Study On Fatigue Performance Of EA4T Axle Steel With Surface Damage Defects

EA4T is a typical axle steel for electric multiple unit(EMU).Because of the aerodynamic effect,foreign bodies such as gravel and ballast are easily rolled up and impacted on the axle during the operation of the train,resulting in a certain degree of surface damage,resulting in a decline in the fatigue performance of the axle,leading to serious accidents.In view of the actual axle surface damage,different methods were used to prepare the damage defects on the axle steel sample surface,and the relationship between the defects and fatigue performance was explored.The purpose of this paper was to provide some reference for the formulation of the axle external damage assessment and maintenance standards of EMUs in China.The following conclusions was drawn:(1)The surface layer of EA4 T axle was tempered sorbite with a hardness of 218 HBW.After 30 mm radial direction,it entered the transition layer and began to appear ferrite.With the increase of depth,the hardness began to decrease.The hardness of the matrix was about 200 HBW.Conventional mechanical properties(strength,plasticity and impact toughness)met the requirements of EN13261-2009.(2)According to GB/T 24176-2009 standard,the fatigue limit of smooth specimen and notched specimen was 366.0 MPa and 196.7 MPa respectively.The stress concentration caused by notch was reflected in the S-N curve,which not only decreased the fatigue limit,but also increased the absolute slope of the fitting line in the finite life zone.The cracks of smooth specimens originated from the surface and were single crack source.The cracks of notched specimens originated from the root of the notch and were multi-crack source.(3)Among the impact defects of spherical projectile air gun,the defects produced in low velocity impact(100 m/s)presented regular spherical pits,and the fatigue limit decreased slightly.The main causes of fatigue crack initiation were micro-cracks at the edge of the defects and residual tensile stress.In high-speed impact(200 m/s~300 m/s),the defect morphology became crater-like,and some micro-damages(micro-cracks,micro-notches,local cracks)occured on the edge and surface of the defect.Under the action of alternating stress,the initiation and propagation speed of fatigue cracks would be accelerated,thus the overall fatigue limit would be significantly reduced.(4)The bottom of the static indentation defect was smooth,and there was uniform plastic deformation marks around it.Under the same defect depth,the fatigue limit of the static indentation defect specimen was not much different from that of the defect specimen impacted at 100 m/s speed.Therefore,the static indentation defect can approximately simulate the low-speed impact damage.The surface of EDM defect was rough and there was no plastic deformation around it.The fatigue limit was about 70 MPa lower than that of air gun impact defect with the same depth(impact velocity was 100 m/s~200 m/s).Many micro-holes caused fatigue cracks to sprout from the bottom and propagate together,resulting in a significant reduction of the overall fatigue limit,even lower than the static indentation defect specimens.(5)In the impact defect of cubic air gun,the sharp structure(angle and edge)of the missile body caused the radius of curvature at the bottom of the defect to decrease,the stress concentration was more obvious under the alternating stress,and the fatigue limit was reduced.At the same time,the accumulation of materials,micro-notches,micro-cracks,mosaic of external objects and lamellar structure at the bottom and edge of the defect led to rapid fatigue initiation of the specimen.Cracks propagate in different planes.Finally,the measured fatigue limit was about 60 MPa lower than that of spherical impact defect specimens with the same depth(impact velocity was 100m/s~300 m/s).In practice,the monitoring of such defects on axle surface should be strengthened.