| The high–strength steel(HSS)thick plate with the higher strength,better toughness,corrosion resistance,crack resistance and weld-ability,has been widely employed for the large-scale offshore platform structure,such as jack-up drilling platform,semi-submersible lifting and dismantling platform.However,HSS thick plates are usually butt-welded together by multi-pass welding process.The multi-thermal cycles make it more difficult to accurately estimate the hardness property of the butt-welded joint because of the complicated microstructure evolution process,which also make it difficult to research the welding residual stress in butt-welded joint.What’s more,welding residual stress seriously affects the fracture performance of welded joint,but the effect mechanism is still unclear.In this paper,experimental testing and Finite Element Analysis(FEA)were combined to investigate the microstructure evolution,welding residual stress and fracture performance of thick-plate butt-welded joint.The 75 mm-thickness Q690 HSS used for offshore platform was taken as research object.The welding procedure specification was formulated beforehand,then a multi-pass butt-welded joint was fabricated using Shielded Arc Metal Welding and welding transient temperature was measured.The microstructure,hardness and longitudinal residual stress in mid-section butt-welded joint was measured,and tensile performance of base metal,weld metal and butt-welded joint sample was tested.Microstructure in weld metal was mainly granular Bainite with a small amount of Ferrite and Martensite,while microstructure in heataffected zone(HAZ)was mainly lath Martensite,thereby making the hardness value at HAZ higher than that at weld metal.Due to the complex solid-state phase transformation(SSPT),peak value of longitudinal residual tensile stress in weld metal and HAZ was lower than the yield strength of Q690 HSS.Longitudinal residual tensile stress at mid-section base metal was attributed to the initial stress induced by rolling.Transverse residual stress in uniaxial tension sample of butt-welded joint can be neglected,and the fracture occurred at base metal.Based on the irreversibility of Martensite and Bainite transformation,the microstructure evolution model and hardness algorithm was modified.The ThermalMetallurgical FEA was proposed,and used to analyze the welding temperature field,microstructure distribution and hardness value of thick-plate butt-welded joint of Q690 HSS.The microstructure evolution mechanism was revealed.Volume fraction of microstructure in HAZ and weld metal agreed well with the measurement,and the maximum deviation between predicted hardness and measured data was 6%.It followed that the modified microstructure evolution model and hardness algorithm were reliable.The welding Metallurgical-Mechanical FEA with considering the effect of SSPT and initial stress was developed,and employed to research the welding residual stress in thickplate butt-welded joint of Q690 HSS.The effect of SSPT and initial stress on welding residual stress was analyzed.Moreover,the evolution of welding residual stress was analyzed.Predicted result of longitudinal residual stress in mid-section butt-welded joint had a good agreement with the measurement,verifying the accuracy of the predicted method.SSPT and initial stress significantly changed the distribution and magnitude of welding residual stress in thick-plate butt-welded joint of Q690 HSS,and peak value of tensile stress can be found at the junction of HAZ and base metal.The GTN damage model parameters for thick-plate butt-welded joint of Q690 HSS were identified by experimental data and ABAQUS/Explicit FEA software.With the application of transverse residual stress as predefined stress,transverse tension behavior of thick-plate butt-welded joint of Q690 HSS was simulated.The influence of transverse residual stress and welding defects on fracture performance was analyzed.The effect mechanism of transverse residual stress on fracture performance was revealed.It can be found that the larger transverse residual tensile stress in weld metal can accelerate the void growth and aggregation,making the fracture performance significantly decrease.Research on the microstructure,hardness and welding residual stress in multi-pass butt-welded joint of Q690 HSS,and deeply exploring the effect mechanism of transverse residual stress on fracture performance will provide scientific theoretical basis for mechanical performance and safety evaluation of HSS thick-plate welded joints of offshore platforms. |
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