## 3 Results and Discussion
### Hardness HV10 Measurements  
For hardness HV10 measurements, the Sinowon V-10 stand (Sinowon, Dongguan, China) was used. The hardness was measured in three points in the weld and in six points in the HAZ--three in the left side from the axis of tested specimen and three in right side. Measurements were undertaken in two cross-sections of each specimen (e.g., 1.1. and 1.2. for Specimen 1). [PERSON] et al. [56] proved that decreasing hardness can result from tempered martensite occurring in the HAZ. The same results were observed during our investigation. Hardness HV10 measurements confirmed the microscopic observations. The highest HV10 values were measured in the HAZ, whose structures were identified as brittle bainitic and martensitic structure. [PERSON] and [PERSON] [57] stated, that high hardness of welded layers was caused by a high cooling rate during welding process. The significant effect of in situ local heat treatment was observed for Specimen 5, modified by two additional stitches. The hardness HV10 values in this Specimen in HAZ were lower by 70-90 compared to the Specimen 1, which was performed without additional stitches. The proposed technique caused decreasing of the hardness in HAZ on both sides of weld axis. In Specimen 5, the biggest tempering effect was observed during microscopic testing. For Specimens 4 and 6, where an additional stich was non symmetrical, the hardness decreased in heat treatment areas (HAZ on right side from weld axis). The aim of using such a low value of overlapping like in Specimen 6 (10%) was tempering the area near the edge to check the influence of this value on the weldability of HSLA steel. The prepared examinations confirmed that 10% overlapping caused microstructural changes and a decrease of hardness in the HAZ of Tekken test weld. It proved that the applied method of in situ local heat treatment can be used during underwater welding as a technique, which improved the weldability of the HSLA steel in wet welding conditions. However, it was observed that overlapping higher than 80% of one stitch generated higher hardness and should be avoided. The results of HV10 hardness measurements are presented in Table 4. The hardness distribution in each specimens is presented in Figure 6.  
Figure 5: The exemplary results of microscopic testing: (**a**) S355J2C+N structure; (**b**) Specimen 1 without in situ heat treatment—the view of the weld and heat-affected zone (HAZ); (**c**) Specimen 2, one additional stitch with 100% overlapping—the view of the weld and HAZ; (**d**) Specimen 3, one additional stitch with 90% overlapping—the view of the tempered weld and HAZ; (**e**) Specimen 3, one additional stitch with 90% overlapping—the view of the tempered HAZ; (**f**) Specimen 4, one additional stitch with 80% overlapping—the view of the weld and HAZ; (**g**) Specimen 5, two additional stitches with 100% overlapping—the view of the tempered HAZ; (**h**) Specimen 6, three additional stitches with 10% overlapping of the las—the view of the tempered HAZ t; (**i**) Specimen 6, three additional stitches with 10% overlapping of the las stitch—the view of the weld. The effect of decreasing of HV10 hardness due to in situ local heat treatment can be observed in areas where heat from additional stitch tempered the microstructures of the base Tekken test joint. The measurements showed that there are significant differences in HAZ on both sides of the Tekken joint. These differences resulted from different heat influence on the joint. In the HAZ near the additional stitch (Specimens 4 and 6) the heat provides microstructural changes, which produced lower hardness. The HAZ on other side from the weld axis have not been tempered as much and the hardness is similar to the hardness in Specim 1 welded without in situ heat treatment. [PERSON] et al. [58] proved that hardness level in the welded joint can be related to the microstructure constituent of bainite and Widmanstaten ferrite.