Microstructure-based approach to the evaluation of welded joints of bainitic rails designed for high-speed railways

Bainitic rails, due to their wear conditions, favorable mechanical properties and mechanical response under contact fatigue, have been widely investigated in recent years. In this research, the microstructure of a bainitic rail was analyzed after joining using the flash butt welding method. To date, the structural changes of bainitic rails after welding processes that directly affect their mechanical and functional properties have not been analyzed. The microstructure of the base material consisted of degenerated upper bainite, retained austenite and a negligible amount of cementite. No unfavorable martensite was found in the Fusion Zone (FZ), and its microstructure consisted of coarse degenerated bainite and retained austenite with a blocky and film-like morphology. In the High Temperature Heat-Affected Zone (HTHAZ), the bainitic structure was refined in comparison to the FZ, which resulted in a slight increase in hardness. In contrast, a drop in hardness was found in the HTHAZ. The microstructure of this zone consisted of bainite, polygonal ferrite, retained austenite and M/A constituents. The highest decrease in hardness was found in the Low Temperature Heat-Affected Zone (LTHAZ). In this zone, the retained austenite was completely decomposed into a mixture of ferrite and cementite particles.