## 1 Introduction
In an air environment, the modeling of a hydrogen-inducted delayed intergranular fracture in high strength steels can be used [31]. However, the prediction of diffusible hydrogen content in deposited metal, in a water environment, is impossible due to unpredictable process conditions, which change during welding. These changes result from the instability of the welding arc in the water, which is the other problem in this environment. [PERSON] et al. stated that ultrasonic wave-assisted during underwater FCAW welding has the potential to control the dynamic bubble and then improve arc stability [32]. [PERSON] et al. [33] proved, that droplet has significant influence on the stability of underwater welding process. [PERSON] et al. [34] stated also that ultrasonic energy decreases the porosity and diffusible hydrogen content, which can improve the quality of underwater welded joints. Problems of underwater wet welding generate the high susceptibility of HSLA steel to cold cracking. The cold cracks can occur in the welds and in the heat-affected zone (HAZ) near the fusion line [35]. There are a couple of methods which provide to decreasing the susceptibility to cold cracking of underwater wet-welded joints. It was proved that welding with austenitic consumables, which provides welds with a good plasticity, can reduce the susceptibility to cold cracking. The temper bead welding can also decrease the number of cold cracks in pad welds, made in the water [25]. In previous experiments the bead sequence was investigated. It was stated that a proper bead sequence during underwater wet welding can contribute to decreasing the hardness, tempering the HAZ and reducing the number of cracks in the area of pad welds [36]. A further method reducing the susceptibility to cold cracking is submerged-arc welding applied to marine steels. [PERSON] et al. proved, that this method can also reduce the tendency to corrosion of underwater welded joints [37]. [PERSON] et al. proposed welding with higher heat input, which provides a large possibility for a better protective effect and a larger weld penetration [38]. Another method for improving the quality of underwater welded joints is modification of filler materials. [PERSON] et al. [39] proposed silicate and polymer agglomerated electrodes, which caused higher arc voltage values in comparison to the conventional electrode with the same polarity. This may contribute to decreasing of the number of cracks in the welded joints, which results from lower diffusible hydrogen content in the deposited metal. The use of traditional heat treatment possibilities is very limited in water due to the negative influence of the environment. [PERSON] et al. [40] proposed real-time induction heating during wet FCAW welding. Authors stated that the addition of induction heating could reduce the cooling rate of the joint in a water environment to improve the microstructural properties of the joint. The content of brittle structures such as martensite and bainite decreased while the proeutectoid ferrite and acicular ferrite phases increased. However, the induction heating process caused major problems with the stability of the welding arc. This can affect the quality of the welded joints. The local heat treatment as an effect of multiple passes has a positive effect in HSLA steels in air [41]. However, this method has not been verified sufficiently in a water environment. The aim of this research was to study the influence of in situ local heat treatment on the weldability of high-strength low-alloy S355J2C+N steel. As a method for improving the weldability of steel local heat treatment, provided by the heat from additional stitches, was chosen. Weldability tests are commonly used to evaluate the possibilities of the welding in the air for different materials [42; 43], and in the water [35]. A Tekken weldability test was chosen for investigation. This test allows assessment of the weldability for a butt-welded joint.