| Recently, china are play clear energy strategy,LNG industry increased rapidly because of its low-cost storage and transportation. Cryogenic 9Ni steel and its welding technology have a huge demand. But its assembling and welding technology are relay on foreign country, it has huge restricted to the development of our country's industry and energy's safe. So, master cryogenic 9Ni steel and its welding technology and push the national process of LNG equipment is urgent need to our country's energy development. 9Ni steel as a material of inside storage worked at -162℃environment. So, in order to guarantee the safe of pressure vessel's welding, the research of the welding joint of domestic super-lower carbon welding joint has important significance.According to domestic low carbon 9Ni steel, at the basic of analysis 9Ni steel's weldability. According to BS7448 and GB2358 standard, the CTOD (crack tip open distance) test of welding metal, fusion line and heat affected zone were measured; at the same time, the trend of crack tip has observed by HIROX long focal distance microscope. Revise the Wells calculation formulation of CTOD test. According EN875 and GB/T 299 standard, The charpy energy of different part of welding joint, different groove, different percents of welding metal were measured in -165℃,the microstructure and fine structure was investigated. Fractured surface scanning and energy spectrum analysis of welding joint was investigated.The result shows: the cryogenic toughness of super low carbon 9Ni steel welding joint reach ASME and BS7448 stardand, The CTOD test and charpy impact test all shows that the cryogenic toughness of fusion is the lowest, the heat affect zone has the best cryogenic toughness. The cryogenic impact toughness of V-groove is higher than X-groove, the higher the percent of welding metal, the less the energy of cryogenic impact tests. The metallographic structures of fusion line are massive lath martensite, the fine acicular tempered martensite and reverted austenite in heat affect zone. The grain size increased with distance of fusion line, where grain size decreased. The cryogenic impact toughness increased with the decrease of grain size. The fracture mode of welding joint is ductile fracture which full of dimple and second phase particle. Energy spectrum analysis shows there are much element diffusion near the fusion line area. Segregation of impurity is easy happens in fusion line which area is the most weak part in super lower carton 9Ni steel welding joint. |
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