## 3 Review of Welding Methods
Regeneration of the weld joint involved two stages, and the cycles are shown in Figure 2. Different recrystallization temperatures were used, ranging from 700 \({}^{\circ}\)C, 750 \({}^{\circ}\)C, and 800 \({}^{\circ}\)Cwith times of 10 s, 30 s, 50 s, and 100 s. It was found that the conducted static recrystallization process in the impact zone reduced the grain size from \(106\pm 42\)\(\upmu\)m to \(36\pm 13\)\(\upmu\)m. The recrystallized grains were also smaller than the base material, where the grain size was around 50 \(\upmu\)m. In addition, it was found that a higher volume of bainitic ferrite was obtained in the recrystallized grains, which also confirmed the reduction of the bainitic transformation time and the same time of regeneration. Welded joints after the ITW process showed a higher ultimate tensile strength (2100 MPa) when compared to the process without ITW (1400 MPa) [23]. In Reference [26], for welding high carbon bainitic steels, laser welding with post-veld rapid heat treatment (PWRHT) is proposed. When the welded joint after the welding process approaches the beginning of the martensitic transformation (Ms), it is quickly heated to a temperature lower than A\({}_{1}\) (in less than 10 seconds). As a consequence, disadvantageous, brittle martensite in the welds and cold cracks are avoided. The structure after laser welding and PWRTH consisted of ferrite, austenite, and cementie, and its hardness was comparable to the base material. The obtained structure was different to the correct bainitic structure and, despite a comparable hardness level, it showed lower mechanical properties. [PERSON] et al. [21] proposed a laser beam welding process with regeneration, similar to the TIG method. It was found that low input energy (60 kJ/m) in laser welding, when compared to the TIG method where high input energy was used (908 kJ/m), allows for the achievement of high ultimate tensile strength (33 MPa lower than the strength of the base material). The high tensile strength of  
Figure 1: Scheme of the welding process using the two-pass impacting Trailed welding (ITW) method [23]. Figure 2: Two-pass regeneration with static recrystallization [23]. -welded joints is due to the high stability of the microstructure in the heat-affected zone at low input energy processes. Another welding method used for nanostructured CFB steels is friction stir welding (FSW) [27; 28]. The joining process takes place in a solid state, which reduces the formation of distortions and defects associated with thermal contraction. In the FSW method, a rotating tool containing the shoulder and pin is plunged into the joint between the two front-mounted plates. As a result of friction and plastic deformation of the pin on the surface of the plates, heat is generated, which causes the softening of the material in the area of the tool. The softened material then flows to the rear edge of the shoulder, where it is stirred. The material after cooling forms a joint between the plates [29]. In References [27; 28], it was found that the parameters of the FSW process affect the mechanical properties and microstructure of the obtained joints. The research concerned silicon-rich steels with a relatively low carbon content (0.34 wt.%). The increase in rotational speed increases the hardness of the zone, defined in the work as the stir-zone (SZ), located in the area of the moving shoulder. However, the hardness obtained at the lowest rotational speed was also high when compared to the thermo-mechanical affected zone (TMAZ) and base material. This increase in hardness is explained by the appearance of martensite under the influence of the thermo-mechanical effect (TRIP) and suggests the use of heat treatment after the joining process. However, the presence of martensite was not confirmed and the change in the volume of retained austenite in this zone was not determined. Scanning electron microscopy does not allow microstructures to be unequivocally identified. Carbide precipitates, which are likely in the TMAZ zone, were not analyzed.