## 5 Conclusions
### Problems and Perspectives  
Welding of high-strength and high-carbon nanobainitic steels is difficult in terms of many factors, including welding parameters, heat treatment, choice of weld-metals, and microstructure. The main problems, as well as prospects resulting from the conducted research, were identified:  
* The weld materials so far proposed have shown satisfactory resistance for cold cracking when using pre-heating, an ultimate tensile strength of up to 1200 MPa, and a structure consisting only of austenite and ferrite phases in the weld. However, welding of high-carbon steels requires stronger weld metals. Designing stronger weld metals, including those with a higher carbon concentration, would allow for easier joining and a wider range of welding methods for nanobainitic steels, which, until now, were welded without weld materials. * The weakest zone of welded joints is the low-temperature affected-zone (LHAZ), where softening occurs due to the decomposition of austenite. Welding parameters directly affect the resumption of bainitic structures to the equilibrium state. However, the mechanisms of the austenite decomposition process and the precipitation of cementitte require further research. This research should also involve real welding conditions. The development of a theory regarding these processes could allow for the reduction of unfavorable zones. * A disadvantage which is difficult to avoid is the grain growth due to the influence of high temperatures, which results in lower mechanical properties. Analysis of the grain growth of bainitic steels (containing silicon and the lack of elements forming carbides inhibiting the growth of austenite grains) has not yet been described in detail. * The required regeneration time to complete the bainite transformation is long (up to a few days), which is a problem in industrial applications. Shortening the time requires further research regarding the design of the chemical composition of base materials and weld metals, and also improvement of the deformation process (austempering) and the kinetics of phase transformations that depend on the regeneration parameters and grain size. * To date, no residual stress research [46] on NB CFB welded joints has been presented. Due to the complex welding process and the focus on maximizing mechanical properties, the effect of residual stressors can be significant. * Investigations on the mechanical properties of NB CFB welded joints did not include the aspect of fatigue. Due to the possibility of using welded structures during fatigue conditions, such research should be carried out in the future. A.K. ; writing--original draft preparation, A.Z. ; writing--review and editing, A.A.; supervision. POWR.03.02.00-00-I003/16. Figures 1 and 2 copyright (c) Institute of Materials, Minerals and Mining reprinted with permission of Taylor & Francis Ltd. ([[http://www.tandfonline.com](http://www.tandfonline.com)]([http://www.tandfonline.com](http://www.tandfonline.com))) on behalf of the Institute of Materials, Minerals and Mining. [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON]; [PERSON], [PERSON], \"Acceleration of regeneration treatment for nanostructured bainitic steel welding by static recrystallization\", Materials Science and Technology. Figures 3-5 copyright (c) reprinted with permission of the publisher Taylor & Francis Ltd. [PERSON]; [PERSON], [PERSON]; [PERSON]; [PERSON], [PERSON]; [PERSON] [PERSON]; [PERSON] [PERSON], \"Study on tempered zone in nanostructured bainitic steel welded joints with regeneration\", Science and Technology of Welding and Joining, 26 June 2014. The authors declare no conflict of interest. * _[PERSON] et al. (1983)_ [PERSON]; [PERSON] Bainite in silicon steels: New composition-property approach Part 2. _Mel. Sci._**1983**, _17_, 420-425. [CrossRef]
* [PERSON] et al. (2001) [PERSON]; [PERSON] [PERSON]; [PERSON]; [PERSON]; [PERSON] Design of novel high strength bainitic steels: Part 1. _Mater. Sci. Technol._**2001**, _17_, 512-516.