| The modern marine economy and the development of large domestic ships have put forward higher and higher requirements for the comprehensive performance of high-strength low-alloy steel weldments.Both 907 A and 921 A steels are materials for ships and belong to high-strength weldable structural steels.Double-wire submerged arc welding has become one of the efficient welding methods for ship construction due to its high efficiency and energy saving.In order to provide a theoretical basis for the practical performance of 907 A and 921 A steels in modern large-scale ship structures,this topic focuses on the microstructure and seawater resistance of 907 A welded joints of the same type of steel and907A/921 A dissimilar steel welded joints of double-wire submerged arc welding.Corrosion performance,stress corrosion performance,and crack growth before and after seawater corrosion were studied.Using metallographic microscope(OM)and scanning electron microscope(SEM)to observe the microstructures of the welded joints of the same type of steel of 907 A and the welded joints of 907A/921 A dissimilar steels,it was found that the structure of the weld 1(WB1)of the two types of welded joints was slightly coarse.,The structure of weld 2(WB2)is relatively fine,and a certain number of columnar crystals appear in both welds.The weld structure of the same type of steel of 907 A is ferrite,bainite and a small amount of pearlite,and the weld structure of the dissimilar steel of 907A/921 A is mainly acicular ferrite and bainite.The heat-affected zone(HAZ)structure of the two welded joints is a mixed structure of coarse bainite and martensite.Static immersion experiment,dynamic potential polarization and electrochemical impedance test results show that the HAZ and WB1 of the same type of steel welded joints of 907 A have poor corrosion resistance.The worst corrosion resistance of 907A/921 A dissimilar steel welded joints is the heat-affected zone(HAZ-907A)on the 907 A side.The seawater corrosion resistance of the base material(BM),WB2,and HAZ of the 921 A steel side is significantly better than that of the 907 A.BM,WB1,HAZ on the steel side.Through slow strain rate tensile test(SSRT),it is found that in air and sea water,the fracture position of 907 A same steel welded joint and 907A/921 A dissimilar steel welded joint is on HAZ-907 A,and the fracture is characterized by ductile fracture.The loss of area reduction(IZ)and elongation loss(IA)of 907 A welded joints of the same type of steel are 23.18% and36.54%,respectively,which are higher than the corresponding values of 16.20% and 7.45%for 907A/921 A welded joints of dissimilar steels,that is,the former has higher Stress corrosion sensitivity.In-situ tensile experiments have found that the inhomogeneity of the welded joint structure has a significant impact on the crack propagation.The heat-affected zone with a coarse structure can often crack at a lower stress value,and the crack propagation direction in the adjacent area will be affected.Obviously biased toward the heat-affected zone;in-situ tensile specimens corroded by seawater,the number of micro-cracks initiation increased,the corresponding stress value of crack initiation and the stress peak value of crack growth became smaller,the crack growth rate accelerated,and the fracture time of the specimen Decrease,indicating that the crack resistance of 907 A welding joints of the same type of steel and 907A/921 A dissimilar steel welding joints will decrease after seawater corrosion. |
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