Of X80 Pipeline Steel Gas Welding Wire Composition And Weld Properties

A new type of welding wires matched with X80 pipeline steel for gas metal arc welding was developed by the micro-alloyed theory and grain refining mechanism. The deposited metals of this welding wire not only have high strength(yield strength R_p0．2 =550MPa, tensile strength R_m=620MPa), but also have excellent low temperature toughness(low temperature Charpy impact value A_KV-20 =70J ),which meet the demand of the strength and toughness of X80 welding pipe. The welding joint properties can reach the level of reality pipeline engineering using X80 pipeline steel.The microstructure of weld metal, weld joint and the morphology of fracture appearance was observed with OEM, SEM and TEM.The effects of alloy element on the microstructure and mechanical properties were investigated. Experiment results indicated that Ceq should be hold in rational rang, which can ensure high strength and high toughness of deposited metal. With increase of C content and Mn/Si ratio and decrease of O content and Mn/C ratio, the toughness is improved. It was effective to obtain fine and homogenous ferrite by adding proper Ti and B element to the welding wire.The effects of the size and chemical nature of inclusions on the nucleation of acicular ferrite were studied. As the nuclear of acicular ferrite, the inclusions the size of which is between 300～600nm are chemical heterogeneous compound inclusions that contain various elements such as C, O, Al, Si, Ti and Mn. In many circumstances, multiple acicular ferrite plates can be nucleated at one inclusion. Thus, it is suggested that the inclusions promote the nucleation of acicular ferrite because of its inert interface reducing the energy barrier to nucleation.