Study On Microstructure Of Invar Alloy Weld For LNG Carrier

The liquid cargo tank of the membrane LNG carrier is welded by invar steel with a thickness of only 0.7 mm.The temperature of LNG in the tank is lower than-163?,and its welding process requirements are very high.TIG welding is often used in practice,but the problem of welding hot cracking has not been solved thoroughly.Reducing the heat input can reduce the welding deformation and improve the accuracy,so the laser welding method is suitable for the welding of Invar alloy sheet.In order to study the microstructure of the weld and optimize the welding process parameters,the laser welding and TIG welding experiments of Invar alloy were completed.The microstructure characteristics of the weld were observed and the mechanical properties of the weld were measured.The specimen of fish-bone welded by preheating pulse and post heating pulse laser were analyzed.The hot cracks are concentrated on the upper surface of the weld,which is due to the final crystallization at the upper surface,at this time,the liquid phase filler metal is insufficient.The preheating pulse wave affects the welding temperature gradient,which makes the crystal nucleation and growth more uniform and reduces the hot cracking tendency.Generally,the hot cracking tendency decreases first and then increases with the increase of preheating pulse duration,which is consistent with the test results of preheating pulse laser welding fishbone.A reasonable post heat pulse can reduce the cooling rate of the molten pool,make the liquid metal fully backfill between the crystals,the crystals grow in all directions,stop growing and form a large number of equiaxed crystals after encountering the solid grains,reducing the hot crack susceptibility.The hot cracking tendency is consistent with the test results of fish-bone sheet laser welding with post heat pulse.The optimum welding parameters of TIG welding were welding current 40 A,the pulse frequency 120 Hz,and the welding speed 35 cm/min,the welding effect was relatively best.The optimal tensile strength was 446.9Mpa,which was 88.33% of the strength of the base metal.The micro analysis of TIG welding shows that the central line area is mainly composed of fine cellular dendrites due to its fast cooling rate,the transition area is mainly composed of coarse dendrites,and the fusion line is mainly coarse columnar crystals.The microstructure of the base metal is very uniform and fine austenite equiaxed crystal,and the volume of HAZ crystal increases under the action of welding thermal cycle,which leads to the increase of welding hot cracking tendency.The correct adjustment of welding current and welding speed can control the welding heat input and welding arc force,which can effectively reduce thetendency of welding hot cracking.The increase of welding pulse frequency in a certain range can refine the weld crystal and reduce the susceptibility of hot cracks.The content of S,P and other elements around hot cracking is higher than that in the base metal area,and S,P may form a eutectic liquid phase with nickel element at the grain boundary,which increases the susceptibility of welding hot cracking.The grain structure of TIG welding seam of Invar alloy is larger than that of pulse laser welding,and the crystal transition in the region is relatively abrupt,which causes the strength of the joint to be slightly lower.