| With the development of advanced manufacturing,the requirements for the lightweight of steel structure and actual service safety requirements are constantly upgrading.The high grade high-strength low alloy(HSLA)structural steels,such as Q420,Q460 and Q550,have been widely applied to engineering components.Fusion welding is an important way to connect the modern steel structure,and the welding quality directly affects the mechanical properties and actual service safety of HSLA steels.At present,arc welding,submerged arc welding,gas shielded welding,electroslag welding and other welding methods are widely applied in the connection process of HSLA steel structures in different fields for their respective advantages.However,welding leads to the local microstructure changing near the welded joints,which will directly affect the mechanical properties of the welded components.Furthermore,the improving strength needs the contribution of bainite or martensite phases for the high strength level HSLA steels.The tempering softening phenomenon will occur in the heat-affected zone(HAZ)of the welded joint when heated under the effect of welding thermal cycle.On the other hand,large welded structure requires the implementation of multi-pass welding in order to weld and join the base metal(BM).The microstructure heterogeneity of the fusion zone(FZ)will occur due to the complex process,resulting in formation of microcracks because of the factors such as cyclic thermal stress and slag residue,which will seriously affect the strength,toughness and service safety of the joints.In the present study,the various welding processes including submerged arc welding(SAW)and CO2 gas metal shielded arc welding(GMAW)with solid wire and flux-cored wire were selected for the fusion welding connection of the HSLA Q460 steel thick plates used for advanced buildings and traffic structure.The microstructure,hardness distribution,matching of strength and toughness under quasi-static and dynamic conditions of the welded joints of HSLA steel thick plates under different welding methods and welding processes were investigated.The mechanism of deformation and fracture behavior of the welded joints were discussed.The results showed that the HAZ width of the SAW joint of Q460 steel increased with the increasing of the heat input,the proportion of columnar zone in FZ was increased,and the micro structure in the fine grained region of FZ near the fusion line was obviously refined.With the increasing of heat input,the microhardness of FZ increased.It was found that there was no significant influence of SAW heat input on the tensile strength of Q460 steel joint,while the fracture elongation of the joint increased with the increasing of heat input.Meanwile,excellent strength and toughness of joint can be obtained.Under the condition of SAW heat input of 19.0 kJ/cm,the deformation of the interface between the FZ and HAZ was uncoordinated,which was the main reason why the fracture occurred early.The results showed that the primary dendrites were fine in the columnar crystal zone of FZ of the GMAW joint of Q460 steel with flux-cored wire under the same welding heat input.With the increasing heat input,the columnar crystal characteristics of FZ weakened gradually,the austenite grain refined,the proportion of acicular ferrite decreased and the Widmanstatten ferrite with large size increased significantly.The microhardness of FZ of the Q460 joint welded with solid wire was between those of the BM and HAZ.The microhardness of FZ of the Q460 joint welded with flux-cored wire decreased significantly with increasing heat input and the phenomenon of FZ softening occurred.The yield strength of the welded joint with solid wire was higher and the tensile strength was lower than that of the BM.The increasing welding heat input mainly increased the fracture elongation of the joint welded with solid wire.While with the increasing heat input,the yield strength and tensile strength decreased significantly of the joint welded with flux-cored wire,especially the tensile strength.The reaction and movement of the slag in the molten pool of CO2 GMAW Q460 steel thick plate with flux-cored wire was the main reason that influenced the processes of solidification of metal in the molten pool and the subsequent solid phase transformation,changed the microstructure and microhardness distribution,which resulted in the changing of plastic deformation and fracture mechanism under the tensile stress.The results showed that the CO2 GMAW of Q460 steel with flux-cored wire increased the ratio of equiaxed grain in the FZ,refined the prior austenite grains and decreased the width of HAZ of the joint compared with those of the weld with solid wire under the same heat input.The HAZ hardening phenomenon occurred in both welded joints of Q460 steel.The impact absorbed energy of both welded joints with different welding processes decreased with the decreasing of the temperature.In the range of RT to-40?,the Q460 welded joint with flux-cored wire showed higher toughness.The crack initiation and propagation behavior under the impact loading at RT was mainly controlled by the size and distribution of the acicular ferrite.With the decreasing of temperature,the difference of plastic deformation ability between the grain boundary allotriomorphic ferrite and acicular ferrite became significant.The tensile deformation behavior of Q460 steel and its SAW joints were sensitive to the strain rate.With increasing strain rate,both of yield and tensile strength of the welded joint increased,while the fracture elongation decreased first then increased.As the welding heat input increased,the strain rate sensitivity of tensile property increased.With increasing heat input,the proportion of acicular ferrite in the FZ of the joint increased,which is the main reason why the strain rate sensitivity of tensile property increased.The effects of heat input of CO2 GMAW with solid wire and flux-cored wire on the microstructure and dynamic fracture toughness(Jid)at different temperatures of Q460 steel joints were studied.The results showed that the joint exhibited the excellent dynamic fracture toughness under the condition of moderate heat input while showing the lowest value when the heat input was low within the temperature range of-70? to RT.When the temperature decreased from RT to-70?,the dynamic fracture mechanism of Q460 joint changed from ductile fracture to brittle cleavage fracture.Under the condition of low welding heat input,the allotriomorphic ferrite characterized by the planar growth at the columnar interface in the FZ of the joint led to the rapid intergranular crack propagation at low temperature.The fine acicular ferrite in the FZ of the joint obtained at moderate heat input which can hinder the crack propagation during the dynamic fracture at low temperature was the reason why the joint exhibited high dynamic fracture toughness. |
PDF Full Text Request