Influence of Flux Composition on the Performance of a NickelBased Alloy Covered Electrode for 9% Ni Steel Welding The effect of the flux coating ingredients on the welding performance, weld quality, mass transfer coefficient, and cryogenic impact energy is presented Introduction As one of the world’s three pillars of energy, liquefied natural gas (LNG) has occupied a very important position in the world energy pattern. The global demand for LNG has been increasing continuously in recent years. According to an energy report (Ref. 1), the global demand for LNG will reach 350 million tons in 2020. The storage and delivery of LNG will continue to increase. Because LNG is stored at or below its boiling temperature, the material for the inner walls of LNG storages should securely serve at the cryogenic temperature. Thus, it must have high strength and suitable fracture toughness at the cryogenic temperature. In 1942, 9% nickel steel was developed by an international nickel company as a structural material at cryogenic temperatures. Its low-temperature (77 K) impact value reached 200–300 J. It is the best ductile material that can serve in cryogenic conditions due to its high strength, large fatigue resistance, excellent corrosion resistance, small thermal expansion coefficient, and good weldability (Ref. 2). So far, 9% Ni steel has been comprehensively investigated. Its microstructure consists of fine martensite and retained austenite. The addition of 9% Ni keeps the unstable austenite from austenitization temperature to the room temperature. The existence of the WELDING RESEARCH retained austenite results in excellent mechanical properties at cryogenic temperature (Refs. 2–5). As well known, the welding consumables are key factors affecting the quality of the weld joint. The covered electrode of a Ni-Cr-Fe alloy with a Ni content more than 55 wt-% can be used for 9% Ni steel welding (Refs. 6–8). Although the nickel-based alloy covered electrodes are expensive and show poor welding performance, their strength and low-temperature toughness are excellent. The thermal expansion coefficient of the nickelbased alloy is also similar to that of 9% Ni steel. In the industry, each welding consumable producer has its own flux ingredient that strongly affects the weld quality, mass transfer coefficient, bead geometry, slag detachability, and other welding performance. Generally, the flux coating composition includes gas formers, slag formers, binders, extrusion aids, alloy elements, and so on. The role of the flux coating includes protecting the weld metal from the invasion of air, keeping the arc stability, ensuring good slag detachability, and reducing spatter. In general, it is very difficult to take into account all these aspects when drawing up a flux ingredient. In recent years, several types of flux coatings have been investigated in our laboratory for developing high-level stainless steel and nickel-based alloy covered electrodes, such as E316L (Ref. 9), CaO-CaF2, CaO-CaF2-TiO2, BY H. WANG AND G. HE ABSTRACT A nickelbased alloy covered electrode with two different types of flux coating was investigated in terms of weldability, weld quality, mass transfer, microstructure, and mechanical properties. It was found that the strong basic CaF2CaOSiO2 type electrode exhibited a larger deposition rate, smaller spatter loss and fumes, better fluidity of slag, and wider weld bead. The alkalescent TiO2SiO2SrO type exhibited larger penetration and excellent slag detachability. The metals had the larger mass transfer coefficients in the strong basic slag system if they could form basic oxides. However, the elements had the smaller mass transfer coefficients in the strong basic slag system if they could form acidic oxides. The impurities, S, P, and O, were in the relatively lower levels in the deposited metals of the CaF2CaOSiO2 type electrodes because of the higher contents of CaF2 and CaO in the slag. The Charpy impact values of the CaF2CaOSiO2 type at 77 K were in the range of 81–83 J, but that of the TiO2SiO2SrO type were in the range of 66–70 J. KEYWORDS • NickelBased Alloy • Covered Electrode • Flux Coating • Cryogenic Impact • 9% Ni Steel H. WANG and G. HE (ghe@sjtu.edu.cn) are with the Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai, China. HE is also with the Collaborative Innovation Center for Advanced Ship and DeepSea Exploration, Shanghai, China. DECEMBER 2016 / WELDING JOURNAL 467-s
Welding Journal | December 2016
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