*3.1. E*ff*ect of Heating Rate on Transformation Temperatures (AC1 and AC3) and the Nucleation of Austenite*

It is well-known that the transformation temperatures, AC1 and AC3, and the nucleation of austenite, are strongly affected by the heating rate. The effect of heating rates on the transformation temperatures can be calculated using a commercial thermodynamic software program J-MatPro (version 7 [11]), the results are presented in Figure 4. In this figure, according to the predictions of J-Mat Pro, it is shown that in order to obtain a fully homogeneous austenite transformation, the reheating temperature must be increased well above the AC3 temperature, especially at high heating rates. The formation of austenite is well-accepted to be a diffusion-controlled process, controlled by the slower diffusion process of interstitial elements, i.e., C, in austenite when compared to the diffusion rates in ferrite. Therefore, for heat treatments in the temperature range of 845 and 895 ◦C, or even at higher temperatures, i.e., 1000 ◦C, it would not be expected that austenite will transform into 100% martensite during fast quenching. This view is independent of heating rates (2.5, 30 and 200 ◦C/s) prior to austenite transformation. For example, Figure 5 shows the EBSD-IQ results of ferrite-pearlite samples reheated at 1000 ◦C using 2.5 and 200 ◦C/s heating rates respectively, with a holding time of 30 s prior to rapid quenching. The results shown on this figure clearly support the theoretical predictions that a fully martensitic microstructure can't be obtained using slow or fast heating rates and short holding times at the supercritical temperatures. The resulting microstructure was a combination of martensite + bainite. This behavior can be explained by the effect of a heating rate and holding time on the nucleation and growth of austenite and the dissolution of Fe3C carbides. Several studies [12–14] have indicated that during rapid reheating the classical view of phase transformations will deviate markedly from those observed during equilibrium conditions. That is, the kinetics of transformation will have a different behavior. For example, faster heating rates favors nucleation of austenite, while slower heating rates leads to a significant growth of austenite. That is, slower heating rates permit C diffusion through the austenite, enabling its growth. An additional effect is the holding time at a given temperature, fast heating rates and short holding times, promotes substantial local compositional differences in austenite.

**Figure 4.** Shows the effect of heating rate on the transformation temperatures of 22MnB5 steel [11].

**Figure 5.** EBSD-IQ showing the microstructural balance of martensite and bainite after reheating a ferrite-pearlite microstructure at 1000 ◦C using heating rates of 2.5 ◦C/s and 200 ◦C/s and WQ. (**a**) and (**b**) represent the reconstructed EBSD-IQ microstructure and inverse pole figure and grain boundary character distribution for the sample reheated at 2.5 ◦C/s WQ sample. While (**c**) and (**d**) represent the EBSD-IQ analysis of the % final microstructure for 2.5 ◦C/s and 200 ◦C/s heating rates, respectively.
