*3.3. Precipitation State*

Precipitation state analysis has been performed on selected specimens corresponding to the highest hardness values, according to Figure 12, in order to focus on the most critical scenario on toughness and fatigue behavior perspective. In this paragraph, the analysis of the Variant II (0.10% V) and Variant III (0.03% V and 0.02% Nb) with second peak temperature in the inter-critical range at 735 ◦C is reported. Analysis of Variant II specimen shows the presence of cementite regions (as expected) (Figure 21) and others with very fine V-rich precipitates in the matrix (Figures 22 and 23). The precipitates size distribution is reported in Figure 24, taking into account the chemical composition of precipitates. Results show that V-rich precipitates range sizes is below 60 nm (vanadium content in largest precipitates is lower than 0.5% as compared to a value larger than 30% for the precipitates smaller than 60 nm). In addition, more than 50% of V-rich precipitates have a size below 15 nm. This means that in such a condition vanadium addition does not appear to be critical in terms of fatigue resistance, as it would be expected in the case of its presence in largest precipitates [50].

Similarly, the analysis of Variant III shows the presence of cementite (Figure 25) and areas with precipitates rich in Nb-V (Figure 26) and Nb (Figure 27). However, in this specific case, the distribution of the frequency of the size of the precipitates (Figure 28) shows a different behavior of the precipitation, compared to Variant II. V is always present combined with Nb, in precipitates smaller than 90 nm, whereas the larger precipitates, which size is up to 250 nm, are only rich in Nb. Furthermore, only 30% of the precipitates of Nb-V are smaller than 15 nm in size, evidencing that the combination of V and Nb micro-alloying could compromise the fatigue performance in the HAZ of a welded joint [51].

**Figure 21.** TEM micrograph of Variant II steel after inter-critical treatment with second peak temperature at 735 ◦C. Highlighted are the areas of cementite.

**Figure 22.** TEM micrograph of Variant II steel after inter-critical treatment with second peak temperature at 735 ◦C. Highlighted are the fine V-rich precipitates in the matrix.

**Figure 23.** TEM micrograph detail of Variant II steel after inter-critical treatment with second peak temperature at 735 ◦C. Highlighted are the fine V-rich precipitates in the matrix.

**Figure 24.** Precipitates size distribution (Variant II, 735 ◦C).

**Figure 25.** TEM micrograph of Variant III steel after inter-critical treatment with second peak temperature at 735 ◦C. Highlighted are the areas of cementite.

**Figure 26.** TEM micrograph of Variant III steel after inter-critical treatment with second peak temperature at 735 ◦C. Highlighted are the Nb-V rich precipitates in the matrix.

**Figure 27.** TEM micrograph of Variant III steel after inter-critical treatment with second peak temperature at 735 ◦C. Highlighted are the Nb rich precipitates in the matrix.

**Figure 28.** Precipitates size distribution (Variant III, 735 ◦C).

#### **4. Conclusions**

The behavior of the inter-critical region of a S355 grade steel with different vanadium content is reported in this paper. Double-pass welding thermal cycles were simulated using a dilatometer, with the maximum temperature of the secondary peak in the inter-critical area, in the range between 720 ◦C and 790 ◦C.


In conclusion, in this work it has been demonstrated that vanadium addition in HSLA steel does not lead to the formation of a significant percentage of residual austenite in IC GC HAZ of a double-pass welding process. The combination of a more fine-grained microstructure, higher fraction of HAGBs and the formation of fine precipitates, can be promising for the improvement of fatigue and toughness behavior. This makes the adoption of high strength vanadium micro-alloyed steels very promising in structural applications, also enabling the use of a reduced quantity of raw-materials, hence mitigating the environmental impact of the resulting formulations.

**Author Contributions:** G.S.: Conceptualization, Methodology, Formal analysis, Investigation, Writing—original draft preparation, Writing—review and editing, A.T.: Conceptualization, Methodology, Writing—review and editing, Supervision, D.M.G.: Investigation, Writing—review and editing, M.M.: Investigation, Writing—review and editing, R.S.: Conceptualization, Methodology, Writing review and editing, Supervision, M.S.: Investigation, Writing—review and editing, C.T.: Investigation, Writing—review and editing, G.Z.: Investigation, Writing—review and editin, Supervision, A.D.S.: Conceptualization, Methodology, Formal analysis, Writing—original draft preparation, Writing review and editing, Supervision. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Vantage Alloys AG, Zug, Switzerland.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** The data presented in this study are available on request from the corresponding author.

**Acknowledgments:** Authors thank Dario Venditti (RINA Centro Sviluppo Materiali SpA) for TEM analysis.

**Conflicts of Interest:** The authors declare no conflict of interest.
