| In order to further enhance the effectiveness of the existing line passenger train operations,and improve transport efficiency,to meet the growing travel demand,the 160km/h EMU will replace the operation of general speed trains comprehensively.Weathering steel materials are now widely used in the design and production of 160 km/h EMU structures due to their high strength,good corrosion resistance,easy repair,and other characteristics.Laser fillet welding is a promising new technology with high energy density,small welding deformation,weld metallurgical defects easy to improve,etc.The welding of weathering steel sheets can solve the problem of conventional arc welding deformation,improve the working environment,and facilitate the realization of automated production.For the quality assurance and service life requirements of the welded joints of key structural parts of 160 km/h EMU,this thesis takes 2.5 mm thick Q310NQL2 and 3 mm thick Q345NQR2 weathering steel laser fillet welded T-joints as the object and conducts metallographic tests,hardness tests,fatigue tests,and fracture scans respectively,analyzes the microstructure,hardness and fatigue properties of the weathering steel laser fillet welded T-joints.The microstructure and hardness distribution of laser-welded T-joints of weathering steel were analyzed,the fatigue performance of each group of T-joints was studied,and the effects of different plate thicknesses,gaps,and processes on the fatigue limit were analyzed.The test results show that:Weathering steel laser welding T-joint weld is narrower,deeper,and wider,the joint is not fused,and other defects,the weld is well-formed.The weld zone of the welded joint has a coarser grain size and columnar crystal distribution.The organization has first eutectic ferrite,needle-like ferrite,a small amount of carbon-free bainite and pearlite;the grain near the weld in the heat affected zone is relatively coarse,and the grain in the recrystallization zone is more smallest and uniform,which is similar to the structure of the base material and is composed of ferrite and pearlite.Laser filler welding T-joint hardness distribution is not uniform,hardness curve showing saddle-shaped,from the weld side to the base material side of the hardness value of the heat-affected zone is the highest,the weld next,the base material is the smallest.The median fatigue limits of T-joints were 180 MPa,185 MPa,240 MPa,and 187.5 MPa for the four cases of the double-sided weld without penetration,double-sided weld with penetration,single-sided weld with penetration,and double-sided weld with penetration(0.2mm gap)for the specified life of 1×10~7 cycles,and 162.5 MPa,170 MPa,222.5 MPa,and167.5 MPa for the plate thickness combination of 3.0 mm+2.5 mm,respectively.The test data obtained the median S-N curves and the P-S-N curves with different survival rates by the least-squares method and the maximum likelihood method,respectively.The fatigue limit of the T-joint with a plate thickness combination of 3.0 mm+2.5 mm was the highest using the single-sided weld-through process,while the assembly gap had little effect on the fatigue limit value.The fatigue fracture of the T-joints with laser fillet welding of weathering steel is toe cracking,which is cumulative damage caused by cyclic stress load in the toe area with high-stress concentration.The fracture mechanism of each group of T-joints is relatively similar,and the microscopic morphology of the fatigue fracture is composed of the initiation zone,extension zone,and transient fracture zone.The cracks sprouted at the edges of the fracture,and obvious fatigue glow lines could be seen in the extension zone.Furthermore,some of the fractures had tearing ribs.The transient fracture zone showed tough nests of different sizes and unevenness,which indicated that the fracture mode of crack extension,to a certain extent,led to structural instability was a ductile fracture. |
PDF Full Text Request