In situ study on fracture behaviour of white etching layers formed on rails

Abstract

Failure in engineering materials like steels is strongly affected by in-service deleterious alterations in their microstructure. White Etching Layers (WELs) are an example of such inservice alterations in the pearlitic microstructure at the rail surface. Cracks initiate in the rails due to delamination and fracture of these layers and propagate into the base material posing severe safety concerns. In this study, we investigate the microscale fracture behaviour of these WELs. We use in situ elastic-plastic fracture mechanics using J-integral to quantify the fracture toughness. Although usually assumed brittle, the fracture toughness of 21 -25 MPa√m reveals a semi-brittle nature of WELs. Based on a comparison of the fracture toughness and critical defect size of WELs with the undeformed pearlitic steels, WELs are detrimental for rails. In the micro fracture tests, WELs show crack tip blunting, branching, and significant plasticity during crack growth due to their complex microstructure. The fracture behaviour of the WELs is governed by their microstructural constituents such as phases (martensite/austenite), grain size, dislocation density and carbon segregation to dislocations and grain boundaries. We observed dislocation annihilation in some martensitic grains in the WELs which also contributes to their fracture behaviour. Additionally, the straininduced transformation from austenite to martensite affects the crack growth and fracture.

The fracture toughness of martensite islands in dual-phase DP800 steel

“…However, in our case almost the entire carbon is present in the martensite (see Fig. 2) with an average amount similar to carbon content of 3 at.% present in WELs [35]. Besides, both show a heterogeneous distribution of carbon segregating at defects like dislocations and boundaries.…”

“…This partly results in an inferior fracture toughness [33,34]. The nominal carbon content in our material (0.13 wt%) is much lower than that in the pearlitic rail steel (0.72 wt%) where WELs are formed [23,35]. However, in our case almost the entire carbon is present in the martensite (see Fig.…”

“…For lath martensite, containing abundant substructures as in our case, block boundaries act as the most efficient obstacles for dislocation motion [36,37]. The block size is in the order of 100 nm in our sample, substantially finer than the one in the literature [20,35], which strongly impedes dislocation motion in martensite islands and deteriorates ductility. In addition, grains of martensite in WELs exhibit almost equiaxed morphology, while martensite islands in dual-phase steels have a hierarchical structure with lath, blocks and packets arranging themselves complying with orientation of the prior austenite grain.…”

Influence of the evolution of near-surface rail wheel microstructure on crack initiation by micro-bending investigations

“…25,26 Regarding the evaluation of fracture behaviour and toughness, the micro-bending method is a promising testing method for rail and wheel materials in recent years. [27][28][29] Recently published work of the author demonstrates the applicability of the presented micro-bending testing method for the evaluation of evolved martensitic microstructures in wheelrail contacts, imitated in laboratory. 29 Within this work, other characteristic local microstructures occurring in wheel-rail contacts are investigated, namely SPD and corrosion affected microstructures, by applying micro-bending tests to show the potential of this method in testing local evolved microstructures of field samples.…”

EBSD Study on Proeutectoid Ferrite and Eutectoid Ferrite Refinement Mechanism of D2 Wheel Steel Under a Rolling Condition

“…With the rapid development of high-speed railways, friction and wear between wheels and rails and rolling contact fatigue damage are becoming more and more serious [1][2][3] . Such as the white layer will accelerate the peeling of the material [4] , and the wheel polygonisation wear will increase the vibration of the train [5] .Those problems shorten the repair and maintenance cycle of wheels and rails, and increase railway operating costs [6,7] . Therefore, higher requirements are put forward for the service performance of wheel rail materials.…”