*3.3. Formation of Austenite during Supercritical Reheating*

Prior to studying the effect of reheating temperatures, heating rates and initial microstructure on the formation of austenite during supercritical annealing, the prior austenite grain size (PAGS) was determined. The results of the PAGS from different initial microstructural conditions and reheated at 845 ◦C and 895 ◦C, held 30 s at temperature and rapidly quenched in an ice brine solution are shown in Figure 8. The results show that the heating rate doesn't have a strong influence on the average PAGS. Meanwhile, as expected, the average PAGS value increases slightly with the reheating temperature (see table in Figure 8). The scale in the micros is (50 μm).

**Figure 8.** Average PAGS in μm as function of heating rate, reheating temperature and initial microstructural condition; (**a**) OM of PAGS from 845 ◦C and (**b**) 895 ◦C. While (**c**) shows the PAGS values.

The decomposition products of austenite as function of initial microstructure, reheating temperature, heating rate and 30 s holding time prior to fast quenching are shown in Figure 9. The microstructural balance was obtained using the EBSD-IQ method described by Wu et al. [10], this

method was also used in Figure 5. The results shown in Figure 9 indicate that the microstructures consisted of a mixture of martensite + bainite + undissolved Fe3C carbides and small amount of martensite-austenite (MA) microconstituents. As expected, the amount of undissolved carbides and the MA seems to decrease as the reheating temperature increases. This is supported by the theoretical prediction (Figure 4) and the results presented in Figure 5. It is important to indicate that 100% martensite was not observed in any of the samples observed in this study.


(d) (e)

**Figure 9.** SEM-TEM and EBSD-IQ of WQ microstructure after reheating at 895 ◦C at a heating rate of 200 ◦C/s and fast quenched. Initial microstructure fully spheroidized; (**a**) Table of microstrucrual components based on the EBSD-IQ technique; (**b**) SEM micrograph showing martensite and bainite; (**c**) TEM micrograph showing undissolved Fe3C carbides and retained γ at the carbide/matrix interface; (**d**) shows the inverse pole figure (IPF) and grain boundary character distribution, and (**e**) are the results from the EBSD-IQ analysis showing the percent of microstructural components.

## *3.4. Mechanical Properties*

The tensile properties of a selected number of fully processed samples (ferrite-pearlite) from Figure 9 were tested and the resulting mechanical properties were evaluated, see Figure 10. As expected, the flow stress was continuous for all the samples tested. A comparison of the UTS shows that the samples reheated at 845 ◦C with a heating rate of 2.5 ◦C/s and those reheated using 30 ◦C/s exhibited a slightest difference in UTS value 1591 MPa versus 1648 MPa, respectively. This behavior can be explained by the increased amount of martensite + bainite and less Fe3C + MA in the overall microstructure observed in the samples after reheating at 30 ◦C/s compared to those reheated using 2.5 ◦C/s. Interestingly reheating at higher supercritical temperatures, i.e., 1000 ◦C, did not increase the mechanical properties, as can be seen in Table 2. In this table, the average mechanical properties of the ferrite-pearlite and spheroidized samples reheated at 845 ◦C and WQ (water quenched) are also shown for comparison purposes. The YS and UTS for both starting conditions were very similar, the total elongation of the spheroidized samples was slightly lower compared to that of the ferrite-pearlite samples. The explanation for this behavior could be that the presence of some undissolved Fe3C carbides acted as nucleation sites for the onset of the diffusive necking. Reheating at higher supercritical temperatures and fast cooling produces an increase in the percent of martensite, lower percent of bainite and the presence of Fe3C + MA was not observed, compare Figures 5 and 9. These results are in strong agreement with the common knowledge that optimum microstructural combinations of martensite + bainite are stronger than 100% martensite.

**Figure 10.** Flow behavior of ferrite-pearlite samples reheated at 845 and 895 ◦C using 2.5 and 30 ◦C/s respectively followed by rapid cooling (300 ◦C/s). All the results shown on this figure correspond to samples with an initial ferrite-pearlite microstructure.


**Table 2.** Average tensile properties of samples after reheating and WQ.
