Research On Laminar Plasma Surface Strengthing And Fatigue Property Of High-speed Railway Wheel Material

Railway wheels are one of the most important components of railway trains,concerning safety and performance.With the rapid development of high-speed and heavy-haul railway transportation,wheels experience complex service conditions during the wheel-rail rolling contact process,subjected to random dynamic multiaxial loading of longitudinal,lateral and tangential forces and the coupling effect of multi-physicals field.Consequently,the wear and fatigue damage behavior of railway wheel under the effect of multiaxial loading become more and more complicated.Railway wheels are the typical consumable components.Rolling contact fatigue(RCF)and tread damage are the main factors resulting in wheel turning or replacement,which accounts for nearly 70% of the all wheel damage types,resulting in a serious reduction of wheel service life and an increase of the cost of railway operation and maintenance.Nowadays,laminar plasma(LP)surface treatment technology is widely used in the surface strengthening of mechanical components.Therefore,in this thesis,the surface treatment of high-speed wheel and rail materials were performed by LP with the advantages of high power,long jet and high precision,improving the properties of wear and RCF,as well as systematically investigating the multiaxial loading fatigue damage behavior of wheel material,so as to improve the service performance and prolong the service life,which has important engineering significance on the development of railway wheel.Firstly,the LP surface treatment process of high-speed wheel and rail materials was studied based on the orthogonal experimental method,and the influence of process parameters on microstructure and properties of heat affected zone(HAZ)was analyzed.The results shown that the maximum surface hardness is more than three times of the substrate when the wheel and rail materials were treated by LP surface quenching.Importantly,the scanning speed is the main factor affecting the surface treatment.The microstructure of HAZ transformed into a mixed structure composed of lath martensite,undissolved cementite and retained austenite.Secondly,high-speed railway wheel and rail materials were treated by LP scanning quenching,and the temperature evolution during the quenching process and contact stress and strain distribution were simulated by COMSOL,and the RCF property and damage behavior of wheel material were studied.The results shown that the RCF life of wheel material could be effectively improved due to the residual compressive stress produced by scanning quenching,which has a linear relationship with the depth of HAZ.The treated wheel surface tended to fail in thin layer detachment,while the untreated presented big and deep spalling pits.The martensite with high density dislocation in the HAZ effectively impeded the plastic deformation,delayed the RCF crack initiation and decreased the depth of fatigue crack growth.When the depth of HAZ was less than 1 mm,the fatigue crack tended to grow along parallel to surface,otherwise,it would grow along the shear stress extension direction,which was the same as that of the untreated.During the RCF test,the retained austenite in the HAZ could absorb some mechanical energy resulting in plastic strain accumulation,which induced some of that to transform into twin martensite.However,the surface of untreated wheel under the combination effect of normal and shear stresses,numerous dislocations slipped and gathered in the region with larger von Mises stress,which formed lots of dislocation cells and some transformed into a magnitude of sub-grains.Further,high-speed railway wheel and rail materials were treated by LP spot quenching,and the temperature evolution during the quenching process and contact stress and strain distribution were simulated by COMSOL,and the wear and RCF properties as well as the damage behavior of wheel material were studied.The results shown that the wear properties and RCF damage performance of wheel and rail materials could be effectively improved due to the residual compressive stress produced by spot quenching,as well as the effect of untreated regions surrounded and pinned by HAZs.Those results are better than that of scanning quenching.The friction coefficient,the number of surface wear cracks and the depth of plastic deformation were significantly reduced.Meanwhile,there was a non-monotonicity relationship between the total wear rate of wheel-rail system and the spot quenching time.The HAZ moved as a whole region with the plastic flow in both untreated region and substrate due to the perfect coordination mechanisms between them.During the wear test,plastic strain accumulation induced some retained austenite in the HAZ transformed into twin martensite.However,the untreated wheel produced numerous sub-grains in the region with larger von Mises stress.Moreover,the RCF life showed a small reduction,but the fatigue damage and spalling were remarkably improved,which mainly occurred in the untreated region.There was a larger decrease on the number of fatigue cracks,the depth of crack growth,the size and number of spalling pits.Furthermore,the other surface fatigue crack easily initiated on the interface between the HAZ and untreated region,then grew along the boundary,which induced some secondary cracks that grew along the shear stress extension direction towards to HAZ and substrate.Finally,the tension-bending multiaxial loading fatigue performance of high-speed railway wheel material was studied by a self-designed multiaxial fatigue fixture,and the multiaxial loading stress and strain distribution were analyzed by COMSOL,and the relationship between surface temperature evolution and fatigue damage was established.The results shown a reduction of fatigue life and an increase of fatigue limit under multiaxial loading,however,the corresponding loading force significantly decreased.The surface temperature evolution of fatigue specimen could be divided into three stages: Rapid Increase Stage(RIS),Stability Stage(SS)and Mutation Stage(MS),corresponding to rapid plastic work,micro-plastic deformation and crack initiation,and crack growth,respectively.In the crack growth region,there were numerous tearing edges tangled with secondary cracks,which represented local toughness and local brittleness fracture characteristics,respectively.During multiaxial loading fatigue,the inhomogeneous distributed normal stress failed to start enough slip systems.Meanwhile,the shear stress stepped up the inhomogeneous distribution of local stress and strain,causing ever greater local brittleness and inducing shear strain of microstructure,which generated numerous deformation twinning.However,for uniaxial fatigue,the larger normal stress started plenty of dislocation slip systems,causing that numerous dislocations multiplied and gathered at phase and grain boundaries,forming dislocation walls and cells,then generating sub-grains.