In-situ micro-cantilever bending studies of a white etching layer thermally induced on rail wheels
A complex and untraceable mechanical and thermal loading situation in rolling-sliding contacts can lead to the formation of white etching layers (WELs), depicting critical crack initiation sites. For our detailed studies, to obtain a holistic view of the microstructural characteristics and micro-mechanical properties, we prepared a WEL on a decommissioned (after 200,000 km service life) rail wheel by laser surface treatments with a defined energy input. This WEL is predominantly martensitic down to a depth of 30–40 μm, after which a transition to the deformed ferritic-pearlitic microstructure of the hypoeutectoid rail wheel steel is present. The martensitic region is with 6.98 ± 0.68 GPa significantly harder than the transition zone (5.17 ± 0.39 GPa) and the deformed ferritic-pearlitic base material (3.30 ± 0.33 GPa). In-situ V-notched micro-cantilever bending experiments of the martensitic (and thus most brittle) region show crack initiation and propagation – mostly along the boundaries of the martensitic grains – besides a plastic behavior. Applying the elastoplastic fracture mechanics allows to derive the local fracture toughness KIQ, which is 16.4 ± 1.2 MPam1/2 for this martensitic region of the WEL. The results outpoint the application of micro-cantilever bending tests in addition to hardness testing as a promising tool to discuss the relationship of microstructural characteristics with its micro-mechanical properties.
M. Freisinger, L. Zauner, R. Hahn, H. Riedl, P.H. Mayrhofer
Materials Science and Engineering: A