Optimization Of Narrow Gap Welding Process Of 9%Ni Steel And Exploration Of Ultra-Narrow Gap Welding Technology

With the increasing demand for liquefied natural gas(LNG)worldwide,.9%Ni steel(06Ni9DR)has gradually replaced Ni-Cr stainless steel due to its excellent ultra-low temperature toughness at-196? and is widely applied in the manufacture of LNG storage tanks and transport ships.At present,the welding of 9%Ni steel storage tanks and transport ships mainly uses shield metal arc welding(SMAW),submerged arc welding(SAW)and gas tungsten arc welding(GTAW),among which gas tungsten arc welding has significant advantages such as stable process and high quality.But there are still some problems in GTAW such as low welding efficiency and large number of filling layers.In this thesis,9%Ni steel narrow gap welding process experiments are carried out based on the rotating arc GTAW,which significantly improves the welding efficiency and reduces the heat input.Weld joints with good overall performance are obtained and the 5 mm ultra-narrow gap GTAW technology are further realized.The results have important theoretical significance and application prospects for the high-quality construction of LNG storage tanks and transport ships in our country.According to the principle of non-axial tungsten electrode rotating arc,the rotating arc narrow gap GTAW system is developed,and the welding process experiments of 9%Ni steel are carried out using this system.The experimental results reveal the changing laws of weld penetration width,weld penetration depth and the size of heat-affected zone under different process parameters,and determine the reasonable range of process parameters.Using 8 mm narrow gap groove and suitable anti-deformation angle,the welding efficiency is improved under the premise of ensuring the fusion of the side walls by controlling the heat input of different welding layers.Welding of 16mm plate thickness workpiece with good weld formation and high-quality welded joints is achieved with a single pass of 6 layers.Microstructure analysis,XRD and EBSD tests are performed on the joints,and it is found that the weld structure is a single austenite dendrite structure grown along the temperature gradient direction without other impurity phases,while the heat-affected zone is a mixed structure of martensite,tempered martensite and a small amount of retained austenite.The mechanical properties and corrosion resistance of the weld joints of 9%Ni steel are further tested,and it is found that the tensile strength of the joints is equivalent to that of the base metal under reasonable heat input.The low-temperature impact toughness of-196? is good,reaching 140 J/cm2.Both the forward and back bending angles can reach 160°,which meets the requirements of the storage tank standard.The hardness of the cover weld layer and the filler weld layer in the weld zone is higher than that of the bottom weld layer.The hardness value of the heat affected zone is the largest in the fine-grained zone,and that of the two-phase mixing zone gradually decreases to the same as the base metal.9%Ni steel joints undergo selective corrosion in a simulated seawater environment,and the corrosion tendency is related to the distribution of Ni and Cr elements in the joint.The corrosion resistance of the weld and the heat affected zone is better than that of the base metal.When the heat input is too low,the corrosion resistance of heat affected zone becomes poor.In order to further improve welding efficiency and reduce heat input,the rotating arc welding technology under 5mm ultra-narrow gap conditions is explored.It is found that the side wall has a stronger restraining effect on the arc under the condition of ultra-narrow gap.The arc is ellipsoidal and its size is smaller than that of the narrow gap welding.The side wall and the molten pool area can be heated by arc at the same time,and thus the thermal efficiency is further improved.Due to the change of arc heat,the metal transfer behavior is also different from that of the narrow gap GTAW.During the arc rotation,the end of the welding wire remains in contact with the molten pool during the first half cycle,and two transfer phenomena occur in the second half cycle.The welding wire undergoes a process of "contact-necking-separation-elongation-contact again" in each transfer period,and the process is stable.Ultra-narrow gap welding with rotating arc further improves welding efficiency and the deposit metal filling efficiency,significantly reduces heat input,increases sidewall penetration and reduces the size of heat-affected zone.The strength value of obtained welded joint is higher than that of the base metal and gains good comprehensive mechanical properties.