Research On Microstructure And Properties Of Deposited Metals For 420MPa Grade Weathering Bridge Steel

In the context of"The Belt and Road"initiative,more than 3000 projects are contracted by China along the belt.Steel for infrastructure construction projects,such as construction and transportation,needs more excellent comprehensive properties.Ordinary steel exposed to the atmosphere is prone to corrosion,causing huge losses per year.Paint-free weathering bridge steel can be directly exposed to the atmosphere for hundreds of years,which greatly reduces maintenance costs and use costs.More and more steel materials will be used in stringent working conditions in the future.Therefore,it is of great economic and engineering significance to study weathering steel and welding materials with good comprehensive properties.Five kinds of manual arc welding electrodes for 420MPa weathering bridge steel with high strength,high toughness and good corrosion resistance were designed.The composition,microstructure,mechanical properties and corrosion resistance of deposited metals were analyzed.The effects of microstructure on the toughening and weathering properties were investigated.The results are as follows:The microstructures of deposited metals which were match with 420 MPa weathering bridge steel mainly consisted of acicular ferrite(AF),grain boundary ferrite(GBF),ferrite side plate(FSP),polygonal ferrite and martensite–austenite(M-A)constituents.The proeutectoid(including FSP)of columnar crystal region accounted for the least proportion when the contents of Si and Mn were respectively 0.2wt.%and 1.12wt.%.While that accounted for the largest proportion when the contents of Si and Mn were respectively0.32wt.%and 1.34wt.%.As the proeutectoid in the columnar crystal decreased from15.5%to 8.3%,the impact energy at-40?increased by 131J,the strength decreased by55MPa,and the plasticity changed little.M-A components of all deposited metals were fine,and no obvious chain M-A component observed,leading little harm to impact toughness.The size of more than 70%of the inclusions in the deposited metals was0.2?m~0.6?m,which effectively promoted the nucleation of acicular ferrite,improved the crack resistance of deposited metals.By electrochemical test,the self-corrosion potential of deposited metals was equivalent to that of 420MPa weathering bridge steel.The difference of self-corrosion potential between the two materials was less than 15mV.Suitable electrode was selected for a butt test with weathering bridge steel.The results showed that strength of welded joint was 617MPa,bending qualified and impact energys at-40?of weld metal and fusion zone were 160J and 180J respectively.A good match of the mechanical properties of the welded joint was achieved.Corrosion rate of welded joint was 4.593 g/(m~2·h)in the simulated Marine atmospheric corrosion environment,while the corrosion rate of base metal was 4.640g/(m~2·h).Relative corrosion rate between welded joint and base metal was 98.99%,which achieved the goal of corrosion rate of joints comparable to that of base metal.The safety of the 420MPa grade weathering bridge steel welded structure exposed to the atmospheric corrosion environment was assured.