Microstructural investigation along the transverse section of the lower gauge corner flow lips formed in high rails of mild curves

Abstract

The microstructure of the ‘flow lip’ formed at the lower gauge face in high rails in curved tracks has been investigated along the transverse section. The rails served in tracks having radii of curvature of 1500 m(S1) and 2000 m(S2) respectively. Optical images reveal the formation of an outermost white etching layer (WEL) followed by a brown etching layer (BEL) and deformed sublayers. Cracks originated from the WEL regions and from the notches at the extreme end of the flow lip. The cracks originate from the WEL region due to exhaustion of ductility, whereas, at the extreme end of the flow lip the cracks originate due to stress concentration caused by the notches generated due to uneven plastic flow of material. Using SEM (scanning electron microscopy) and EBSD (electron back-scattered diffraction), the WEL region is identified to consist of fine grains of ferrite and particles/fine fragments of cementite. The cementite in the WEL region is present as particles in S1 and as broken fragments as well as particles in S2. From the SEM images, the BEL has a similar appearance to WEL but with intermittent patches of lamellae-type features. The lamellae type features are much larger in S2 than S1 implying that the radius of curvature can significantly affect the level of deformation. The deformation was lower in S2 than S1 for similar distances from the outer edge, possibly due to less severe shear loading in a less sharply curved rail.