Welding Process And Joint Performance Of 9Ni Steel For LNG Low-temperature Ship

Liquefied Natural Gas(LNG) was a very clean type of energy. Along with the implementation of the national green shipping project of “Gasifying the Yangtze River Region” and the construction of the Maritime Silk Road under “the Belt the Road”initiative, the industrial development in the coastal region will drive up the demand on LNG. In the whole process of LNG transportation, LNG carrier ship was a key link.Usually LNG carrier ship was made of 9Ni low temperature steel. 9Ni steel with10 mm and 14 mm thick was used in this paper, and the influence of power type,welding energy input and welding positions on microstructure and mechanical properties of the joints were studied. The results showed that:1. AC power welding equipment would be helpful for obtaining welding joints that meet operational requirement. At 15 mm away from the welding seam on both sides, magnetic remanence after welding with a DC welder will be 3 times of magnetic remanence after welding with an AC welder. Large magnetic remanence after welding with DC power was an important contributor to the welding joints failing both in tensile strength and bending performance.2. Different welding energy input would be applicable for different welding positions. For flat welding, using welding joints with energy input of 9.8KJ/cm would be a good choice; too small or too big energy input would cause different extent of defects to the welding seams and generate different degrees of break or local cracks of the welding joints during bending; meanwhile, along with increasing energy input, the tensile strength of the welding joints would increase first and decrease later, while welding joints formed under low energy input would show brittle fracture; welding seam’s low temperature impact toughness would rise first and fall later. Vertical welding usually used lower welding energy input. When using 12.1KJ/cm, 16.5KJ/cm and 25.2KJ/cm in turn for welding, along with the increasing energy input, the tensile strength of the welding joints gradually decreased, the elongation rate of the fully welded joint was gradually reduced, and the low temperature impact toughness of the welding seam and the heat-affected zone kept going down.3. The welding positions and energy input were important in this study. The welding seam structures comprised of all of austenitic, plus a small amount of precipitation of carbide, and the obtained microscopic structures in the heat-affected zone were all of martensite. In terms of hardness, the heat-affected zone was the highest, followed by the mother plate and then the welding seam, all were smaller than the standard value of 400 HV.4. Comprehensive results showed that, for 9Ni steel, to obtain good welding joints,the welding energy input values for different welding positions would be as follow: flat welding: 9.8KJ/cm; horizontal welding: 9.5KJ/cm; vertical welding: 12.1KJ/cm...