Design of high-strength and damage-resistant carbide-free fine bainitic steels for railway crossing applications

Abstract

A novel high-strength steel design is proposed, with a fine bainitic microstructure free from inter-lath carbides, for railway crossings applications. The design is based on the phase transformation theory and avoids microstructural constituents like martensite, cementite and large blocky retained austenite islands in the microstructure which are considered to be responsible for strain partitioning and damage initiation. The designed steel consists of fine bainitic ferrite, thin film austenite and a minor fraction of blocky austenite which contribute to its high strength, appreciable toughness and damage resistance. Atom probe tomography and dilatometry results are used to study the deviation of carbon partitioning in retained austenite and bainitic ferrite fractions from the T0/T0ʹ predictions. A high carbon concentration of 7.9 at.% (1.8 wt%) was measured in thin film austenite, which governs its mechanical stability. Various strengthening mechanisms such as effect of grain size, nano-sized cementite precipitation and Cottrell atmosphere at dislocations within bainitic ferrite are discussed. Mechanical properties of the designed steel are found to be superior to those of conventional steels used in railway crossings. The designed steel also offers controlled crack growth under the impact fatigue, which is the main cause of failure in crossings. In-situ testing using micro digital image correlation is carried out to study the micromechanical response of the designed microstructure. The results show uniform strain distribution with low standard deviation of 1.5% from the mean local strain value of 7.7% at 8% global strain.