*3.2. Formation of Austenite during Intercritical Reheating (Annealing)*

The effect of the initial microstructure on the formation of austenite during intercritical reheating has been extensively studied. These studies provided a comprehensive view of the metallurgical reactions of austenite formation that take place during intercritical annealing; (1) nucleation and growth of austenite [15,16]; (2) the role of the initial microstructure [17]; (3) the incomplete dissolution of Fe3C carbides [18,19]; (4) the non-uniformity of carbon content in intercritical austenite [20]; (5) partitioning of interstitial and substitutional solutes between αand γphases [21]; and (6) the effect of heating rate [22].

Figure 6 shows the volume fraction of austenite formed during intercritical annealing (in the temperature range 732 to 792 ◦C) as function of heating rate and initial microstructure at very short holding times, i.e., 30 s. The results shown in this figure seem to indicate the influence of the initial microstructure and heating rate on the kinetics of transformation. That is, in a ferrite-pearlite microstructure, slower heating rate leads to higher nucleation and growth of austenite formation compared to faster heating rates for a given intercritical annealing temperature. When the initial microstructure is ferrite-100%spheroidized Fe3C carbides, the formation of austenite doesn't have a similar dependence on the heating rate. This might be related to the fact that in a fully spheroidized microstructure, not all the Fe3C particles nucleate austenite. The Fe3C carbides located at the ferrite grain boundaries nucleate austenite preferentially, while those Fe3C carbides located in the matrix do not contribute to the nucleation of austenite. These carbides dissolve and the carbon contributes to the growth of austenite. In summary, the kinetics of austenite formation in a ferrite-pearlite microstructure can proceed in one or two stages depending on the heating rate. The results shown in Figure 6 also seem to support the view that the formation of austenite from an initial ferrite-spheroidized Fe3C microstructure tend to exhibit lower kinetics of austenite transformation compared to ferrite-pearlite or fully martensitic starting microstructures [19,23,24].

**Figure 6.** Formation of austenite during intercritical annealing as function of initial microstructural condition and heating rate.

It is well-accepted that the reheating temperature, holding time, and the effect of substitutional elements on the activity of carbon, controls the diffusion of carbon at the dissolving Fe3C/γ interphase, hence the growth rate of austenite can be described by the equation shown below [25].

$$w = D \frac{d\mathbb{C}}{d\mathbb{x}} \Big( \frac{1}{\Delta \mathbb{C}^{\gamma \leftrightarrow a}} + \frac{1}{\Delta \mathbb{C}^{\mathbb{C} \leftrightarrow a}} \Big) \tag{1}$$

where *v* is the velocity of the austenite phase boundary, D is the diffusion coefficient of C in austenite, *dC*/*dx* is the carbon concentration in the austenite matrix, Δ*C*γ↔<sup>α</sup> and Δ*CC*↔<sup>γ</sup> are the differences in carbon concentration between austenite and ferrite and carbide and austenite, respectively. Mn segregation at the Fe3C/γinterphase will decrease the diffusion of C through the austenite, hence decreasing the growth rate of austenite. The segregation of Mn in the lamellae pearlite and at the Fe3C/αinterphase, i.e., spheroidized carbides is shown in Figure 7. This segregation affects the dissolution of Fe3C and hence the kinetics of carbon diffusion in austenite during intercritical and supercritical heat treatments.

**Figure 7.** SEM-EDS line scan showing the segregation of C and Mn in the pearlite and Fe3C carbides.
