• Impact test results

Table 5 depicts results of the impact tests at −40 ◦C, showing that the Charpy impact energy of the WM (16.4 J) is less than the HAZ (18.5 J) samples employed in the LBW joints. Additionally, the Charpy impact energy of the WM (15.1 J) is significantly less than the HAZ (19.5 J) for all SMAW samples. The Charpy impact energy of the WM for the SMAW samples is lower than that of the LBW samples. The presented post-weld heat treatment in the SMAW process could influence part of the HAZ and improve HAZ toughness. Because of this, the Charpy impact energy of the HAZ for the SMAW samples is a little higher than that of the LBW samples.



Additionally, the prescribed table values came from the unstandardized impact specimens (2.5 × 10 × 55 mm) compared to ISO standard samples (10 × 10 × 55 mm); an equivalent Charpy impact energy value is equal to the presented value multiplies 2 or 4 [27]. Therefore, even though the minimum value of the WM for the SMAW sample is equivalent to 26.92 J, it meets the requirement for the Charpy impact energy value (27 J).

Figure 14 shows the impact fracture photo of the WM of the LBW joint. From Figure 14a, it can be seen that there is almost no fiber area, the fracture surface is relatively flat, and pits occasionally exist, showing brittle fracture characteristics. Figure 14b is an enlarged view of the fracture fiber area. It can be seen that there are relatively few dimples and a pore with a size of 10 μm. Figure 14c is the morphology of the fiber region at higher magnification, and dimples vary in size between 0.5–3 μm. Figure 14d shows a high-magnification dimple photo in the radiative region, where the shear characteristics are more significant compared with Figure 14c. On the whole, the fracture impact energy of the WM is low, with 11.50 J, and the mode of failure is brittle.

**Figure 14.** The WM impact fracture surface morphology (**a**) Macro-morphology (**b**) Fiber area morphology (**c**) Dimples details in fiber area (**d**) Dimples details in radiation area.

#### **4. Conclusions**

In the present work, the mechanical properties and microstructural features of SMAW and LBW welded joints for 16-mm-thick HSLA Q890 steel were discussed. The microhardness of the welded joint results showed that the LBW was slightly higher than the SMAW, meaning that the post-weld heat treatment associated with the SMAW process can improve microhardness. Furthermore, the tensile results showed that tensile failure occurred in the base metal; the ultimate tensile strength and percent elongation of the LBW welded joint (973.5 MPa and 10%) were higher than those of the SMAW joint (951 MPa and 2.9%) due to the filler filling of the SMAW process. Finally, the impact results showed that the Charpy impact energy of the weld metal (16.4 J and 15.1 J) was lower than that of the heat-affected zone (18.5 J and 19.5 J) in the LBW joint and the SMAW joint.

**Author Contributions:** Conceptualization, H.C.; validation, Y.L.; writing—original draft preparation, Y.L. and Y.C.; writing—review and editing, Y.C. and X.X. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by National Natural Science Foundation of China grant number 51809161 and 52005315.

**Institutional Review Board Statement:** This research did not require ethical approval.

**Informed Consent Statement:** This research did not involve humans.

**Data Availability Statement:** We are willing to share our research data in MDPI journals.

**Acknowledgments:** The authors gratefully acknowledge the financial support by the National Natural Science Foundation of China (No. 51809161, No. 52005315), Shanghai local colleges and universities construction project supported by the Shanghai Committee of Science and Technology (No. 23010501100) to carry out the present research work. The authors would like to particularly acknowledge to Lu Fenggui from Shanghai Jiaotong University for fruitful discussions and suggestions.

**Conflicts of Interest:** The authors declare that they have no conflict of competing financial interests in this paper.

### **References**


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