STP1548

    Comparison of the Microstructural Changes and X-ray Diffraction Peak Width Decrease during Rolling Contact Fatigue in Martensitic Microstructures

    Published: Oct 2012


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    Abstract

    The statistical evaluation of bearing endurance life tests requires a sufficient number of repetitions. Rolling contact fatigue (RCF) damage assessment based on an X-ray diffraction (XRD) residual stress analysis enables the alternative materials engineering approach by representative examination of only one run without losing significant informational value. The calibrated correlation between the loading induced decrease of the minimum-to-initial XRD peak width ratio, b/B ≤ 1, and the statistical parameters of the Weibull failure distribution depends on the condition of the steel matrix. Different low-tempered martensite microstructures are investigated by rig tests of ball bearings subjected to subsurface RCF. For standard heat treatment condition, the XRD equivalent value of the actual L10 life, which refers to 10% failure probability, is known to be b/B ≈ 0.64. The fatigue induced microstructural changes are characterized by metallography and scanning electron microscopy and roughly assigned to the XRD calibration curve. A dark etching region occurs at b/B ≤ 0.84, from which flat white bands evolve at b/B ≤ 0.60. The density of the parallel ferritic ribbons increases with further advancing RCF. Steep white etching bands are not observed until the L50 life. L50 corresponds to a failure probability of 50% and is more than five times as large as L10. For a carbonitrided martensite microstructure, similar bearing life is suggested by the evaluated rig tests. The L10 calibration is estimated from the RCF induced microstructural changes with decreasing XRD peak width ratio. The development of dark etching regions and flat white bands already at b/B ≤ 0.90 and b/B ≤ 0.72, respectively, indicate a higher XRD L10 equivalent value of b/B ≈ 0.75 of the nitrogen stabilized steel matrix. Martensite, hydrogen charged in-service, e.g., by high-frequency electric current passage or dense surface cracks, suffers from accelerated RCF, leading to early failures. Flat and steep white bands are observed at b/B ≈ 0.71 inside an extended dark etching region. Also, increased fatigue crack nucleation and faster butterfly-like phase transformations occur. Cold working does not result in visible microstructural changes. The different metallographic findings for the investigated martensite conditions at a reference level of b/B ≈ 0.71 are discussed in the framework of a microscopic model for rolling contact fatigue, including reheating experiments.

    Keywords:

    bearing endurance test evaluation, X-ray diffraction material response analysis, X-ray diffraction bearing life calibration, subsurface rolling contact fatigue, microstructural changes, carbonitrided bearing steel, hydrogen accelerated fatigue, metal physics rolling contact fatigue modeling, cold working, reheating


    Author Information:

    Gegner, Jürgen
    SKF GmbH, Dept. of Material Physics, Schweinfurt,

    Univ. of Siegen, Institute of Material Science, Siegen,

    Nierlich, Wolfgang
    SKF GmbH, Dept. of Material Physics, Schweinfurt,


    Paper ID: STP104650

    Committee/Subcommittee: A01.28

    DOI: 10.1520/STP104650


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