Study On Quality Inspection Of A In-Service Rail Dispersedly Quenched By Laser Beam And Its Process Optimization

The laser dispersed quenching(LDQ)can significantly increase the service life of a rail.However,fatigue cracks are easy to initiate in the rail strengthened by laser during the service.The phenomenon of propagation and local spalling will occur for the cracks in a quench spot as the service time increases.In order to solve the problem of quantitative detection of micro-cracks on the rail surface and ensure the safe operation of the rail,a systematical investigation on the eddy current testing of the micro cracks on the rail surface were performed in this paper.The comparison relationship diagram of the signal amplitude and the comparison relationship diagram of the width of the time base duration wave are summarized and formulated under the conditions such as excitation frequency of 500 KHz,gain of 19 dB,scanning angle of 90° for probe and scanning speed of 1m/min.And the calculation formula was derived.The classification and quantization parameters of the crack can be obtained according to the two comparison relationship diagrams and the calculation formulas with the detection parameters specified above.The method solves the technical problem of quantitative detection of micro cracks on the surface of a rail.The test results of the in-service rails show that the cracks in the surface quench spot are small,dense cracks(1.288 mm)with the same depth.Metallographic section analysis shows that the crack depth is 1.188 mm.The test results are in good agreement with the actual conditions of the crack.The railway is allowed to have cracks below 3 mm.The above test results show that the cracks in the quench spot of the rail are within the safe range and do not affect the safe operation of the railway.The laser strengthening process with hardness of 30 HRC,40HRC,50 HRC and 60 HRC is designed respectively in the investigations so as to optimize the laser strengthening treatment process of the rail.The test results show that the quenched structure with a hardness of 50 HRC has a tensile strength of up to 1593 MPa and its elongation ratio is better than that of a quenched structure of 60 HRC.The stress distributions of quench spots and nearby substrates on different strengthened rails are analyzed by the ANSYS simulation software.The stress concentration near the quench spot increases with the increase of the hardness of the quench spot when the shaft weight is 20 t.The maximum tensile stresses near the quench spots are 324 MPa,327 MPa,340 MPa,and 346 MPa,respectively.The results of the rolling contact fatigue test show that the wear resistance of the strengthened rail increases with the increase of the hardness of the quench spot.After comprehensively analyzed the mechanical properties of the quenched structures and the stress state near the quench spot under each process,the results show that the strengthened rail with quench spot hardness of 50 HRC has the best comprehensive performance,which is more conducive to extending the service life of the rail.