| Self-shielded flux cored wire (SSFCW) is a new type welding material, which is capable of providing weld metal free from welding defects when welding in the open air without any intentionally protect. Due to its good wind resistant, simple equipment make it especially suitable for field, high-rise building and other fields. Using self-shielded flux cored wire for field pipeline construction has the characteristics of high efficiency, good weld quality and so on. As a result, long-distance pipeline construction costs can be greatly reduced. In this paper, BaF2-Al-Mg slag system is selected as basis slag system. Through the study of flux ingredients selection, wire manufacturing process,self-protection mechanism, welding usability, deposited metal strengthening and toughening design in detail, a new kind of self-shielded flux cored wire with high strength and toughness was developed, which can be used for grade X80 pipeline steel welding.For BaF2-Al-Mg slag system, gas, slag and alloy elements are jointly used to protect the droplet and molten pool from the contamination of air. By gas and slag protection, nitrogen dissolved in weld pool is reduced as much as possible. For the dissolved nitrogen, aluminum is used to form AlN, which is a stable nitride, so that nitrogen porosity and aging embrittlement are eliminated. Aluminum and magnesium both are strong deoxidizer, which can control the oxygen content in the weld effectively.In this paper, flux pretreatment is a process. BaF2,LiF,Fe2O3 flux are mixed and granulated, then sintered at 700℃850℃for 30min, 60 to 150 mesh flux is kept after grinding and sieving. Pretreated flux has the features of uniform particles, increased density, improved flux fluidity and low moisture absorption. As a result, the flux uniformity and wire feed rate are ensured.The effect of flux ingredients (including BaF2, LiF, Fe2O3, Al-Mg, Li2CO3) on slag coverage rate, spattering rate and welding fume have been studied by mixture regression design, by which mathematical models were established. The effect of flux ingredients on as mentioned parameters can be predicted and controlled by the mathematical models.A new droplet, molten slag and wire shell relationship mode has been proposed in this paper. The kinds of droplet transfer modes and the effect of Fe2O3,Li2CO3,Al-Mg flux ingredients on them have been studied by means of high-speed photography technique. Suitable quantity of each ingredient was obtained.Thermodynamic analysis shows that aluminum-nitrogen reaction in the molten pool has a critical value for aluminum content. The critical value is decreased with the increasing of nitrogen content in molten pool. Lower the critical value, the aluminum in molten has no nitrogen-fixation effect. Beyond it, nitrogen-fixation effect is increasing with the increase of aluminum in molten pool.The effect of Ni, Mn, Zr, Ce on the strength and toughness of deposited metal have been studied by orthogonal design. The optimal alloy system for as studied SSFCW was obtained, that is Ni: 1.6wt.%, Mn: 1.1wt.%, Zr: 0.02wt.%, Ce: 0.009wt.%. With this alloying system, the tensile strength and Charpy-V impact value at -40℃are 594MPa and 164J respectively.Welding usability and weld joint mechanical properties of the developed wire have been evaluated by means of welding at X80 pipeline steel. The evaluation results meet the specifications of the second"West-East Gas Transmission"project for grade X80 girth weld requirements.
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