Research On Subsurface Damage Mechanism Of GCr15 Contact Fatigue

GCr15 bearing steel is a high carbon chromium bearing steel which is widely used in the manufacture of bearings,gears and other rolling elements.Normally,bearing in service subject to cyclic loading which often called rolling contact loading including radial compressive stress and friction along the rolling direction.The actual life of the bearing is often lower than the designed life in this complex condition.There are still some differences in its failure mechanism although the study of rolling contact fatigue performance of bearing steel has been conducted for a long time.The device are always running under high temperature,high speed and high load conditions.Therefore,contact fatigue has become a hot issue for many scholars again.This thesis simulate the working environment of bearing by experiment to study the worn morphology of the surface and subsurface microstructure damage mechanism of the specimens under different load and slide-to-roll ratio for tracing the origin of fatigue and prolong the fatigue life of the bearing providing fundamental theory.This thesis taking GCr15 bearing steel material as the research object,line contact rolling fatigue tests were conducted by using the MJP-20 rolling contact fatigue tester.The damage mechanism of subsurface under contact fatigue was studied by changing the experimental parameters such as loading and slide-to-roll ratio.Uniform microstructure was obtained under the same heat treatment condition,the microstructure and composition of the materials were analyzed before the experiment;the worn surface,cracks and microstructure of the surface were observed by optical microscope(OM)and scanning electron microscope(SEM)after the experiment,the transmission electron microscope(TEM)was used to observe the subsurface microstructure changes,the hardness of matrix near the white etching area was measured by nano-indentation finally.Through the analysis of the contact surface morphology,subsurface cracks and microstructure changes.We can conclude as follows:(1)There is a great differences in the surface worn morphology of GCr15 bearing steel specimen under different contact pressure and slide-to-roll ratio.The results showed that the loading and slide-to-roll ratio are the common factors that affect the abrasion of each specimen.The crack is the main cause that lead to surface spalling failure in bearings.Theunstability propagation of cracks will lead to a variety of failure modes,such as pitting,spalling and fracture(2)Rolling contact fatigue is a very complex process with the cycling variation of loading.The propagation of fatigue crack is a continuous or discontinuous progress under the influence of many factors,and the propagation direction changes with the change of working condition.The wear debris in the cracks is the result of the relative movement and repetitious friction of the crack surface.The twisted and rotated microstructures was observed in the cracks,so Hertzian contact is a complex multi-axial stress state.(3)WEA was formed under the combined action of contact pressure and slide-to-roll ratio,and WEA is produced at different contact pressures.No carbides were observed in the WEA and a distinct interface exists between the matrix and WEA.Elongated grains and massive dislocation clusters are present in the matrix next to the WEA.The degree of grain refinement is more serious as approaching the interface.The hardness of WEA has a significantly increase than matrix material due to grain refinement.(4)Rolling contact fatigue can induce microstructure alternation in both ways:nanocrystallization and amorphization.Accordingly,WEA can be categorized as deformed WEA and transformed WEA.The deformed WEA mainly consists of nanocrystallites,while the transformed WEA is a coexistence of amorphous phase and nanocrystallites.Both types exist depending on the degree of plastic deformation accumulation in RCF.