A Study On Welding Process Of 9Ni Steel For LNG Cryogenic Tanks

With the rapid developing of Chinese economy,the need for energy is urgent.LNG as green energy has become the focus of development.But,cryogenic 9Ni steel as Key materials of and welding process is no any welding process in our country.Test and research has an important engineering significance.According to the new domestic 9Ni steel,By study and discussion a great deal of science and technollgy information about 9Ni steel at home and abroad,and comprehensively analyses weldability,weld materal ,weld mathed and weld process. Have recerached weld process of 9Ni steel by weld thermal simulation test and the welding of test plates.Testing cryogenic mechanical properties of the sample by using JBD - 300 cryogenic Charpy impact , hardness-testing instrument,MTS 809 Axial test system. The relations between welding process and cryogenic toughness has been proved up by metallography,SEM,XRD.The results showed that at 560℃has the best cryogenic toughness (257J)in single-run welding thermal simulation test.In coarse grain heat-affected zone (CGHAZ) cryogenic impact enegy is only 50-90J,great lower than base metal.And cryogenic impact enegy was decrease with the increzse of heat input energy. Double-runs welding thermal simulation tests show that there were some brittle zones along the fusion line. Coarse grain heat-affected zone have bing divided into small zone under the following welding thermal cycles. Remain CGHAZ, UA CGHAZ, IC CGHAZ have very low cryogenic toughness.Crack initiation and propagation easily in these zone.This study alos found that 9Ni steel with very low C and extremely cleanliness would take CGHAZ from microstructure hereditary phenpmenon for the quick cooling rate after weld.Thus,CGHAZ will be refined more marked and improve properties in the second welding thermal cycle.Some results are found that cryogenic toughness of 9Ni steel will be enhance with little heat input , low interpass temperaturea and multi-layer and multi-passes.If the heat input arrive at 37kJ/cm,the cryogenic toughness of 9Ni steel still at a high level(about 50J at -165℃).So efficiency of weld would be enhanced by increase heat input properly based on multi-passes welding.