Study On Phase Transformation And Fatigue Properties Of Martensite White Etching Layer Of Railway Rail

Rail squat,which refers to the failure of the rail surface cracks and collapse,is a very common rolling contact fatigue(RCF)defect of railway rail.Squats often occur on rail surface in accompany with“White Etching Layers(WELs)”,extensive studies have revealed that the formation of WELs will promote the occurrence of rail squats.Compared with rail matrix,the WELs have distinct microstructural characteristics,and its formation mechanism is by now still controversial.Meanwhile,the influence of WELs on the RCF properties of rail is still rarely reported.Based on the thermally induced phase transformation mechanism of WELs,the present thesis studies the influence of thermal-mechanical coupled effects on the temperature changes of rail surface during wheel/rail contact,and the phase transformation process of WELs by conducting microstructural analysis,dilatation test and thermo-simulation compression test of rail surface materials from ex-service railway rails,as well as the finite element modelling.Furthermore,the influences of the existence of martensite WELs on the RCF properties of rail surface,and the crack propagation behaviors around WEL regions are mainly investigated.The main content and conclusions are as follows:1.Microstructural analysis on ex-service rail sample from practical operational track line is conducted,optical microscopy is used to observe the morphologies of WELs around rail squats.The microstructural characteristics of WELs,crack distributions and morphologies are analysed,the microhardness of WELs and rail matrix are also measured.Phase transformation and thermal-mechanical parameters of pearlitic rail material are obtained by dilatation test and thermo-simulation compression test,respectively.Results show that the WELs occurred in the form of martensite structures,the shape of the WEL is circular,and its maximum depth is around 20μm.Obvious cracks can be found in leading and trailing edges as well as the middle positions of WELs.Cracks in leading and trailing edges of the WEL propagate along the interface of WEL and rail matrix,while the middle cracks propagate vertically through rail matrix boundary.The starting temperature of pearlite rail material austenitization is719℃,and the temperature of martensite transformation is 255℃after fast cooling.The thermal expansion coefficient of austenite is the highest,while martensite has relatively lower thermal expansion coefficient than pearlite.The yield strength of rail material decreases with the increase of temperature.2.Based on the microstructural observation results of the WEL,an advanced FE modelling method is proposed with the application of a temperature-dependent rail material model,which comprehensively considering the temeperature-induced thermal stress and thermal softening.The wheel/rail contact pressure and temperature distributions are equivalent as a coupled moving pressure patch and heat source.Taking into account of a certain locomotive configuration,the study investigates the changes of temperature and stress-strain on rail surface due to different slip ratios during multiple wheel passages on rail surface.The influence of temepratrure rise on the austenitization of rail surface materials are evaluated,and the fatigue life and wear rate of the rail are calculated.Results show that the flash temperature of rail surface increases with the increase of slip ratios,meanwhile the heat affected depth becomes larger.When the slip ratio is 9.43%,the flash temperature of rail surface reaches to 776.05℃after the ninth wheel passage of the coupled locomotives,which is sufficient to austenitize rail materials.When the slip ratio is larger than 2.38%,temperature rise will lead to materil softening,hence the stress and strain are significantly increased.The RCF life of rail decreases with the increase of slip ratios,when the slip ratio is 9.43%,the rail life is1.07×10~6 cycles.But when the slip ratio is less than 2.38%,the rail life is larger than 5×10~6cycles.A larger slip ratio will result in higher temperature rise,and then severer wear.According to different temperature rise,the rail surface wear is categorized as:mild wear,wear transition and severe wear.Comparisons between FE modelling results and the experimental observations verify the reasonbale accuracy of the proposed FE model.3.A systematic rail material model is developed,which is dependent on the phase transformation parameters from pearlite via austenite to martensite by experimental test.Multiple wheel passages are conducted to analyse the temperature and phase evolutions of rail surface.In addition,the volume fraction and depth changes of the martensite WEL are calculated according to different wheel passages.Results show that when the accumulated rail surface temperature exceeds the austenitization limit,the fast cooling rate leads to a complete phase transformation from austenite to martensite.Due to the extremely short wheel/rail contact time,a single wheel passage is only able to make very little pearlite to transform via austenite to martensite.After the first train passes(12 wheels in total),the volume fraction of martensite generated on rail surface is1.89%.Then the structure and thermal-mechanical properties of rail surface materials are altered accordingly.During the subsequent train passages,in addition to heating martensite transformation into austenite,more heat will be absorbed by pearlite rail matrix,resulting in the increase of austenite volume fraction,and the martensite volume fraction increases as well.After the third train passes,the following train passages lead to a stable increase of martensite volume fraction,which is about 2%.After 10 passages of the train,the volume fraction of martensite increases to 19.78%.4.The longitudinal cross-section(parallel to wheel’s rolling direction)is selected,and a 2D plane strain wheel/rail rolling contact FE model with the existence of WEL on rail surface has been developed to analyse the influence of WEL on the stress-strain and residual stress-strain distributions of rail materials.The study also systematically analyse the influences of different strength grades rail steel(standard carbon and head hardened),friction coefficient,loading pressure,slip ratio and WEL geometries on the RCF properties of the WEL.Results show that when the wheel rolling from rail matrix surface to WEL surface,the contact stress increases significantly and it will also enlarges with higher rail steel strength.The increase of friction coefficient and loading pressure will lead to a larger effective stress variation between rail matrix and WEL.Compared with rail matrix surface and WEL/rail matrix interface areas,fatigue life in WEL surface is much lower.The residual stress of rail surface materials is compressive in the longitudinal direction,where the maximum value reaches to around 600-700MPa,this is in consistent with results from experimental measurement.In the WEL area within 1mm depth,three distinct residual stress gradient regions can be found.Region-I has the largest residual stress with very high dislocation density and fine grains.The residual stress decreases gradually in Region-II where a mixed microstructures can be seen.In region-III,there is a pearlite lamellar area,the changes of residual stress are inobvious.The results are well in line with the microstructural observations from material’s metallurgical test.There are greater influences of loading pressure,friction coefficient and WEL geometries on the stress distributions of rail matrix surface and the leading edge of the WEL.In contrast,the influence of slip ratio is smaller.A critical WEL geometry of the length-depth ratio of 5 is determined.The fatigue life in different positions around WEL increase with the length-depth ratio,but it shows a slight decrease when the length-depth ratio is larger than 5.5.Based on the experimental observation of WEL crack morphologies,the initial cracks with different lengths and inclinations are pre-fabricated in the leading edge,middle position and trailing edge of the WEL in the 2D model.The crack propagation properties in different positons of WEL during wheel/rail rolling contact are evaluated by stress intensity factor and J-integral criterion.The influences of crack length,oblique angle,loading pressure and friction coefficient on WEL cracking properties are investigated.Results show that crack in leading edge of WEL is the most likely to grow,while crack in the trailing edge is the least to propagate.With the increase of crack length,the maximum of J-integral decreases in each position of the WEL.For crack in middle position,when it extends to rail matrix interface,the J-integral significantly increases due to the discontinuity of materials.Crack in leading edge of WEL propagates along the interface between WEL and rail matrix,crack in the middle position propagates with a very slight inclination after it crosses the rail matrix boundary.Crack in trailing edge of WEL has two propagation possibilities,one is along the WEL/rail matrix interface,and the other is growing towards rail matrix but along the plastic flows.In the propagation behaviors of WEL cracks,the shear model plays the dominant role.The loading pressure and wheel-rail friction coefficient have a great influence on the crack propagation properties in the WEL.The present thesis includes 76 figures,15 tables and 155 references.