**3. Results and Discussion**

## *3.1. Microstructural Characterization*

Figure 2 represents typical light microstructures along with super imposed electron micrographs (at top right hand side) of SQ specimens heat-treated at 720 ◦C for various holding times. These micrographs show a significant increase in volume fraction, polygonality, grain size, and continuity of ferrite as a consequence of longer isothermal holding time at 720 ◦C. For more information, the progress of prior austenite decomposition in various SQ heat-treated specimens was measured in term of microconstituent volume fractions, and the associated results are given in Table 2. A careful microstructural analysis indicated that the microstructures of SQ1, SQ5, and SQ15 heat-threated samples were characterized by just a mixture of ferrite and martensite microphases, introducing ferrite-martensite DP microstructures, while in the case of SQ30 heat-threated samples, a maximum volume fraction of 15% ferrite along with the remaining 85% pearlite microconstituents formed in the microstructures, representing the typical ferrite-pearlite DP microstructures. In other words, with the increase in SQ holding time up to 30 min in the intercritical α+γ region, there was enough time available for carbon atoms to diffuse from ferrite into the prior austenite and equilibrate and this enabled enough carbon concentration within the prior austenite regions for subsequent pearlite formation.

**Figure 2.** Light and FE-SEM micrographs of various SQ heat-treated specimens depicting the formation of ferrite, pearlite, and martensite microphases in the microstructures. The blocky ferrite grains were mostly surrounded by martensite areas in the SQ1 and SQ5 heat-treated specimens, while continuous grain boundary ferrite was developed in the SQ15 and SQ30 heat-treated samples. Microstructural features were characterized by ferrite-martensite DP microconstituents for SQ1, SQ5, and SQ15 samples, while ferrite-pearlite microphases were formed in the SQ30 heat-treated ones. Ferrite, pearlite, and martensite microconstituents are marked with F, P, and M symbols, respectively.


**Table 2.** The progress of ferrite and pearlite formations realized at 720 ◦C for various SQ holding times. The pearlite formation started after 15 min isothermal holding time.
