White Layer in M76 High Carbon Rail Steel: Formation Mechanism and Properties

Abstract

High-frequency vibrations resulting from many factors under loads existing in the wheel-rail system have a huge impact on the structure and properties of rail steel: there are signifi contact stresses in the surface layer that affect strength characteristics and overall fatigue of the railway track structure. Such an impact results, in particular, in the formation of the so-called White Layer (WL): a hardened layer on the surface of the base material, resistant to chemical etching, and having high hardness (above 1000 HV) and brittleness.The objective of the research was to study the features of the formation mechanism, as well as the properties of the white layer formed on the metal surface using an integrated approach, namely, of destructive testing methods, electron microscopy, XRD, metallography, and microhardness measurements.The reliability of experimental studies is due to the use of standardised test methods, developed methods of destructive and non-destructive testing in the main areas, the involvement of an accredited and certified laboratory, which makes it possible to fulfill in full the tasks set with appropriate quality.The results of the research allowed to present an analysis of the white layer, the formation of which took place in high-carbon M76 rail steel after cyclic tests with a 20 kHz frequency. The morphology, phase composition, and microhardness of these inclusions have been studied in detail in comparison with the base metal. It is shown that the white layer is highly dispersed, pearlitelike, featureless inclusions of a ferrite-cementite structure, while their microhardness is 3–4 times higher than the original steel and is of 1000–1200 HV. A possible explication of a mechanism of the formation of a white layer is suggested: crushing of ferrite and cementite, which are part of pearlite, without intermediate phase transformations.