomon–Jaffe parameter, which is derived from conventional treatments, is not appropriate for designing tempering using a Joule heating device.