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.