PSI - Issue 42

Mihai A. Popescu et al. / Procedia Structural Integrity 42 (2022) 1626–1633 M.A. Popescu et al. / Structural Integrity Procedia 00 (2022) 000–000

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Table 4. V-notch specimen type and designation. Specimen type Surface

Root

Surface

Root

VWT VHT

A1 A3

A2 A4

B1 B3

B2 B4

Fig. 3. Transition zone from BM to HAZ to WM.

3. Results and Conclusion

3.1. Results

The microstructural analysis led to the identification of acicular ferrite and grain boundary ferrite in the WM, for the HAZ upper and lower bainite with aligned and non-aligned carbides, as well as pro-eutectoid ferrite. Possibly, a small amount of martensite could be detected, although the hardness values support the claim that if martensite is present, it was not revealed during the testing and a low percentage is present. The HAZ was revealed and shown in Figure 3. Because the thermal cycle is dependent on the heat input and distance from the fusion boundary, the areas closer to it will have harder microstructures that will progressively soften while moving away from the fusion boundary. Near the fusion boundary, the coarse-grained super-critical (CGSC) area shows rapid austenite grain growth, typically occurring above 1000 ° C, while further away the fine-grained super-critical (FGSC) area does not reach similar temperatures, and thus does not produce the same e ff ect. The inter-critical (IC) area of the HAZ is far enough from the fusion boundary to present only partial transformation to austenite and substantial grain refinement. The sub-critical (SC) HAZ is far enough from the fusion boundary to only show some spheroidal pearlite and little grain refinement. S2 has a larger grain size in the HAZ and WM, especially noticeable in the CGSC zone of the HAZ. Figure 4 reveals the bevels geometries that were used as well as the number of passes. For S1 the bevel is double-V with 30 identified passes, while for S2 the bevel is single-U with 25 passes. Because the thickness of the plates is the same, but fewer welding passes were utilized for S2 the weld beads have a larger area, with less overlapping in comparison with S1, where a larger number of passes allowed for smaller and wider weld beads that overlap frequently and have a tempering e ff ect. The size of the HAZ is similar for both samples, except for the root area of the weld, where it is much smaller in S1 and the grains appear to be more refined. The Vickers hardness values (Figure 5) obtained are mostly in accordance with the literature. This points towards the fact that rapid cooling was partially avoided, and the successive passes may have re-heated the weld metal de posited previously and tempered it, thus resulting in a softer, more ductile microstructure. The values tend to be lower for S1 in nearly all the locations except the HAZ (Figure 6), which can be a result of the increased number of weld passes and possible higher input used for S2. Lower values were expected for the WM in both samples.