The microstructure of AHSS Gen3 980T was analyzed after the metallography procedure using the SEM.
Figure 10a shows the microstructure of the material, in which arrows are used to indicate the martensite (M) and ferrite (F).
According to the image scale of
Figure 10a, it is possible to state that the average material grain size is less than 5 μm. The literature focused on the study of the third-generation steels, and reports the presence of three phases, namely martensitic, ferrite, and retained or residual austenite, [
4,
5,
18]. Li et al. [
6] and Guo et al. [
7] analyzed the microstructure of AHSS Q&P 980 steel, and they also found these three material phases. To identify each phase, both authors combine techniques of optical microscopy, SEM, and XRD. To visualize the phases of martensite and ferrite for the AHSS, the SEM with samples etched by Nital reagent is generally applied. The authors [
6,
7] used a concentration of 4%. This analysis indicates that the lightest phase represents martensite, and the darkest phase is ferrite. However, the SEM adopted in this work did not indicate the presence of the retained austenite phase. We describe all phases contained in the material as a non-trivial microstructural characterization that requires special techniques, such as different etching and powerful microscopes. According to Wu et al. [
19], the retained austenite and carbide precipitates could not be distinguished by optical microscopy due to limited resolution in metallography. To see the retained austenite, Radwanski et al. [
20] employed two etching methods of color metallography knows as Klemm and Le Pera. According to the authors, Klemm’s etchant suggests the presence of small particles of retained austenite that was confirmed by XRD analysis. On the other hand, Le Pera highlights martensite + retained austenite of the rest of micro-constituents. However, there is no visible distinction between martensite and retained austenite.
Figure 10b presents the XRD spectrum of the as-received material, where were observed non- prominent peaks describe the phase related to retained austenite, while the more significant peaks indicate the ferrite + martensite. The diffraction peaks observed at 44.7° (110), 65.1° (200) and 82.4° (211) are composed by the presence of BCC (body-centered cubic) phase inherent in both martensite and ferrite, whereas the diffraction peaks observed at 43.9° (111), 50.5° (002), and 74.2° (022) are associated with the FCC (face-centered cubic ) phase corresponding to austenite. These intensity peaks are similar to the results found by [
21], which are presented in the XRD for the undeformed TRIP and Q&P AHSS steels.
Figure 11 compares the material microstructure in different regions, namely (
a) base metal, (
b) HAZ, and (
c) fusion zone, using the SEM images. Both regions display a distinct pattern. However, this fact is more evident when
Figure 11a,c are compared directly because the microstructure in the fusion zone presents oriented acicular grains, and the structure is predominantly martensitic. Thus, Gen3 980T steel has its hardness increased in the fusion zone. This observation was confirmed with the nano-hardness profile in the bead zone,
Figure 11d. The result of the nano-hardness test was displayed in terms of [GPa], a procedure similar to the work of Fernandes et al. [
22] that studied the optimal parameters for laser-welding of Dual-Phase (DP600) AHSS. The hardness in the fusion zone is higher than in the base metal. The Vickers test also was carried out for Gen3 980T steel in the base metal and the fusion zone, and the average values and the standard deviation are exhibited in
Table 6. The result shows an increase of 53.7% of hardness in the fusion zone. For Sample 3, with a bead diameter of 1.1 mm, a HAZ around 0.5 mm for each side of the weld bead was verified.