Consequences of thermite welding on the microstructure of the heat affected zone of a carbide-free bainitic steel rail

Abstract

Due to their excellent Rolling Contact Fatigue (RCF) resistance, Carbide-Free Bainitic (CFB) steels rails have recently received increasing attention to replace pearlitic steels in the railways sector. This is mainly attributed to the TRansformation Induced Plasticity (TRIP) effect, which mechanically transforms austenite into martensite during wheel/rail contact. However, the welding of these carbide-free bainitic steels could lead to a local decrease in mechanical properties, as well as RCF resistance, inside the Heat Affected Zone, which limits the use of these steels. The present work aims to explain the reason of the loss of RCF resistance of these steels after welding. To do so, the microstructure evolution along the Heat Affected Zone is investigated thoroughly, and is linked with its thermal history.In the most critical areas in terms of RCF resistance, the consumption of all retained austenite has been evidenced, thus suppressing TRIP effect. This loss of austenite is mainly observed in a critical range of temperatures and has been linked to the low thermal stability of austenite. The microstructural investigations performed lead to a proposal of improvements in terms of rail chemical composition and/or heat treatments, in such a way that carbide-free bainitic steels rails could be less sensitive to RCF resistance loss after welding.