Study On X120Pipeline Metal-cored Wire And Toughness Mechanism Of Deposited Metal

With the rapid development of the oil and natural gas industry, the adoption ofpipelines to transport oil and natural gas has become the primary technical solution.To reduce the cost for long-distance transportation, the strength level of the pipelinesteel is rising continuously, and the research and application of X120pipeline steel,which possesses the highest strength level, will further reduce the cost and increasethe gas traffic volume. X120pipeline steel is made of ULCB steel, with strength of650¹000MPa and excellent low-temperature toughness, bringing a huge challenge tothe weld metal. Few researches are aimed at the strengthening and toughening of theweld deposited metal presently. Utilizing present welding materials, the weld metalwill get a higher carbon content and hardenability than the base material, with highdegree of alloying, and cold cracking will tend to occur in the weld metal. As a resultself-developed ULCB metal core wire series were adopted in this paper, the variationof the structure and performance of the MIG weld deposited metal with a strength of650⁹00MPa were studied, and the strengthening and toughening mechanism of theULCB weld deposited metal were analyzed.Focused on these problems, a weld deposited metal structure with lath bainite asthe main strengthening phase, and less amount of granular bainite and acicular ferriteas the toughening phase was designed in this work, adopting ultra low carbon design（carbon content of0.02⁰.04%） when developing low C-Mn-Mo-Cr main alloysystem to obtain the weld deposited metal structure. A X120MIG metal core wirewas developed, and the weld metal possesses a tensile strength of922MPa, with meanimpact toughness of64J at-40℃.On this basis, the effect of the weld deposited metal microstructure on thestrengthening and toughening mechanism of the ULCB deposited metal was furtherinvestigated. Characterization methods including OM, SEM and TEM were adoptedto compare and analyze the deposited metal microstructures with different mechanicalproperties. The results show that: the ULCB microstructure is a multi-phase structure,consists of AF, GB, DUB and low-carbon martensite. For the deposited metal, in theultra low carbon condition （C≤0.04%）, when the tensile strength reaches900MPa,DUB and GB become the main phase in the deposited metal, with the totalcomposition of60⁸0%. The AF and GB nucleated at higher temperature can split the original austenite grains effectively, restraining the growth space for the subseque ntphase transformation structure, refining the size of the lath structure and creatingdiverse orientations for the sub-structure in the original austenite grains, throughwhich the impact toughness of the weld metal can be effectively improved.Finally, the controlling of the inclusions in the deposited metal and theirinfluences on the mechanical properties of the deposited metals were investigated.Using Ar-CO₂mix with various CO₂contents as the protecting gas and wires wereZr-Ti micro alloyed. The results indicate that with the increasing of CO₂content inthe protecting gas, composition of granular bainite which is the high temperaturephase transformation product increases, as well as the size and volume of theinclusions. When the inclusions in the900MPa level ULCB deposited metal composemainly of ones with size more than1μm, the impact toughness of the metal declinessignificantly. Through the addition of trace amount of Zr, under the combined effectof Zr and Ti, size of the inclusions in the deposited metal falls into0.4μm⁰.8μmrange, with their declined area percentage, providing abundance potential nucleationpositions and improving the impact absorbing energy of the deposited metal.