Research On Influence Of Surface Pit Defect On The Fatigue Property Of High-speed Train Axle Steel

Safety of electric multiple unit (EMU) is the most important factor concerned during its operation and it depends strongly on the fatigue life of their axles. The present research mainly focused on the influence factors of the surface pit defects, caused by high speed foreign matter, on the fatigue life of surface induction hardened S38C high-speed train axle in order to provide not only the evaluation law or references between the axle fatigue life and their surface pit defects but proposals for surface repairing of these defects.The permitted lifetime of EMU axle is about 30 years and the corresponding running cycle is around 2×109. Generally, the key factors leading to the axle fracture failure are local stress corrosion, metallurgical defect and inclusion, and fretting damage on mounting surface.With the increasing passenger transportation application of EUM and the developments of corresponding maintenance faculties of EUM, it is found that surface pit defect by foreign matter on axle surface is the current failure defect. However, there are no detailed references on these aspects yet. To evaluate the influence of surface pit defect on the fatigue failure of the actual axle service in detail,the following studies have been carried out.Firstly, the surface pit defects on the surface of axle were classified according to their geometrical morphology. Chemical composition, metallography and hardness of the surface layer and matrix of S38C axle were characterized by direct-reading spectrometer, optical microscopy, transmission electron microscopy and hardness tester. Following experiment design was based on the above test results. Secondly, artificial surface pit defects were introduced on the standard S38C fatigue specimen, the electric discharging machining (EDM)pit defect and the artificial indentation formed by hardness tester. Influence of two kinds of defects on the fatigue property was studied by applying fatigue testing machine, harness tester, optical microscopy, transmission electron microscopy, scanning electron microscopy,EDS, and XRD. Further, fatigue property of four point bending specimen from S38C surface with tempered martensite layer was investigated and the influence of two kinds of defects was studied. Meanwhile, the fatigue crack threshold and fatigue crack propagation rate were tested by special design method. At last, laser cladding repairing of electron discharge machining notch and artificial indentation on axle surface was explored.Conclusions are as follows:1. The pit defects on S38C axle surface includes groove caused by perpendicular impact and scratch caused by tangential impact. Microstructure of hardened layer on surface of S3 8C axle is tempered martenite with high density dislocation and lattice distortion,and that of matrix is pure pearlite and ferrite.2. Fatigue limit of both kinds of S38C axle steel specimen with electron discharge machining notch or artificial indentation declines with the increasing of projected area of defect. Fatigue limit of former specimen is almost the same as the value predicted by Murakami's Equation and that of later specimen can be predicted by modified Murakami's Equation by introducing a strengthening constant.3. Fatigue limit of S38C axle steel specimen with indentation introduced before or after induction hardening declines with the increasing of projected area of defect. However,the fatigue strength of the later drops by a larger scale.4. Fatigue limit of smooth nitrocarburized specimen is 12% more than that without heat treatment. Fatigue crack originates from porous oxide layer on the surface. Indentation leads to pre-crack of compound layer and fatigue limit of nitrocarburized specimen declines rapidly with increasing of indentation size, even lower than that of untreated specimen with similar-sized indentation. Fatigue limit of nitrocarburized specimen could be improved by polishing out compound layer.5. Four point bending fatigue limit decreases with the increasing of depth of notch.Indentation poses no influence to fatigue limit. The tungsten steel ball is crashed,leading to secondary indentation around with sharp corner and pre-crack, resulting in decline of fatigue limit. The fatigue crack propagation threshold of hardened layer of axle is 1.43 MPa?m,much lower than that of matrix 5.66 MPa?m.6. The new defect caused during laser cladding repairing leads to discrepancy of fatigue limit of repaired specimen.In the light of above work, material around the artificial indentation was hardened with plenty of dislocation and fatigue limit of S38C axle steel specimen can be predicted by modified Murakami's Equation by introducing a strengthen constant. Nano crystal formed under multi-direction shear deformation during the formation of artificial indentation.Nanocrystallization leads to deterioration of plasticity and cracks formed when the surface get impact with multi-direction shear deformation. What is more, fatigue crack propagation threshold of surface layer of S38C axle is lower and failure is prone to occur.