Welding of carbide-free bainitic steels for railway applications

Carbide Free Bainitic (CFB) steels B360 and B320 show an outstanding behaviour in terms of fatigue deformation mechanisms, mainly when analysing its resistance to head checks. This, results in a lower necessity for maintenance and therefore, in a reduction of costs. However, problems have been encountered when welding CFB rails, showing cracks when in track, in both the entry and the exit site of the weld. The mechanisms of failure of these rails is not entirely clear. Nowadays, with the rise in use of high-speed trains and high loads, Continuously Welded Rails (CWR) are gaining importance; as other joining mechanisms such as fish-plates are becoming obsolete. Almost 90% of the CWR are welded by Flash Butt Welding (FBW); which is known as one of the most reliable welding processes; nevertheless, the welds are a weak link between rails due to mechanical and microstructural variations with the parent rail. Welding of conventional pearlitic rail steels, has been under investigation on numerous occasions; however, the same has not been made as extensively on CFB rail steels. On this thesis, an extensive examination of the Heat Affected Zone (HAZ) of three FBW rails (B360, B320 and R260) is discussed. CFB FBWs showed much wider HAZ extension than conventional pearlitic steels, of importance will be the extension of the tempered or Subcritical HAZ (SCHAZ), as CFBs, mainly B360 has shown low resistance to tempering; which could result on preferential wear and plastic deformation in track applications. With the purpose of reducing the extension of the tempered SCHAZ, weld trials were carried out on B360 rails using a stationary FBW GAAS 80 / 580 Schlatter welding machine in Scunthorpe, UK. Those, were produced by variations on the number of Pre-Heat (PH) cycles, on the preheating time (s) (ON) and delay period (s) (OFF) and additions of Post Weld Heat Pulses (PWHP) at the end of the welds. Finally, an analysis of the welded rails mechanical properties was made.