1 Introduction
Fatigue failure in bearings involves in the initiation and propagation of damage originating from contact surface or subsurface.1,2 Although surface damage, such as wear, can be relieved by careful surface treatment and effective lubrication, subsurface failure caused by alternating shear stress is inevitable under the theoretical framework of Palmgren and Lundberg.3 This mode of fatigue failure is Rolling Contact Fatigue (RCF). RCF refers to a localized cumulative damage that occurs between two tangent and cyclically loaded parts.4,5 The raceways and rolling elements of a normal rolling bearing are generally subjected to billions of rolling contacts during service. With the increase of service time, cumulative damage gradually develops at a depth of 200-400 μm under contact surface,6 eventually leading to the failure of bearings. The initiation of damage prefers to appear when the region corresponding to the peak of alternating shear stress coincides with the defect region in materials, and further results in fatigue failure such as macroscopic spalling.7,8
Under the assumption of macroscopic homogeneity, bearing steels will not suffer from plastic deformation and damage during nominal elastic loading. However, the presence of phase interfaces and carbide inclusions introduces heterogeneity at microstructure level. Local yielding and micro-plastic strain accumulation will occur within bearing steels under RCF. And such micro-plastic flow is often referred to as ‘ratcheting’ phenomenon.2,9,10 The damage evolution of RCF is generally divided into three stages. In stage I, micro-plastic strain accumulation tends to occur near the inclusions, which indicates the starting point of damage initiation.11 Such damage evolution behavior is inconducive to the fatigue life of bearing steels.
The typical components of bearing steels consist of tempered martensite, homogeneously distributed primary spheroidized carbide, tempered carbide and retained austenite.12,13 Among them, martensite is a supersaturated solid solution formed by the solubilization of carbon in α-Fe matrix.14 In addition to that, sparsely proportional carbon atoms have little influence on plastic deformation. And the crystal structure of martensite is body-centered tetragonal,15 which is similar with the body-centered cubic structure of α-Fe, so martensite phase can be replaced by α-Fe phase to simplify the research. The carbides in bearing steels can be categorized into metallic and non-metallic, among which the main metallic elements of metallic carbides are Cr, Fe and Mo.16 Although the content of carbide can be reduced by fine processing and heat treatment, there still exists a certain proportion of Fe-rich M3C. The primary spheroidized carbide in AISI 52100 bearing steels is (Fe,Cr)3C.13Bhadeshia17 found that 3-4% of cementite cannot be dissolved in the process of austenization during the quenching of AISI 52100 bearing steels. Lian et al.18 found a small amount of Fe-rich M3C when studying the dissolution and precipitation of carbides in carburized M50NiL bearing steels. Due to the mismatch of two phases within materials, dislocations will be generated and emitted on the interface during deformation, thus causing plastic accumulation.19,20 Therefore, the mechanism of fatigue failure can be revealed to some extent by studying the influence of the interface between bcc-Fe and cementite on cyclic plastic accumulation in bearing steels.
Existing relevant atomic models mostly focus on the generation and evolution of defects at ferrite-cementite interface under monotonic loading. Ferrite is a kind of solid solution formed by carbon atoms occupying the interstitial space among the lattice atoms of α-Fe crystal, and the maximum solubility of carbon in α-Fe is only 0.022 wt% below 1000K.21 Therefore, ferrite phase is usually replaced by α-Fe phase in the existing models. Ghaffarian et al.22,23 have reported that the deformation mechanism of ferrite-cementite interface strongly depends on the size, temperature and different loading directions of cementite, and have revealed the hindrance mechanism of cementite layer on dislocation, as well as the influence of temperature and lamellar thickness on the ductility of the model. Guziewski et al.24 have noted that the mechanical properties of ferrite-cementite interface depend on the volume ratio and orientation relationships (ORs) between ferrite and cementite phases. More recently, Liang et al.20 have reported the nucleation and evolution mechanism of dislocation at the ferrite-cementite interface, and have explained the plastic mechanism of fatigue failure under cyclic tensile and compressive loadings in pearlite. However, few existing papers pay attention to the initiation and evolution of interfacial defects under cyclic alternating shear load. In addition to that, Pandkar et al.25 have revealed the accumulation of deformation and micro-plastic strain during RCF due to ratcheting behavior by using finite element method, and have explained the contribution of carbide particles towards ratcheting. Nevertheless, the macroscale studied is insufficient to explain the origin of cumulative damage. And due to the limitations of spatial-temporal resolution of existing experimental instruments and the complexity of atomic modeling of bearing steels, the mechanism of plastic accumulation near inclusion in bearing steels still remains unclear.
In the present work, we focus on the plastic accumulation induced by the interface between bcc-Fe and cementite under cyclic alternating shear stress and attempt to explain the cumulative damage mechanism of bearing steels under RCF at the microstructure level. Fig. 1 shows the framework of this paper. In Section 2 we first establish a finite element model to obtain the distribution and variation rule of subsurface shear stress under bearing contact load. Then a two-phase atomic model of bcc-Fe and cementite is built and pretreated. Monotonic shear loads are carried out to obtain the mechanical properties of the model. Three types of shear loads are designed for cyclic deformation. In Section 3 we firstly analyze the shear stress responses under different loading conditions. Then the periodicity of statistical features, including dislocation density and proportion of irregular structures within the model, under cyclic loadings are discussed, respectively. Afterwards, the evolutions of dislocations under monotonic and cyclic shear loads are explained in detail. The morphology change of cementite phase after ten loading cycles are presented ultimately. The current work has partly explained the mechanism of cumulative damage initiation in bearing steels under RCF at the microstructure level.