Xavier Kleber

Understanding white etching cracks in rolling element bearings: The effect of hydrogen charging on the formation mechanisms

Among prevalent tribological failures, notably in rolling element bearings for wind turbines, an unusual rolling/sliding contact fatigue failure mode has been identified as white etching cracks. White etching cracks are broad subsurface three-dimensional branching crack networks bordered by white etching microstructure, eventually leading to flaking. Reproduction of the failure mode on standard rolling element bearings test rigs has not been mastered yet except with artificial hydrogen charging. Even though these failures have been reported for several decades, there is no evident common denominator in different occurrences. Hence, initiation and propagation mechanisms are not yet fully understood in application. Analyses of the contact conditions of a standard rolling element bearings test rig reproducing white etching cracks on standard and hydrogen precharged inner rings reveal that hydrogen charging seems to modify the white etching cracks initiation mechanism. Based on fractographs, serial sectioning, and scanning electron microscopic analyses, surface initiation and propagation mechanisms are proposed, including influent drivers and possible preventive techniques.

A computational study of the thermoelectric power of 2D two phase materials

In this paper, we report some numerical analysis results obtained in the study of the thermoelectric power (TEP) of 2D two phase materials. Based on the numerical resolution of transport equations, we compute the TEP of different composite structures. These were numerically simulated using a grain growth model. We show that the ratio of the electrical conductivity of the two phases is the relevant parameter for metallic material which is verified by the Wiedeman–Franz law. We observe that for a low ratio, the TEP of the composite follows a simple rule of mixture, whereas for a higher value, a S-shaped curve is obtained. Applied to the case of atoms precipitation in a metallic matrix, we show that for a low fraction of precipitates, their effect can be neglected when compared with the variation induced by the atoms precipitation. We found that an induced anisotropy in the shape of the grains leads to a strong deviation from the rule of mixture.

Thermoelectric power of a two-dimensional metal/metal composite: a numerical approach

Based on the theory of coupled transport of electricity and heat in metals, we present a numerical method allowing us to calculate the thermoelectric power (TEP) (Seebeck coefficient) of a two-dimensional metal/metal composite structure. Depending on the arrangement, the surface fraction and the geometry of the two components, the temperature, the potential and the thermoelectric current distributions are calculated within the structure. The apparent TEP is then deduced. In order to validate this numerical model, various metal/metal composite structures were experimented on. The data resulting from the numerical simulations proved to be in excellent agreement with the experimental results. This method of calculation contributes to a better understanding of TEP measurements when done for the characterization of multi-constituent materials. We show that it may also be applied to calculate the current distribution in inhomogeneous materials subject to a thermal gradient, and hence contribute to a better understanding of results obtained by the thermoelectric method with magnetic readout.

Understanding white etching cracks in rolling element bearings: Formation mechanisms and influent tribochemical drivers

Among prevalent tribological failures in rolling element bearings, a peculiar rolling contact fatigue mode has been defined as white etching cracks, which correspond to broad subsurface three-dimensional branching crack networks bordered by white etching microstructure. White etching cracks tend to appear before other conventional rolling contact fatigue microstructural alterations and eventually lead to premature flaking or radial cracking that remains unpredictable using fatigue life estimations. Even though they have been reported for several decades, occurrences present no common evident denominator and are delicate to reproduce on laboratory test rigs without artificial hydrogen charging. In this study, white etching crack reproductions on two different standard endurance test rigs are compared and analyzed in order to, firstly, propose, an update of the understanding of white etching crack surface affected tribochemical formation mechanisms, and secondly, to identify influent tribochemical and mechanical drivers. If white etching cracks are associated to different combinations of specific non-self-sufficient macroscopic drivers depending on the application or the test rig, evidences demonstrate that they often come down to similar phenomena at a microscopic tribological scale that should all be mastered for efficient white etching crack reproduction and countermeasure design.

Liquid zinc embrittlement of a high-manganese-content TWIP steel

The cracking resistance of a twinning induced plasticity (TWIP) steel with high-manganese content has been studied in relation to the liquid metal embrittlement phenomenon. This phenomenon was investigated by hot tensile tests carried out on electrogalvanised specimens using a Gleeble® simulator at temperatures ranging from 600°C to 1000°C. The results show that this steel can be embrittled by liquid zinc within a limited range of temperature depending on strain rate.

Understanding white etching cracks in rolling element bearings: State of art and multiple driver transposition on a twin-disc machine

Among the prevalent tribological failures affecting rolling element bearings, an unconventional rolling contact fatigue mode has been identified as white etching cracks. Those correspond to three-dimensional branching crack networks partially bordered by white etching microstructure, eventually leading to premature and unpredictable failure. Recent work supports that this failure mode may be associated with various combinations of operating conditions depending on the application or test rig, but that all seem to converge towards similar tribological drivers related to surface-affected hydrogen evolution at asperity scales, which is known to embrittle the bearing steel. Nevertheless, as white etching cracks remain delicate to reproduce without artificial hydrogen charging, the underlying formation mechanisms remain unsettled. The present work aims to better understand how some of the main tribomechanical and tribochemical drivers may trigger white etching cracks and premature failures. In this study drivers such as sliding kinematics, water contamination, and electrical potential and lubricant additives are progressively transposed on a twin-disc machine that provides an enhanced control of contact parameters. Various attempts advocate that the tested drivers are not self-sufficient to reproduce the failure mode in such apparatus, but confirm that specific lubricant additives may reduce the fatigue life by promoting surface-initiated embrittled cracking similar to white etching cracks. A local criterion accounting for the local sliding frictional power dissipation and the lubrication regime is further proposed to assess the risk of white etching cracks based on the analysis of various reproduction and occurrences.

Influence of grain boundary cementite induced by gas nitriding on the rolling contact fatigue of alloyed steels for gears

It is well-known that rolling contact fatigue life can be increased by modifying gear surfaces. For instance, gas nitriding is a thermochemical treatment enhancing superficial hardness, and introducing compressive residual stresses but nitriding alloys can lead to the precipitation of cementite at grain boundaries. In this study, experiments were performed on a twin-disc machine to investigate the influence of the microstructure of nitrided layers on rolling contact fatigue failure modes. Nitrided layers with small grains, thin and short grain boundary cementite filaments showed better resistance to surface originated pitting compared with nitrided layers with coarse precipitates, for similar mechanical properties in terms of hardness and residual stresses. Moreover, examinations carried out on cross sections throughout crack networks revealed the influence of these precipitates on the propagation of rolling contact fatigue cracks, which is at the origin of the differences in performance between thin and coarse microstructures.

Low Temperature Solubility Limit of Copper in Iron

The solubility limit of copper in iron at temperature lower than 700°C is not precisely known because copper diffusion is too slow to reach an equilibrium with classical experimental techniques involving long range diffusion. However, fine precipitation of copper can lead to an equilibrium in a reasonable ageing time. Hence, coupling ThermoElectric Power and Small Angle X-ray Scattering techniques leads to a precise estimation of this solubility limit in the temperature range 500°C-700°C. Values obtained are confirmed by Tomographic Atom Probe and give results much higher than what is usually extrapolated from high temperature experiments.

Embrittlement of a High Manganese TWIP Steel in the Presence of Liquid Zinc

In the past decade, new steels have been developed for the automotive industry in the framework of environmental requirements. Among them, high manganese austenitic steels combining exceptional properties of strength and ductility are particularly promising. These exceptional properties stem from a fully austenitic structure at room temperature and a twinning deformation mode in addition to the classical mechanism of dislocation gliding, known as the TWinning Induced Plasticity (TWIP) effect. In this study, the cracking resistance of the Fe22Mn0.6C TWIP steel was investigated in relation to the liquid metal embrittlement (LME) phenomenon. Indeed, liquid zinc has been found to have an embrittling effect on such steels. Electro-galvanized specimens were subjected to hot tensile tests using Gleeble® thermo-mechanical simulator. The influence of different parameters such as temperature and strain rate on embrittlement was studied. The results show that this steel can be embrittled by liquid zinc within a limited range of temperature depending on strain rate. A critical stress for cracking has been defined for each embrittlement condition.

Rolling contact fatigue crack propagation in nitrided alloyed steels

Experimental investigations were carried out to better understand the rolling contact fatigue mechanisms in nitrided layers of the 33CrMoV12-9 steel grade. Surface-initiated pitting failure mode was reproduced on a twin-disc machine to analyse crack growth and compressive residual stress behaviour within the nitrided layers. Metallographic examinations, 3D observations by means of high-resolution X-ray computed tomography and residual stress analysis were realised on nitrided 33CrMoV12-9 specimens before and after rolling contact fatigue tests. The study revealed that if the initial compressive residual stresses associated with the surface treatment are released during the process of rolling contact fatigue, pre-existing superficial cracks propagate in the nitrided layers along the intergranular carbides. These precipitates induced by the nitriding process therefore act as preferential crack propagation sites.