Study On E71T8-NI1J Self-shielded Flux-cored Wire And Toughness Mechanism

Pipeline industry, which is one of the five transportation industries combined withrailway, highway, air and waterway transportation, is considered as the safest,inexpensive, efficient, energy saving and simple way of transportation. It hasadvantages such as great conveyance ability, long transport distance, low cost, highsafety and the like. Self-shielded flux-cored wire （SSFCW） is widely applied inpipeline steel installation in such industrial fields as oil pipelines, high-rise buildings,large storage tank and ocean platforms because of its excellent all-position weldingperformance, high low-temperature toughness, deep weld penetration and strong windresistance. However, the main welding problems of SSFCW are that the improvementof the strength and toughness of steel at low temperature requires high impacttoughness of welding joint at low temperature. Meanwhile, under stringent conditionand in practical pipeline welding joints, welding quality is not stable and the impactdispersion degree is high. Therefore, low temperature toughness of SSFCW becomesthe most important index of its mechanical properties. In addition, there are fewreports on toughness mechanism of SSFCW deposited metal. Evaluation on SSFCWall-position welding performance depends solely on the experience of welders, thusobjective and quantitative evaluation is needed. Therefore, it is necessary to study onthe SSFCW and its toughness mechanism systematically.To solve the above mentioned problems, this paper develops E7T8-Ni1J SSFCW,with555MPa of tensile strength and300J of average impact toughness value at-40℃as well as0.4of dispersion degree of deposited metal. According to QSY standard,the tensile test, impact test, nick-break test, side-bend test, macrostructure test andhardness test conform to the requirements of welding technical specification. Theminimum value of tensile strength of welded joints is625MPa and the minimumimpact toughness value at-20℃is131J. With excellent strength and toughness,mechanical property and technology performance of this kind of wire have reachedinternational leading level.On that basis, this paper also studies on the effects of physical and chemicalproperties of inclusions on the toughness mechanism of E71T8-Ni1J SSFCW. Analyze inclusions in deposited metals by using OM, SEM, TEM, nano indentationand other modern analysis. The result shows that inclusions in deposited metals aremainly Al₂O₃, MgO and certain elements of Mn, Ti. MgO inclusion is more likely topromote acicular ferrite （AF） nucleation than Al2O3inclusion; the more inclusions arein the range of0.30-0.80μm, the more AF are likely formed; the hardness value ofinclusions is3.58-4.62GPa, and their elastic modulus is231.1-276.7GPa, soinclusions with high hardness and elastic modulus are favorable for AF nucleation andthus toughness is improved.Finally, quantitative evaluation is conducted for arc physical propertied ofE71T8-Ni1J SSFCW in all-position welding. The electrical parameters for four kindsof wires in all position welding are tested and analyzed with Analysator Hannover,and relevant information of four wires such as arc voltage, welding current,shot-circuiting time, arc burning time is accurately described. Meanwhile, all-positionwelding index （APWI′） of SSFCW is established. In conclusion, all-position weldingperformance of four wires from good to poor is P2, S27, P1, S06. The developed S27wire has better all-position welding performance which ranks only second to that ofP2.