Microstructural and mechanical characterization of the tribologically transformed zone of pearlitic and bainitic railroad wheel steels: Analysis of WEL, TL and NS

Abstract

Rail transport is one of the logistics solutions that presents the lowest environmental impact, greatest safety, efficiency and competitiveness. Therefore, it is a strategic choice to promote the economic development and infrastructure of emerging countries, such as Brazil. As it is increasingly challenging to optimize the mechanical and tribological properties of pearlitic railway steels on an industrial scale, bainitic microstructure steels are an attractive solution to solve wheel-rail contact problems. However, there are few studies on the influence of bainite morphology on railway contact, especially in high-carbon steels. Thus, this research aims to analyze the effects of local plastic deformation caused by twin-disc rolling-sliding cycles on the microstructural changes that occur in the tribologically transformed zone. Microalloyed Class D forged railway wheel steel with pearlite (350 HV_0.5), upper bainite (350 HV_0.5) and lower bainite (450 HV_0.5) microstructures was used and their relationship with wear and RCF performance. To support the characterization of the materials, analyses were performed using optical microscopy (OM), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), electron backscattered diffraction (EBSD), transmission electron microscopy (TEM) and hardness measurements. The contact conditions promoted cementite decomposition, which initiated in the NS, intensified in the TL, and saturated in the WEL across all microstructures. The occurrence of martensitic transformation at greater depths in the bainitic microstructures compared to pearlite resulted in thinner deformed layers with higher stress accommodation capacity. These features provided the bainitic discs with superior resistance to RCF and wear.