Studies On Fretting Wear Behaviors Of Axle Steels Under Varied Modes

Up to now, the studies on fretting wear mostly focused on tangential fretting mode and the contact configuration of counter-pairs mainly was ball-on-flat. However, the actual fretting damage phenomena usually include fretting modes (radial, rotational and torsional) besides the traditional tangential mode. In addition, the rotary fretting damages (torsional and rotational fretting) are often not to occur under the condition of ball-on-flat contact configurations, whose fretting running and damage behaviors are still unclear. At the present time, the fretting wear behaviors of the axle steels of train are understood insufficiently yet. Therefore, the requirement of improved the basic theories of fretting wear in depth and the need for actual engineering applications, to carry out the studies on fretting running and damage behaviors under different fretting modes and contact configurations is very urgent and necessary.In this paper, for the counter-pairs of axle steels (LZ50 and 35CrMo) against 52100 (GCrl5) steel balls under different normal loads, (angular) displacement amplitudes and the number of cycles, the tangential, torsional and rotational fretting wear tests have been carried out under the ball-on-flat contact configuration, and the rotational fretting wear tests were carried out under the ball-on-concave contact configuration. Dynamic analyses in combination with the microscopic examinations through the means of micro-hardmeter, optical microscopy (OM), scanning electron microscopy (SEM), profilometer and energy dispersive spectroscopy (EDX), the fretting running behaviors and damage mechanisms of the axle steels have been systematically investigated under the varied fretting modes and contact configurations. The obtained main conclusions are as follows:1. Fretting wear characteristics under varied fretting modes(a) For the all fretting modes (tangential, torsional and rotational fretting) in this research, their kinetic curves (F\-D curves, T-θcurves and Ft-θcurves, respectively) only presented three shapes of linear, elliptical and parallelogram. The linear and elliptic curves corresponded to the partial slip of fretting, while the parallelogram curves corresponded to the gross slip condition. For the both tangential and torsional fretting modes, there were only three regimes, i.e. partial slip regime (PSR), mixed fretting regime (MFR) and slip regime (SR). However, for the rotational fretting mode, because the fretting running regimes were strongly dependent upon the test materials, the MFR of the axle steels cannot be observed.(b) The normal load and (angular) displacement amplitudes presented significant influences on the friction dynamic behaviors of materials under different fretting modes. When the normal load was constant and the (angular) displacement amplitude increased, the fretting running state changed from the partial slip to gross slip accompanied with the increment of the damage; for the constant (angular) displacement amplitude, a reverse result was obtained with the increase of the normal load. The fretting running behaviors and damage mechanisms can be revealed by the fretting maps for different materials under varied modes. However, the fretting maps for different modes were quite different, it is maybe the reason that the deformation behaviors of varied modes are quite different.(c) Under different fretting modes, the damages in the PSR were slight, and the wear mechanisms of the axle steels were the combination of oxidative and abrasive wear; in the MFR and SR, the wear mechanisms all were abrasive wear, oxidation wear and delamination.(d) The values of the friction'coefficient, the friction dissipated energy and the friction torque were different with the changing of material properties under the same fretting mode. The LZ50 steel presented lower hardness, lower strength, and higher plastic deformation capacity, so it presented higher friction coefficients, the friction dissipated energies and the friction torques than that of the 35CrMo steel.2. Rotational fretting wear characteristics under varied contact configurations(a) The normal load and angular displacement amplitude presented significant affects on the rotational fretting running behavior under both two contact configurations. Under the conditions of the same normal load and the same maximum Hertz contact stresses, the boundary of SR moved to the direction of lower angular displacement amplitudes with the contact configuration changed from ball-on-concave to ball-on-flat.(b) The changing of contact configurations modified the sizes of contact zones and contact stress distributions, and affected on the debris running behaviors on the same time. So, the contact configuration had significantly affected on the evolution of the contact stiffness, friction coefficients and friction dissipated energies and the morphologies of wear scar and the wear degree.(c) When the normal load was identical, to compare with the ball-on-flat contact, the coefficient of friction and friction dissipated energy at steady stage of ball-on-concave contact were smaller, and the wear of materials was also slighter. When the maximum Hertz contact stress was same, to compare with the ball-on-flat contact, the friction dissipated energy of ball-on-concave contact in the stable stage was higher, which corresponded to the severer damage.