Develop Submerged Arc Welding Flux For The F550Platform Steel And Study The Welding Joints’ Mechanical Properties

As the exploitation of marine resources from shallow sea to deep sea, the offshoreplatform steel’s grade and thickness are higher and higher. In this paper, used submerged arcwelding for jointing the offshore platform steel F550Z, developed high basicity and lowhydrogen flux, combined with alloy wire, and researched the effects of heat-inputs on thewelding joints’ mechanical properties by using different heat-inputs. Analysising heat-inputseffects on the welding joints’ tensile strength and low temperature toughness andmicrostructure and hardness, affording experimental and theoretical basis for weldingproduction and selecting welding parameter reasonably.Study the effects of different raw materials on the welding operability; confirm theamount of different raw materials in the formulation, and check the flux properties of slagremoval and welding performance. After lots of trails, the flux formulation is established,and tset the flux components for calculating the basicity, which is3.39and meets the designrequirements. Combining with solid wire, study the mechanical properties, check thediffusible hydrogen of deposit metal and research the reason of low levels of diffusiblehydrogen.The welding material is the combination of developed flux and CHW-SEM2NP cordwire. When the heat-inputs is designed as15kJ·cm^-1and27kJ·cm^-1and38kJ·cm^-1and50kJ·cm^-1, the mechanical properties show certain trends. The research results indicate thatwhen the heat-inputs increased, the joints’ tensile strength exceed the minimum value ofCCS requirement which is670MPa, and no significant changes with the heat-inputs changes,the tensile strength is not sensitive to the heat-inputs. The hardness of welded jointsdistribute evenly, the hardness in the welding center is lower than that in base metal, nosignificant fluctuations.The heat-inputs affect heavily the microstructure in different parts of joints andtoughness at-60℃. The experimental results indicate that toughness decrease firstly andincrease then with heat-inputs increase. When the heat-input is15kJ·cm^-1, the average ofimpact value is65J, as the heat-input reaches38kJ·cm^-1, it can’t meet the minimum value ofCCS requirement which is34J, but the heat-input reaches50kJ·cm^-1, it is110J, which is2to4times than others. By analysing microstructure and SED, the welding metal get theeffect of heat treatment when heat-inputs is15kJ·cm^-1or50kJ·cm^-1, the microstructure areacicular ferrite and fine granular dispersed carbides, especially the heat-input is50kJ·cm^-1.The microstructure in the fusion line and2mm from fusion line are lath bainite and granular bainite. When the heat input is small, a spot of granular bainite disrupt the direction of lathbainite, which is good for toughness. But granular bainite increase and get coarse withheat-input increase, which leads to bad toughness. The impact values in these zonesdecrease with heat-inputs increase. The microstructure and toughness in5mm from fusionline are almost the same with those in base metal.In summary, when the heat-inputs are15kJ·cm^-1and27kJ·cm^-1, all the properties ofjoints meet the CCS requirements, considering the mechanical properties and efficiency ofproduction, Using SAW for F550Z steel, the heat-inputs should be27kJ·cm^-1.