| Based on the welding requirement of large liquid natural gas (LNG) storage tank, the welding process and mechanism of shielded metal arc welding (SMAW), gas tungsten arc welding (GTAW) and submerged arc welding (SAW) of 9Ni steel was systematically investigated. The interfacial microstructure and mechanical properties was studied by means of optical microscope, scanning electron microscope coupled with energy dispersive x-ray detector, strength tests and hardness tests. According to the investigation on the effect of welding parameters on the microstructure and mechanical properties of joint, the welding parameters was optimized. The characteristic of fracture in different distinctive region was introduced. By comparative analyzing the applicability of three welding methods in production of large LNG storage tank, the most suitable welding method for different position in floor and wall plate was determined.The SMAW welding of 9Ni steel was realized using Freezal Ni9 electrode. The welding seam consisted of columnar crystal and intergranular carbides prepeciates. The heat effect zone was mainly composed of martensite lath. The effect of welding parameters essential derived from welding heat input. The tensile strength of joint obtained by flat welding, horizontal welding and vertical welding was similar. The joining using flat welding reached the highest strength 739MPa and presented the best bending properties. The hardness and brittleness of joint with horizontal and vertical welding was relative higher. The impacts properties in low temperature using three welding position were similar. The fracture mainly showed the ductile characteristic.The GTAW welding of 9Ni steel was successfully carried out using NERTAL625 welding wire. The heat effect zone was divied overheating zone, complete normalize zone and incomplete normalize zone. The main microstructure in every part was martensite lath. The welding seam was mainly composed of columnar austenite grains. The segregation phases existed between the austenite grains. When the welding position was changed, the microstructure of welding seam still consisited ofγsolid solution and segregation phases. However, the compostion of segregation phases in the welding seam using flat and vertical welding was higher than that using horizontal welding. The test results of properties showed that the low temperature ductility in heat effect zone was higher than that in welding seam. The highest yield and tensile strength of welding seam reached 692MPa and 740MPa, respectively. All fracture in welding seam and heat effect zone presented ductile characteristic.The ENiCrMo6 electrode and ERNiCrMo4 welding wires were applied to achieve the SMAW assisted SAW welding of 9Ni steel. The microstructural characteristics in the heat effect zone and fusion zone were similar with that using GTAW method. The low temperature ballistic works in welding seam and heat effect zone reached 124.7J and 225.3J, respectively. The average yield and tensile strength of welding seam was 690.5MPa and 732MPa, respectively. All properties fit the requirement of welding standards. The analyses of fracture showed that all fracture in welding seam and heat effect zone was ductile fracture.From comparative analyses of the results using the above three welding methods, it was confirmed that the combination of above three methods was suitable for the actual welding of lage LNG storage tank. The best method for flat welding of floor was SMAW or GTAW method. The SAW method was good choice for horizontal positon welding of wall. The multilayer and multiple welding of SMAW and GTAW method can be applied for vertical position welding of wall. The conection seam between floor and wall was flat fillet welding seam, where the SAW method was most suitable for this position.
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