Role of steel composition and welding parameters in the improvement of fatigue behaviour of high-strength welded rails

The project, focused on high-strength welded rails, is aimed at investigating the influence played by welding parameters and chemical composition of the base material on softening in the HAZ, in order to develop a specific know-how on this topic. Bainitic and pearlitic high-strength rail steels, up to grade 360, were considered and two welding techniques were used, Thermit and Flash Butt welding. Macro-etching and hardness measurements were carried out, together with micro-structural and mechanical assessment of the parent rail and heat softened zone (HSZ) in terms of tensile, impact, fracture toughness, fatigue and fatigue crack growth tests; in addition four point bending and rolling contact fatigue tests were carried out. The influence on the softening mechanism of both welding parameters and the chemical composition of base material was studied by laboratory tests and industrial trials. In parallel, in-service monitoring of sites throughout the UK railways was performed. The main results were as follows. - Detailed information about the properties of HAZ obtained adopting several welding procedures was collected and is at the disposal of rail producers and end users. - The softened zone was clearly highlighted in all the high-strength welded rails and the pearlite globularisation (consequence of the thermal cycles related to welding) was identified as the mechanism controlling softening. - Among welding parameters, the cooling rate was singled out as the most suitable to improve the HSZ properties. - Cr, Nb and V were detected as the main alloying elements giving a reduction of hardness drop in the HSZ. Grade 350 NbV rail steel benefited most when the cooling rate increased.