*3.1. Microstructure and Hardness*

The microstructural evolution detected by SEM for all the considered steels subjected to heat treatments (as shown in Figure 2) is reported in Figures 3–6.

Moving from 790 ◦C to 720 ◦C the microstructure changes from ferrite-perlite to bainite, regardless the chemical composition, as confirmed by EBSD pole figure maps, reported in Figures 7–10. This demonstrated that the formation of certain microstructural constituent, after welding thermal cycles, is not sensitive to micro-alloying addition in the selected ranges (V up to 0.10% and V-Nb addition up to 0.05%). Moreover, it is visible that Variant

II shows the smallest and more uniform grain size among all the other variant for a peak temperature of 790 ◦C.

**Figure 3.** Microstructures as obtained by dilatometric cycles (reference material).

**Figure 4.** Microstructures as obtained by dilatometric cycles (variant I).

**Figure 5.** Microstructures as obtained by dilatometric cycles (variant II).

**Figure 6.** Microstructures as obtained by dilatometric cycles (variant III).

**Figure 7.** EBSD polar figure maps of specimens subjected to dilatometric cycles (reference material).

**Figure 8.** EBSD polar figure maps of specimens subjected to dilatometric cycles (variant I).

**Figure 9.** EBSD polar figure maps of specimens subjected to dilatometric cycles (variant II).

**Figure 10.** EBSD polar figure maps of specimens subjected to dilatometric cycles (variant III).

Figure 11 shows the quantification of high angle grain boundaries (HAGBs %) (φ > 10◦), obtained by EBSD analysis, for each condition. The tendency of all the variants, except for Reference material, is to increase HAGBs fraction with the increase of the intercritical temperature. Variant II has the highest fraction of HAGBs in comparison to other variants. At the same time, Variant I shows the same trend of Variant II and exhibits higher fraction of HAGBs in comparison to Variant III, except for 735 ◦C and 790 ◦C. This suggests that, in regards to the HAGBs fraction, Variant II is expected to show the highest fatigue and toughness performance, since these grain boundaries type are responsible for a higher deflection of cracks during a fatigue cycle and an obstacle to cleavage propagation [44–46].

**Figure 11.** High-angle grain boundaries quantification (HAGBs %) (φ > 10◦) for each condition.

The hardness dependence of all the considered variants as a function of the inter-critical temperature is reported in Figure 12. As expected, after welding thermal cycle, for each steel variant there is an increase of hardness value compared to hot rolled state. Moreover, while the reference material appears to be independent on the tested temperature, Variant I is subjected to a hardness loss starting from a hardness value approximately similar to that of the reference material. A clear effect of micro-alloying is reported for Variant II (0.10% V) and Variant III (0.03% V–0.02% Nb). These results show that an increase in the inter-critical temperature leads to a decrease of hardness. Typically, at a fixed inter-critical temperature of 720 ◦C an increase by approximately 30 Vickers points of each variant in comparison to its initial value in hot rolled state is present. For a temperature of 790 ◦C, Variant I kept the same hardness, Reference material experienced an increase of 30 Vickers points, while Variant III and Variant II experience an increase of hardness by 20 Vickers points. These results are consistent with the ones published by [47,48].

In particular, both for Variant II and for Variant III, a peak of hardness at 735 ◦C is evident and the nature of this behavior can be attributed to both the presence of residual austenite and a different precipitation state. A desired strengthening of the steels due to formation of the residual austenite would be detrimental in terms of toughness [30,31]. Otherwise, an adequate state of precipitation (fine and homogeneously dispersed precipitates) would ensure the strengthening and, at the same time, a better fatigue behavior [49]. The investigation of these aspects (RA and precipitation state) has been conducted and is illustrated in the Sections 3.2 and 3.3.

**Figure 12.** Hardness dependence on inter-critical temperature for the different considered materials.
