Effect Of Welding Current On Low-Temperature Mechanical Properties Of Weldedjoints Of Q690 Steel And Its Mechanism

Q690 steel plays an important role in engineering architecture.This article studies the microstructure,hardness,tensile properties at room temperature,and low-temperature impact toughness of welded joints corresponding to three different processes.It explores the influence of welding current on the mechanical properties and fracture mechanism of Q690 steel welded joints,providing theoretical support for the safe service of Q690 steel welded joints.The research results show that the three welding processes have good welding joint formability,and the weld zone is composed of acicular ferrite and bainite.With the increase of welding current,the acicular ferrite increases and the amount of bainite decreases.The fusion zone represented by the 2 # process is composed of unmelted but heated coarse grains and newly crystallized as cast microstructure;The coarse grain Heat-affected zone is composed of self tempered martensite,martensite and bainite ferrite;The fine-grained Heat-affected zone is composed of fine bainite and local martensite,and the grains are equiaxed fine-grained.According to the hardness curve,the hardness of the weld and base metal of the three processes fluctuates slightly,and the hardness of the Heat-affected zone fluctuates greatly.With the increase of welding current,the hardness of the weld and Heat-affected zone of the three processes increases.According to the tensile data,as the welding current increases,the yield strength and tensile strength of the three welding processes increase,while the elongation after fracture decreases and the reduction of area decreases.The macroscopic morphology of the tensile fracture surface of the three welding processes is composed of fiber zone and shear lip.As the welding current increases,the width of the shear lip on the fracture surface increases,and the microscopic morphology of the fiber zone is composed of dense dimples.The microscopic morphology of the shear lip changes from small and deep dimples to large and shallow dimples with uneven distribution.According to the Boltzmann fitting formula,the ductile brittle transition temperatures of the three process welds are 0.46 ℃,-4.95 ℃,and-4.01 ℃,respectively.The ductile brittle transition temperatures of the three processes Heat-affected zone are-1.7 ℃,-5.3 ℃ and 29.1 ℃respectively,so the weld and Heat-affected zone have the best resistance to cold brittleness in process 2 #.As the temperature decreases,the macroscopic morphology of the three process welds gradually changes from the fiber zone and shear lip to the nearly completely radiative zone,and the microscopic morphology changes from being mainly composed of ductile dimples to being mainly composed of cleavage and tear ridges.For the Heat-affected zone,the change trend of 1 # process and 2 # process is similar to that of the weld.When the temperature of the Heat-affected zone of 3 # process is 60 ℃,there is a radiation zone on the fracture surface.With the temperature decreasing,the proportion of the radiation zone increases.When the temperature reaches-60 ℃,the fracture surface is a near complete radiation zone,and the impact energy value is only 20.1J.Under different temperatures,the impact toughness of the weld and Heataffected zone of the three processes is the best under the 2 # process.If the 3 # process welding joint is applied to the project,there will be great potential safety hazards.