| High-alloy ultra-high strength steel(UHSS)such as AF1410 is widely used in aircraft landing gear and other vital load-bearing components due to the excellent plastic toughness,good stress corrosion resistance,and welding performance.After the welding is completed,the UHSS components generally need to undergo post-weld heat treatment to produce martensite transformation and secondary hardening to meet the design requirements of structural parts.However,UHSS generates great residual stress in the welding and heat treatment processes,which affects the final shape and dimensional accuracy of structural workpieces.The workpiece structure is sometimes so complicated that the subsequent straightening and stress relieving methods are challenging.The carbon equivalent of high-alloy UHSS is very high.The thermal stress and the interal stress due to solid-state phase transformation are generated during the welding and heat treatment processes.However,the SSPT and the residual stress caused by SSPT are difficult to be measured experimentally,which leads to the fact that the influence of SSPT on stress and distortion is often ignored in the manufacturing field.Therefore,understanding the influence of SSPT on residual stress and distortion distribution in the whole manufacturing process is the basis for achieving precise manufacturing of UHSS components.The main research contents and results are as follows:(1)The "thermo-metallurgical-mechanical" coupled model is developed by using the Leblond model and the K-M equation based on Abaqus software,and the welding process of S355J2 steel is simulated by the coupled model.The results show that the phase transformation type in the welding process of S355J2 steel is mainly bainite transformation.Besides,the bainite transformation brings the compressive effect to the residual stress.The simulation results of microstructure and residual stress distribution are in good agreement with the measured results,which verified the accuracy of the developed finite element model.(2)The phase transformation law including CHT curve and CCT curves of AF1410 steel is studied.The thermophysical and mechanical properties of the single phase of AF1410 steel are systematically measured,and the martensitic transformation plasticity behavior of AF1410 steel is studied,thus establishing a complete AF1410 steel material property database.The SSPT model for AF1410 steel is developed and the accuracy of the model is verified by the experiment results.(3)The formation mechanism of residual stress during bead-on-plate welding,multi-layer multi-pass welding,and electron beam welding of AF1410 steel is studied by the coupled model.The results show that the residual stress value in the center of the weld seam decreases when the mechanical properties of supercooled austenite are considered.Besides,the transformation strain of bainite and martensite also brings compressive residual stress.The transformation plasticity caused by martensite transformation can significantly reduce the magnitude of longitudinal and transverse residual stress during welding.In addition,the welding thermal cycle also plays a critical role in the stress formation,which can redistribute the accumulated stress in the welding zone.SSPT has a significant influence on the prediction accuracy of welding residual stress and distortion.(4)The evolution of stress and distortion in water quenching,oil quenching,and vacuum gas quenching of U-shaped parts is studied.The results show that the distortion of the U-shaped sample quenched by water,oil,and vacuum gas is decreased sequentially but the three quenching processes can cause different directions for the Ushaped part.The influence of SSPT on stress and distortion is more significant at fast cooling rate,and the inhomogeneous phase transformation caused by the huge temperature difference completely changes the distortion trend of U-shaped parts.In addition,choosing a low quenching holding temperature could reduce the distortion magnitude of the U-shaped sample.(5)The "thermo-metallurgical-mechanical" coupled finite element model is simulated the entire manufacturing process of the UHSS component.The evolution law of stress and distortion in "electron beam welding-vacuum gas quenching-tempering"processes is analyzed,and the influence of the position and magnitude for the applied load on the distortion of the UHSS component is studied.The simulation results of"electron beam welding-vacuum gas quenching" show that considering SSPT can improve the calculation accuracy and the vacuum gas quenching is the main reason for the severe distortion of the UHSS component.The tempering process has little effect on the distortion of the UHSS component.The distortion induced by the "electron beam welding-vacuum gas quenching" process of the UHSS component can be controlled by applying loads during the tempering process.The correcting distortion under different loads or positions can be predicted through the "thermo-metallurgical-mechanical"coupled finite element model,which provides the possibility to eliminate the original complex procedures in the manufacturing site. |
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