Study On Weld Toughening Mechanism Of CO₂ Gas Shielded Welding Wire For Q500qENH Steel By Ni And Weld Corrosion Property

CO₂ gas shielded solid wire has good crack resistance,technology strong adaptability,low cost advantage has extensive application in the field of weather resistance of steel bridge,However,due to the strong oxidation atmosphere during welding,Si,Mn and other alloying elements are easy to burn,resulting in a significant decrease in weather resistance and strength and toughness.This problem is more prominent in high-strength grade 500 MPa weathering steel welds.In order to ensure strength and weather resistance,the content of Si,Mn,Cr,Cu and other alloys in the welding wire increases,but this also makes the metallurgical reaction of the molten pool more complicated,the brittle and hard phase of the weld increases,the crack sensitivity increases,and the toughness decreases,resulting in welding.It is difficult to take into account the joint strength,weather resistance and low temperature toughness.For this reason,combined with the requirements of the engineering standards for the weld strength,low temperature toughness and weather resistance of Q500q ENH weathering steel,CO₂ gas shielded welding wires with different Ni contents were prepared in this paper from the perspective of alloying.Through deposition test,series heat input welding test and relative corrosion test of weld and base metal,the effects of Ni content and heat input on weld strength and toughness,and the corrosion difference between Ni content wire and base metal were systematically studied.The influence of weld microstructure and alloy composition differences on low temperature toughness and weather resistance was revealed by SEM,TEM,and electrochemical characterization.The influence and mechanism of Ni content on the microstructure of the weld were studied.The microstructure of the columnar crystal region of the weld with different Ni content included proeutectoid ferrite（PF）,acicular ferrite（AF）,bainitic ferrite（BF）and M/A,the weld bead heat affected zone includes PF,AF,BF,M/A and carbides.When the Ni content increases,the phase transition temperature A_r3 decreases,the microstructure and M/A of the columnar grain zone and the weld bead heat-affected zone are refined,and the proportion of high angle grain boundary increases.The effect and mechanism of the heat input process of 15-30 k J/cm on the microstructure of the weld were studied.With the increase of heat input,the microstructure of the columnar grain zone of the weld and the heat affected zone of the weld bead grow up,and the proportion of high angle grain boundary decreases.At the same time,the solidification time of the molten pool increases,and the inclusions are easy to float up and remove,so that the inclusion content in the weld decreases and the size becomes larger.The influence of Ni content and heat input process on the low temperature toughness of welds was studied,and the general law of the influence of alloying elements and heat input process on impact was revealed through the influence of microstructure on the crack initiation stage and crack propagation stage.Both phase M/A and large-sized inclusions can cause microcracks and reduce the crack initiation work.At the same time,the coarsening of the structure reduces the number of high angle grain boundary,which reduces the hindering effect on cracks and reduces the crack propagation work.The relative corrosion performance of the deposited metal and the base metal of the wire with optimized Ni content was studied.With the increase of the corrosion cycle,the rust layer of the weld and the base metal became dense,and the corrosion rates of both increased first and then decreased,and finally tended to be stable.Regularity,and the relative corrosion rate of the two is only 1.7%,at the same time,the electrochemical corrosion performance of the two is similar,and the weather resistance is well matched.