Research On Contact Surface Effect And Fatigue Behavior Of High Alloy Bearing Steel

With its combination of good hot hardness and excellent rolling contact fatigue strength,Cr4Mo4V VAM-VAR steel is widely used for critical aircraft engine bearing.Rolling element bearings normally fail by spalling fatigue,but in high performance aircraft gas turbine bearing significant hoop stress can be developed and these stresses can lead to fracture of the rings.The coarse carbides influence the development of fatigue and are detrimental to formability,which are remnants of the solidification process in.The purpose of this research project is to investigate the evolution of the mechanical properties of Cr4Mo4V bearing steel due to rolling contact fatigue.The global model simulates the ball-on-rod test under RCF loading conditions and provides Hertzian stress distribution in the contact region of the steel rod.The effect of immobilized carbides on the fatigue properties of Cr4Mo4V bearing steels was studied by rotating bending fatigue test.(1)The fatigue and wear characteristics of Cr4Mo4V steel under lubrication of 4050 lubricant were evaluated.Tests were conducted using a rolling contact fatigue(RCF)test rig of the ball-on-rod type with 0.18 slide-to-roll ratio.S-N data showes continuous gradual decline and large dispersion.Rolling contact wear of Cr4Mo4V steel involved mainly abrasive wear,adhesive wear and fatigue wear.Wear volume became larger when time and stress increased.The maximum depth of raceway groove reached up to 17μm.The rolling contact fatigue mechanism analysis of steel with different stress was investigated in detail by observing fatigue cracks of the junction of cross section and wear surface of rod specimens using optical microscopy and scanning electronic microscopy.It is found that there are two kinds of flaking,i.e.surface originated flaking(SOF)and white etching area originated flaking(WSF).The initial etching clearly reveals the RCF affected plastically deformed area at the edges of the test track of Cr4Mo4V steel rod material.With increasing contact cycles and contact stress,dark etching regions(DER),white etching area(WEA)and butterfly wings(BW)formed in the subsurface in sequence.Carbide peeling pits,adhesive wear and fatigue wear pits caused surface originated flaking.Carbides and inclusions in WEA and BW led to the generation of micro-cracks.Cracks extended deeper due to the presence of chain-carbide.(2)To account for large differences in the sizes of test rod,RCF affected zone,and carbide particle,a two-step finite element approach in the form of a ’global model-submodel’ is used.The plastic zone is developed slightly beneath the surface at a depth of about 50μm and it extends up to 150μm due to high level of peak contact stress(≈4 GPa)used in the accelerated ball-on-rod tests.The stress concentration induced by the hard carbide particles in the metal matrix plays an important role in local yielding of the material at the scale of the carbide microstructure.(3)The rotational bending fatigue properties of Cr4Mo4V bearing steels were tested at a stress ratio R =-1,a test frequency of 85 Hz and a room temperature.The safety fatigue limit of smooth cylindrical specimen of Cr4Mo4V steel is 1019MPa by lifting method.The safety fatigue limit of smooth funnel shape sample is 1153MPa.S-N data of Cr4Mo4V steel showed gradual decline and large dispersion.Fracture observation of smooth cylindrical specimen showed that there were five types of initiation,namely surface defects leading to initiation,near-surface carbides leading to initiation,near-surface non-metallic inclusions leading to initiation,internal non-metallic inclusions leading to initiation and internal carbides initiation.Fracture observation of smooth funnel shape specimen showed that there were two types of initiation,internal carbides initiation and hydrogen-induced fracture.Internal crack fractures have fish-eye features.A granular bright area(GBF)was observed in the vicinity around the internal initiation source carbides.The fatigue fracture was observed with broken carbides,and the broken carbide increased the crack growth rate.Carbides cracking could occur in the bearing steel Cr4Mo4V under the action of cyclic stresses.These fractured carbides attract the crack tip leading to faster growth rates.Influence of carbide size on the near surface initiation can be quantitatively analyzed by using the critical volume density of carbides.The influence of the size of the defect on the fatigue life was analyzed by the ratio of the nominal stress amplitude to the fatigue strength of the defect.