Effect Of Si On Low Temperature Toughness And Corrosion Resistance Of High Grade Weathering Bridge Steel Submerged Arc Welding Weld Metals And Process Optimization

Based on the design concepts of modern steel structure bridges with super large spans,safety and durability,green environmental protection,lightweight,and other high parameters,Cu-Cr-Ni uncoated 500 MPa weathering bridge steels have become the preferred materials for steel structure bridges under rural industrial atmospheric environments.However,the alloy system of 500 MPa weathering bridge steels is complex,and its supporting welding materials need to match their strength and corrosion resistance while also considering excellent low-temperature toughness.But in welds of weathering bridge steel,there is a contradiction between high strength and excellent weld metallurgical quality,high strength and high toughness,high toughness and weather resistance,resulting in the comprehensive performance of the existing welded joints of weathering bridge steel are poor,this severely limits the promotion and application of this grade of weathering bridge steel.In this paper,submerged arc welding（SAW）wires of 500 MPa weathering bridge steel were taken as the research object,based on the mechanism of alloy elements in the weld metals and metallurgical principles,a method was put forward to improve the comprehensive properties of weld metals by optimizing the Si content in welding wires,the influence of Si content variation on the metallurgical quality,impact properties,and corrosion properties of weld metal was revealed.The effects of Si content on the welded microstructure and mechanical properties were studied under a typical SAW heat input of 30 k J/cm.The amount of inclusions in the weld metal decreased significantly with an increase in Si content,but the size slightly increased.Acicular ferrite（AF）,granular bainitic ferrite（GBF）,polygonal ferrite（PF）,and M/A constituents were formed in the columnar zone（CZs）and equiaxed zones（EZs）of welds with varied Si contents.With the increase in Si content,the content of PF increased,and AF,GBF,and M/A constituents coarsened.The increase in the size of M/A constituents and inclusions led to an increase in local microstrain concentration and a decrease in critical fracture stress,resulting in a decrease in crack initiation energy;the increase of PF and the coarsening of AF result in a decrease in the density of high-angle grain boundaries（HAGBs）and thereby a decrease in crack propagation energy.Based on the consideration of improving the metallurgical quality of welds and ensuring joint toughness,the amount of Si added to the welds was optimized.Weld metals with different Si contents were selected to study their corrosion mechanisms under typical industrial atmospheric conditions.By analyzing the influence of Si element on the corrosion kinetics,phase structure,microstructure,and electrochemical properties of the rust layer,the corrosion resistance mechanism of Si element on weld metal was explored.The results indicated that the rust layer generated by weld metal with different Si contents under typical industrial atmospheric conditions was mainly composed ofα-Fe OOH,γ-Fe OOH and Fe₃O₄.As the Si content increased,the relative content ofγ-Fe OOH was decreased,while the relative content ofα-Fe OOH was increased,indicating that an increase in Si content could effectively promote the formation ofα-Fe OOH and enhance the weathering resistance of the rust layer.Weld metals with higher Si content promoted the formation of composite oxides of Si,simultaneously formed small nanoscaleα-Fe OOH and three-dimensional Si O₂ network structure were conducive to the repair of cracks and pores in weld metals,improving the resistance of the rust layer.The effects of welding heat inputs of 20-50 k J/cm on the welded microstructure and mechanical properties under SAW wire with Si content of 0.32 wt.%were studied.With the increase in heat inputs,the deposition amount of weld metal increased,the high-temperature residence time prolonged,and the proportion of PF in the CZs and EZs of the weld metals significantly increased;AF,GBF,and M/A constituents experienceed coarsening.With the increase in heat inputs,the coarsening of microstructures led to a decrease in theri mechanical properties.Based on the consideration of improving welding efficiency and ensuring excellent strength and toughness of joints,a suitable heat input ranges for SAW wire had been proposed.Aiming at the above submerged arc welding wire with optimized Si content and suitable welding processes,the factory application experiments of submerged arc welding wire were conducted;and the welding coating tests of submerged arc welding wires with commercial flux,crack tests of inclined Y-groove welding,typical butt weld,penetration fillet weld,slope fillet weld and T-shape fillet weld were carried out.Based on the results of weldability tests and mechanical property tests,the practical application of the submerged arc welding wires were evaluated and the technical conditions of the welding wires were proposed,which provided a reference for the real bridge application of the submerged arc welding wires.