## 3 Results and Discussion
### Metallographic Microscopic Testing  
From each welded joint two specimens were observed using an optical microscope. The observations were carried out in the weld, in BM and in HAZ. The microstructure of S355J2C+N steel is characterized by the presence of fine-grained ferrite and fine-grained pearlite (Figure 5a). The structure of the weld, which was not tempered by heat from additional stitch was the same in each specimen. The dendritic structure was built of bright fine-grained ferrite arranged in columns,  
Figure 4: The exemplary results of macroscopic testing: (**a**) Specim 1; (**b**) Specim 2; (**c**) Specim 5; (**d**) Specim 6.  
from which grew acicular ferrite at the boundaries of dendrites. Inside dendrites were fine grains of ferrite, which is typical for weld made under water (Figure 5b) [25; 27; 36]. The structures in HAZ were different in each specimen. Specim 1 welded without the additional stitches was characterized by brittle, martensitic and bainitic structures (Figure 5b). The observed brittle structures are the result of a high cooling rate generated by the water environment. [PERSON] et al. [40] stated that these structures cause decreasing of the mechanical properties of underwater welded joints, and may increase the susceptibility to cracking. The structures in HAZ in Specim 2 were similar. The application of additional stitch with the 100% overlapping did not affect changes of microstructures in the HAZ, which generated cracks near the FL (Figure 5c). Similar results were found by [PERSON] et al. [51]. They proved, that in some applications, the structure after heat treatment did not change, and still included brittle martensite. Significant changes were observed in the weld of next specimens in areas where the heat from the additional stitch was affected. These areas were characterized by disappearance of the dendritic structure and the formation of a ferritic fine-grained structure (Figure 5d). The changes in overlapping of tempering stitch contributes to tempering the brittle martensitic structure in the HAZ. The structure in this area was mixed of brittle martensite, tempered martensite, ferrite and pearlite (Figure 5e). The 80% overlapping provided a higher content of tempered structures in the HAZ. However, cracks were still observed (Figure 5f). Using two tempering stitches with 100% overlapping (Specimen 5) provided the best results. The in situ local heat treatment generated the normalization structures with fine pearlite and fine ferrite in areas overlapping the HAZ from base stitch and tempering stitches (Figure 5g). It was proved that the temper bead welding technique is able to stop the cracking by the change of structure in bead-on plate welding conditions [52]. The additional stitches were laid in the same direction as the Tekken test weld, within the maximum time of two minutes after competition of previous welding process. It allows to increase the cooling rate of prepared specimens. [PERSON] et al. [53] performed an experiment with in situ quench and tempering for microstructure control and enhanced the mechanical properties of a laser cladded process. They proved that by pausing 80 s between tracks, a partial tempering effect was achieved. In this paper, we achieved a similar tempering effect of the structures in HAZ. [PERSON] et al. [54] proved that longer cooling times show a tendency to improvement of impact toughness of high-strength steel weld metals obtained by GMAW process. It can be assumed that our technique would also improve the mechanical properties. The microstructure in the area of overlapping of three HAZ in Specim 5 was the same as in Specim 6 in the area where the third additional stitch with 10% overlapping generated in situ heat treatment (Figure 5h). The structure in this area was built by normalized structures with fine pearlite and fine ferrite. The microstructure in HAZ (which did not overlap on the other HAZ) in the last stitch is similar to the microstructure of the HAZ for Specim 1 (without in situ local heat treatment).