## 2 Design of Weld Metals  
Obtaining the structure of nanocrystalline bainite in an entire welded joint area determines the most favorable mechanical properties. The homogeneous bainitic structure is difficult to achieve during conventional welding methods. In addition, the NB CFB structure can be obtained for materials characterized by a specific chemical composition, which causes problems with the selection of weld metals. At present, there are no commercially available weld metals that allow for the structure of NB CFB in the weld zone. In Reference [13], the chemical composition of an electrode was proposed, which allows such a structure in the weld zone to be obtained with a small amount of martensite and retained austenite (Table 1). After the welding process, as a result of the high hardenability associated with the designed chemical composition of the weld material, the typical Widmanstatten structure was not achieved. High ultimate tensile strength (approximately 950 MPa) and yield strength (approximately 830 MPa) was not achieved in the joint. It was found that silicon does not significantly affect strength parameters but does have an influence on impact toughness. The increased concentration of silicon caused a decrease in the impact energy, which was explained by the slow development of martensite tempering in relation to the material with a lower content of silicon. [PERSON] et al. [14] proposed weld metals with a higher carbon content (0.32%), which, according to the authors, can replace the austenitic welds when welding armor steels. The used weld metals allowed the desired structure of CFB to be obtained with blocky austenite in the inter-dendritic areas. The presence of unstable blocky austenite reduces the impact toughness of the welds due to the induction of martensitic transformation under loads [15].  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multicolumn{1}{c}{**Chemical**} & \multirow{2}{*}{**Process Parameters**} & **Mechanical Properties** & **Microstructure and Comments** & **Reference(s)** \\ \hline (1) 0.10\% C; 2.2\% & TIG & YS = 830 MPa & & \\ Mn; 0.86\% Si; 2.1\% & Interpass temperature: & UTS = 950 MPa & & \\ Ni; 0.2\% Mo & 250 \({}^{\circ}\)C; & The impact & & \\ (2) 0.12\% C; 2.3\% & Electrode \(\approx\)4 mm; 30 & toughness decreases & & \\ Mn; 1.38\% Si, 2.1\% & runs; & with increasing & & \\ Ni; 0.2\% Mo & Current: 174 \(\pm\) 1 A; & silicon content. No & & \\ (3) 0.1\% C; 2.2\% & Voltage: 25 \(\pm\) 1 V; & significant influence & & \\ Mn; 1.63\% Si; 2.1\% & Input energy: 1.08 & of silicon on weld & & \\ Ni; 0.2\% Mo & kJ/mm. & strength was found. & & \\ \hline \hline \end{tabular}
\end{table}
Table 1: Review of weld metals that allow a carbide-free bainite (CFB) structure to be obtained.  
It was also found that the weld material with the structure of CBF was resistant to hydrogen cold cracking [14] and hot cracking [16]. The obtained ultimate tensile strength was 1200 MPa [14], but this is still insufficient in the context of high-carbon nanostructured bainitic steels (even at 2500 MPa [6]).  
Therefore, weld materials with a higher strength should be designed. On the other hand, increasing the strength requires the carbon content to be increased, which causes worse weldability. Thereat. Therefore, medium and high carbon bainitic steels were welded without weld metals in order to obtain high-strength parameters for the joints.