Investigation Of Essence Of Subsurface White Etching Area And Crack Growth Path Under Rolling Contact Fatigue

Bearing is one of the most important parts in transmission system.It is subjected to rolling contact fatigue loading under normal service condition.As high-speed train and large power wind turbine develop towards heavy load and high rotation speed,the loading and working conditions of bearings are becoming increasingly harsh and complicated.It leads to an earlier failure than expected based on the designed life.The bearing failure of rolling contact fatigue is a great challenge for scientific and engineering communities.One of the main failure modes in bearing failure is subsurface White Etching Area(WEA)accompanied by White Etching Crack(WEC).WEA consists of nanocrystallines.Although extensive and in-depth studies have been carried out by domestic and foreign scientists,a prominent discrepancy still exists in formation mechanism of WEA.The prevailing dominant mechanisms comprise hydrogen induced WEA and WEC at inclusions,two crack faces rubbing induced WEA,carbides dissolution due to large plastic deformation leading to WEA and low temperature dynamic recrystallization,etc.However,WEA presents typical features of strain localization: large sheared microstructure(including elongated carbides)and high localized deformation band.These are much like features of adiabatic shear band generated under high strain rate loading.However,the link between the WEA and SB has never been accepted by researchers due to the two main reasons:(1)Shear band(SB)has to be generated under adiabatic condition,which requires high strain rate loading in order to obtain the adiabatic condition.However,rolling contact fatigue is running under quasi-static loading condition;(2)SB has to be formed instantaneously while WEA is formed gradually under a large number of cycles of rolling contact fatigue.Based on the detailed analysis of the WEA,the current study proposes a new mechanism of WEA formation by generating SB on shear-compression specimens under quasi-static compression.The mechanism is: the shear band essence of WEA.Both WEA and SB are common responses of localized shear deformation.WEA is not a unique feature in rolling contact fatigue.The study contains the following four main contents:(1)Features and formation mechanism of subsurface WEA under rolling contact fatigueVarious morphologies of WEA were generated under rolling contact fatigue loading.WEA was categorized as deformed WEA and transformed WEA according to whether phase transformation occurred in microstructures or not.The deformed WEA presented features of elongated martensite and carbides.It was consisted of nanocrystallines of matensite,substructures as well as high density of dislocations.The transformed WEA showed highly localized and compact microstructures.It was composed of amorphous phase with fine nanocrystallines of 3-50 nm scattered.Phase transformation from bcc martensite to fcc austenite was observed with the retained austenite(RA)rising from 2%-3% to 20%.The results showed that the deformed WEA was formed under the mechanism of large plastic deformation induced grain refinement.The transformed WEA was governed by large plastic deformation,but it was still unclear that whether it was thermally affected,which remained as the biggest challenge in rolling contact fatigue.Both the deformed and transformed WEAs were governed by large plastic deformation mechanism.(2)Shear band formation under quasi-static shear compression loadingLarge patches of SB were generated in shear compression specimen under quasi compression tests.This indicated that SB did not necessarily have to be formed under high strain rate.High temperature softening was not essential for SB formation.SB showed typical white etching morphology.Considerable melting was observed in the fracture surface.SEM and TEM results showed that SB can also be categorized as the deformed SB and the transformed SB.The deformed SB presented elongated martensite and carbides,the same as the deformed WEA.It was consisted of consisted of nanocrystallines of ferrites,substructures as well as high density of dislocations.The microstructure of the transformed SB showed well-developed equiaxed grains with transformed austenite.The deformed SB was formed under the mechanism of large plastic deformation induced grain refinement.The transformed SB was generated due to dynamic recrystallization and phase transformation,which were initiated by high temperature rise at the SB fracture.(3)Shear band essence of WEAThe results from digital image correlation(DIC)showed that the formation of SB was a process where strain localization developed gradually.It went through uniform elastic deformation,plastic deformation localization,development of highly localized deformation and fracture.Although the macro strain in rolling contact fatigue was quite low,the local strain could reach up to 0.6.The detailed comparison between WEA and SB was carried out in terms of shear localization,the relationship with crack,microstructural features and formation mechanism.It displayed that the essence of WEA was SB,both of which were the results of highly localized strain under large shear plastic deformation.(4)Crack growth along retained austenite grain boundary in the subsurface of rolling contact fatigueIn order to rule out the mechanism of crack faces rubbing induced WEA,subsurface crack growth path was studied through etching the prior austenite grain boundary with the solution of picric acid.Surface-initiated crack propagated towards subsurface.Its crack path was significantly influenced by prior austenite grain boundaries.The EBSD results showed that the subsurface crack tended to propagate along prior austenite grain boundaries.Microstructures besides cracks showed apparent texture with the crystal orientation of [101] and [111] which were preferable lath for crack growth.The crack propagated along the grain boundary of the retained austenite for crack growth across the grain boundary.Besides,the retained austenite was uniformly distributed between crack surfaces or on one side of the crack surface.The plastic deformation of the contact surface induced the transformation from retained austenite to martensite.The retained austenite content of the deformed layer decreased from 30% to 3% of the matrix,while that of the matrix which was not affected by the deformation remained unchanged.