| The welded structure is the most common non-detachable connection method used in engineering structures,but the main feature of the welded structure is that the mechanical properties of the welded joint are unstable.The effect of welding thermal cycling deteriorates the mechanical properties of welded joints and reduces their service life.As the application of heterogeneous high-strength steel welded structures is more extensive and the service conditions are getting worse,how to choose a reasonable welding process and strength matching method to obtain welded joints with superior mechanical properties,increasing the structural integrity and service life of welded components is an important factor to consider when designing welded joints.Therefore,this paper selects the ER50-6 welding wire matched with Q390 and the ER76-1 welding wire matched with Q690 and so on to weld Q390 and Q690 respectively.This paper uses the combination of experiment and finite element simulation to study the influence of different welding wires on the comprehensive mechanical properties of welded joints.The main research results of the thesis are as follows:(1)Two different strength matching welded joints were prepared by CO2 gas shielded welding.The results of metallographic experiments,composition testing and microhardness experiments indicate:The heat affected zone on the Q690 side is dominated by coarse bainite,and the M-A component along the original austenite grain boundary appears,while the Q390 side is mainly fine ferrite and pearlite.The weld area of ER50-6 welded joint is mainly polygonal ferrite,while the weld area of ER76-1 welded joint is mainly acicular ferrite.The diffusion distance of three elements of Cr,Mo and Ni in ER50-6 welded joint is 30um,and the diffusion distance of Ni element in ER76-1 welded joint is 35um.The microhardness change trend of ER50-6 welded joint is as follows:Q690 base metal>Q690 side heat affected zone>Q390 side heat affected zone>Q390 base metal>weld metal;HAZ on Q390 side produces local hardening.The microhardness change trend of ER76-1 welded joints is as follows:Q690 base metal>weld metal>Q690 side heat affected zone>Q390 base metal>Q390 side heat affected zone;softening zone on both sides appears softening,Due to the presence of acicular ferrite in the weld zone,the hardness is higher than that of the weld zone of the ER50-6 welded joint.(2)The strength of the R76-1 welded joint is higher than that of the ER50-6 welded joint because the ER76-1 welded joint has a high strength acicular ferrite.The impact toughness of the welded joints of the two welded joints at the same temperature is not much different,probably because the polygonal ferrite and acicular ferrite have little effect on the impact toughness.As the impact test temperature decreases,the fracture mode of the impact specimen changes from ductile fracture to brittle fracture.The apparent fracture toughness value KQ of ER76-1 welded joint is greater than the KQ value of ER50-6 welded joint,which indicates that the apparent fracture toughness of ER76-1 welded joint is better.(3)The GTN damage model of the welded joint was established.The mechanical properties and damage parameters of the GTN damage model were determined by experimental data,and the correctness of the model was verified.The stress distribution and the variation of the plastic zone at different moments of the two welded joints are compared and analyzed.The plastic zone of CT specimens under four thicknesses was analyzed.The maximum size of the plastic zone decreased with the increase of the thickness of the specimen.When the maximum dimension of the plastic zone no longer changed,the effective value of KIC was obtained.To obtain an effective KIC for two strength-matched welded joints,the CT specimen thickness should be at least 75 mm. |
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