Embrittlement Mechanisms Of Thick-plate Weld Metal Under Restraint Welding And The Toughening Mechanisms Through Post Weld Heat Treatment

Due to the restriction of the thick-plate welded structure or the external constraint used for restricting the macroscopic deformation during the welding process,the safety and reliability of the welded structure will be endangered.In this study,the embrittlement mechanisms of thick-plate DH36 weld metal under restraint welding were analyzed from the microscopic points of view.Meanwhile,the post weld heat treatment(PWHT)was used to improve the toughness of the weld metal,and the toughening mechanisms were investigated.The toughness of weld metal were tested through the Crack Tip Opening Displacement(CTOD)test and the Charpy impact test,and the microhardness test and the nano-indentation test were used to measure the mechanical properties of microstructures in weld metal on as-welded(AW)and PWHT conditions.Then,the microstructure observations and the microstructure-based finite element simulation methods were adopted to analyze the influences of microstructures and the microstructure heterogeneity to the toughness of weld metal.The test results show that the fracture toughness of weld metal is poor in AW condition,as the CTOD values are all lower than 0.5 mm.While it is improved significantly after PWHT,and the CTOD values are all higher than 1.5 mm.With the decrease of temperature,the impact toughness of weld metal decreases rapidly on AW condition,while it decreases much slower after PWHT.The microstructures of weld metal are mainly composed of the proeutectoid ferrite(PF)and the acicular ferrite(AF).The mechanical properties of the AF are higher than the PF,which results in the microstructure heterogeneity in the weld metal.The microstructures of weld metal are highly heterogeneous for AW weld metal,but the extent of the microstructure heterogeneity decreases after PWHT.It is analyzed that the microstructures and the extent of microstructure heterogeneity will lead to the toughness differences of weld metal on AW and PWHT conditions.The weld metal on AW condition has high dislocation density(6.027E+14m^-2),and the dislocation entanglement was obvious,which was harmful to the toughness.However,the dislocation density of the weld metal decreased after PWHT(4.511E+14 m^-2),with the dominant dislocation morphology evolving from the dislocation tangles to the movable dislocation lines.And amounts of fine carbides precipitate and spheroidize after PWHT,which also improves the toughness of weld metal.In addition,the extent of the microstructure inhomogeneity in weld metal is relatively high in AW condition.When the microstructure is subjected to external macroscopic strain,the distributions of the stress and strain on AF and PF are heterogeneous,which destabilizes the matrix.So that it is easier to lead to the initiation and propagation of the cracks and thus decreasing the toughness of the weld metal.As the PWHT reduces the extent of microstructure heterogeneity,the distributions of stress and strain on AF and PF homogenized.So the initiation and propagation of cracks are inhibited,which improves the toughness of weld metal.