*3.3. Alloying Element Partitioning between Ferrite and Prior Austenite* 3.3.1. Spot EDS Analysis for Alloying Concentration

The EDS analysis was accomplished widely at several positions within central ferrite grains and the central vicinity martensite (prior austenite) regions in order to investigate in detail the alteration of carbon, Si, Cr and Mn partitioning within ferrite and prior austenite regions developed during the progress of ferrite formation at 720 ◦C using various SQ heat-treated samples. Although, the measurement of carbon concentration within ferrite and prior austenite (martensite) microphases by EDS analysis technique accompanied some overestimation, but this technique can still be used as a comparable study in order to identify the alteration of carbon concentration within ferrite and prior austenite microphases as reported by several investigators [7,26,27]. In this way, the results of spot EDS analysis for carbon, Si, Cr, and Mn concentrations within the central locations of ferrite and martensite microphases are summarized in Tables 3 and 4 for various SQ heattreated samples. For a better comparison of carbon, Si, Cr and Mn partitioning within ferrite and prior austenite with the progress of ferrite formation at 720 ◦C, the concentrations of these alloying elements as a function of SQ holding times are shown in Figure 5 for central ferrite and martensite areas. A careful investigation of the results depicted in Table 3 and Figure 5a illustrates that the mean level of carbon concentration within the central region of ferrite grains decreased continuously from 6.32 EDSNs for the short time ferrite-martensite SQ1 samples to 5.87 EDSNs for the long time ferrite-pearlite ones in conjunction with a concomitant increase in carbon concentration of prior austenite from 10.64 to 11.87 EDSNs, respectively. These qualitative results illustrate that a higher level of carbon concentration occurred within the prior austenite regions associated with the pearlite formation (SQ30) as a consequence of greater ferrite areas, in comparison to the prior austenite formation related to the short time treated SQ1 specimens. The Si concentration within the central areas of ferrite grains increased with a gentle slope from 1.23 to 1.51 EDSNs, while this was almost constant for the central location of prior austenite areas (Figure 5b, Table 3). The mean Cr concentration also increased within the central ferrite grain from 0.83 to 1.09 EDSNs with the increase in SQ holding times from 1 to 30 min (Figure 5c, Table 4). However, the amount of mean Cr concentration in the central region of martensite areas did not change for the SQ1, SQ5, and SQ15 samples with ferrite-martensite microstructures (nearly 0.93 EDSNs), while it increased suddenly to 1.25 EDSNs for the SQ30 sample with ferritepearlite microstructures. These results indicate that Si and Cr atoms are partitioned from growing ferrite-prior austenite interfaces towards the ferrite phase constituents during the progress of ferrite formation, and Cr atoms, on the other hand, promote pearlite formation

at the lateral stages of prior austenite phase transformation in SQ30 specimens heat treated for the prolonged duration. Although, the Mn content of central ferrite grains has been almost constant, the amount of mean Mn concentration increased in the central region of martensite areas from 0.50 to 0.72 EDSNs with the increase in SQ holding time from 1 to 15 min. For longer isothermal holding of 30 min, the amount of mean Mn concentration within the central region of pearlite increased to 0.78 EDSNs, emphasizing that Mn atoms did partition to the prior austenite side during the progress of ferrite formation in the SQ heat-treated specimens (Figure 5d, Table 4).

**Figure 5.** Changes in carbon (**a**), Si (**b**), Cr (**c**), and Mn (**d**) concentrations as a function of SQ holding time for central regions of ferrite and martensite/pearlite areas using various SQ heat-treated samples.

**Table 3.** The results of mean EDS analysis with the related average (Ave.) and standard deviation (S.D.) for carbon and Si concentrations within the central regions of ferrite grains and martensite areas taken from various SQ heat-treated samples.



**Table 4.** The results of mean EDS analysis with the related average (Ave.) and standard deviation (S.D.) for Cr and Mn concentrations within the central regions of ferrite grains and martensite areas taken from various SQ heat-treated samples.
