## 4 Analysis of Microstructure Changes after the Welding Process
### Low-Temperature Heat-Affected Zone  
One of the main problems of welding nanostructured bainitic steels is the precipitation of cementite in low-temperature heat-affected zones (LHAZs). Analogous to tempering processes, there are areas located in the LHAZ that correspond to tempering at different temperature ranges. In Reference [6], it was found that tempering at temperatures of up to 500 \({}^{\circ}\)C did not cause significant changes in the hardness and microstructure of nanobainitic steels. However, during longer period of heating, precipitation of cementite may occur at temperatures above 450 \({}^{\circ}\)C [35; 36; 37; 38]. Areas of blocky austenite at higher temperatures (above 550 \({}^{\circ}\)C) were decomposed to fine dispersive perlite, and austenite with a film-like morphology was decomposed to ferrite and cementite due to the limited volume for pearlite growth [38]. In this temperature range, the austenite completely decomposed [38]. Further increases of the tempering temperature resulted in a ratio of ferrite and cementite that corresponded to the equilibrium state and an increase in the width of the ferrite plates [39], which reduced the mechanical properties. However, according to [PERSON] and [PERSON] [37], long-term tempering also causes a significant decrease in the volume of austenite, which becomes less stable by the precipitate of cementite. Then, austenite with a lower concentration of carbon was transformed to martensite after cooling to an ambient temperature. There was also no segregation of alloying elements between the ferrite/austenite phases during tempering [36]. In addition to research on the nanostructured CFB bainitic stability during tempering, in situ tests were carried out during the continuous heating of two silicon-rich steels [40]. It was found that for a steel of 0.84C-2.26 Mn-1.78 Si-0.25 Mo-1.55 Co-1.47 Cr, austenite began to decompose at 400 \({}^{\circ}\)C, while for the alloy 1.04C-1.97 Mn-3.89 Si-0.24 Mo-1.43 Al, decomposition began to occur at 580 \({}^{\circ}\)C. Therefore, the higher contents of silicon and aluminum in the steel caused a higher stability in the austenite. In addition, tests performed during continuous heating were closer to welding conditions than tempering over longer time periods. The differences in the obtained results indicate the need for further research, especially during continuous heating. In the context of welding processes, cooling to an ambient temperature or a regeneration temperature  
Figure 3: Scheme of welded joint. after subsequent heating stages is also important due to the fact that the mechanisms of austenite decomposition can be significantly different. [PERSON] et al. [21] conducted research using a thermomechanical simulation of the LHAZ zone at 400 \({}^{\circ}\)C for low- and high-energy welding with regeneration (Figure 4). The samples after reaching 400 \({}^{\circ}\)C were cooled to the regeneration temperature (250 \({}^{\circ}\)C). In the initial steel consisting of 0.82C-1.66 Si-2.05 Mn-0.22 Cr-0.36 Mo-1.06 Ni, a typical nanobainitic structure consisting of thin ferrite laths and austenite with a film-like morphology (and a small fraction of blocky austenite) was also identified (Figure 5a). For the low-energy process, no significant differences were found when compared to the base material. It was found that only short austenite structures with a film-like morphology were decomposed to spheroidal precipitations of austenite (Figure 5b). In the case of high-energy welding processes, the microstructure changed substantially. The short film-like austenite completely decomposed, while the areas of long filmy austenite became discontinuous. Many spheroidal precipitations also formed in the bainitic ferrite matrix (Figure 5c). According to the authors, the blocky austenite in a high-energy welding process decomposes into a fine nanostructured bainite with a different crystallographic orientation than the nearby bainite (Figure 5d).