## 4 Conclusions  
In the paper the effect of in situ local heat treatment on the weldability of HSLA steel welded under water in wet welding conditions was studied. The investigations showed that additional stitches can improve the quality of welded joints in the water environment. The additional stitches provided microstructural changes in the HAZ, decreasing the hardness in this area. Prepared investigations showed that the optimal number of stitches with 100% overlapping for S355J2C+N steel welded in wet welding conditions is three.  
The main conclusions resulting from experiments are:  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Specimen No.** & \multicolumn{2}{c}{**Heat Affected Zone (HAZ)**} & \multicolumn{3}{c}{**Base Material (BM)**} & \multicolumn{2}{c}{**HAZ**} \\ \hline
1.1. & 416 & 493 & 491 & 281 & 271 & 260 & 464 & 432 & 426 \\
1.2. & 475 & 488 & 423 & 262 & 275 & 276 & 446 & 451 & 428 \\
2.1. & 491 & 499 & 489 & 259 & 274 & 258 & 497 & 452 & 493 \\
2.2. & 479 & 465 & 439 & 258 & 266 & 242 & 472 & 471 & 477 \\
3.1. & 450 & 499 & 493 & 255 & 238 & 287 & 474 & 475 & 475 \\
3.2. & 482 & 501 & 472 & 260 & 245 & 278 & 404 & 466 & 430 \\
4.1. & 455 & 462 & 419 & 210 & 220 & 225 & 415 & 396 & 401 \\
4.2. & 461 & 452 & 459 & 260 & 286 & 232 & 384 & 371 & 379 \\
5.1. & 428 & 367 & 386 & 261 & 284 & 271 & 394 & 398 & 409 \\
5.2. & 381 & 390 & 382 & 266 & 265 & 284 & 373 & 384 & 378 \\
6.1. & 494 & 486 & 490 & 254 & 258 & 262 & 394 & 408 & 402 \\
6.2. & 480 & 494 & 490 & 277 & 280 & 275 & 387 & 367 & 377 \\ \hline \hline \end{tabular}
\end{table}
Table 4: The results of HV10 hardness measurements.  
Figure 6: The distribution of hardness HV10 measurements for all specimens.  
1. The investigated S355J2C+N steel is characterized by poor weldability in wet welding conditions. The way to improve the weldability in the water environment is in situ local heat treatment provided by additional welded stitches. During non-destructive testing it was observed that these stitches do not cause imperfections on the surface.
2. The additional stitch laid on the face of welded joint contributed to tempering of brittle structures in heat-affected zones which generated lower values of HV10 hardness. The tempered martensite and normalized structures were observed during microscopic testing.
3. For improvement of the weldability of S355J2C+N steel in a water environment, two additional stitches with 100% overlapping should be laid. This technique generated normalized structures with fine pearlite and fine ferrite in the HAZ, which decrease hardness in this area by 70-90 HV10. The higher number of additional stitches provided microstructure changes in the welded joint, which may result from increasing the crucial t8/5 time.
4. The positive effect can also be achieved by bead-on plate welding with the one stitch with overlapping lower then 80%. Welding with higher values increased the hardness and did not affect the tempering of HAZ.  
Conceptualization, J.T.; methodology, J.T.; validation, J.T.; formal analysis, J.T.; investigation, J.T. and A.J.; writing--original draft preparation, J.T.; writing--review and editing, J.T. and A.J.; supervision, J.T.; All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Authors want to thank [PERSON] from Gdansk University of Technology for the support and his guidance during our work.  
The authors declare no conflict of interest.