Microstructure and mechanical properties of a flash butt welded pearlitic rail.

Abstract

The structural changes resulting from the Flash Butt Welding (FBW) of pearlitic rails have been associated with wear/premature failures, despite this, there are no studies applying dilatometry to correlate the welding thermal cycles with the microstructural development of such material. The microstructural evolution of the heat affected zone is clarified with the aid of dilatometry. The increase in the steel hardenability associated with a larger austenitic grain size promotes the austenite-pearlite transformation at lower temperatures in the grain growth region. This explains why this region has larger pearlite colony size but smaller interlamellar pearlite spacing and higher hardness than the grain refined region. Partial cementite spheroidization in the heat affected zone is responsible for significant decrease in hardness and tensile strength and is correlated to localized dipping, rolling contact fatigue and failures. A dilatometry based methodology is proposed to define a process window and control the post-weld cooling rate at the rail head in order to improve the weld performance due to a better hardness profile, without increasing costs or welding time. For the steel evaluated, a 20% increase in the hardness of the softened area at the HAZ was obtained by dilatometric simulation of a safe accelerated cooling (5 °C/s).