Study On Welding Process,Microstructure And Properties Of High Manganese Steel Joints Used In LNG Tanks

As the demand of LNG increases rapidly,LNG storage tanks are valued by countries all over the world.In the process of safe production,transportation and use of LNG,the performance of materials of storage tank is very important.Low temperature austenitic high manganese steel is expected to replace 9Ni to be widely used in the manufacture of LNG storage tanks steel because of its excellent low temperature mechanical properties and good economic advantages.Welding is critical in the manufacturing of LNG storage tanks,and welding defects are responsible for more than 90% of pressure vessel accidents.As a new material,its welding process and performance evaluation of welded joints are still in the exploratory stage in China.In view of this,what was studied was the welding process of the new developed high manganese steel for LNG tanks,which was filled with nickel-based welding materials.In order to obtain a welded joint with good organization characteristics and performances including hardness,tensile properties,bending properties and low temperature impact performance,provide experimental bases and theoretical analyses for the industrial application of ultra-low temperature and low-cost steel of vessels,and then provide the bases for the establishment of the performance evaluation system and the application standard.The main results are as follows:1.Nickel-based welding materials were firstly used to weld the newly developed high manganese steel for LNG storage tanks.Three Welding processes were designed,including the SAW joint with X groove,the SMAW joint with X groove and the SMAW joint with V groove,by changing the welding method,groove form,heat input and inter-channel temperature.Finally,the welding joints were get,which met the performance requirements.2.Fusion ratios and microstructures.The fusion ratios of the SAW joint with X groove,the SMAW joint with X groove and the SMAW joint with V groove were35%,23% and 17%,respectively.The grain size decreased in turn.And the microstructures were the same.The weld metal(WM)were dendritic austenite with a small amount of precipitated phase,and the coarse grained heat affected zone(CGHAZ)and fine grained heat affected zone(FGHAZ)were all cellular austenite with few twin crystals.3.Hardness distribution.The Vickers hardness of base metal,heat affected zone and weld decreased slightly successively under three welding conditions.The vickers hardness of heat affected zone and weld of the SAW joint with X groove,the SMAW joint with X groove and the SMAW joint with V groove were 227.3 HV,210.2 HV,215.3 HV,211.7 HV 208.2 HV and 209.7 HV respectively.4.Tensile properties.The tensile strength,the yield strength and the post-fracture elongation of the SAW joint with X groove were 713 MPa,489 MPa and 27%respectively;these of the SMAW joint with X groove were 773 MPa,469 MPa and 30%respectively,and these of the SMAW joint with V groove were 762 MPa,493 MPa and 21% respectively.The welded joints of three welding processes all met the requirements.5.Bending performance.When the bending angle reached 180°,side bend specimens of the SAW joint with X groove did not meet the operating requirement because of the fracture;SMAW joints with X and V groove met the operating requirement,because no cracks greater than 3 mm appeared on the drawing surface.6.Low temperature impact toughness.Average impact toughnesses of the WM,the fusion line(FL)and the heat affected zone(HAZ)of the SAW joint with X groove were 8.9 J,32.0 J and 15.3 J respectively,the WM and the HAZ were brittle fractures,and the FL was mixed fracture.Average impact toughnesses of the WM,the FL and the HAZ of the SMAW joint with X groove were 32 J,43 J and 50 J respectively,the WM and the FL were mixed fractures,and the HAZ was ductile fracture.Average impact toughnesses of the WM,the FL and the HAZ of the SMAW joint with V groove were 56 J,56 J and 60 J respectively,and all of them were ductile fractures.The performance of the SAW joint with X groove did not meet the requirement while SMAW joints with X and V groove met the requirement.7.Comprehensive evaluation of the welded joints under three welding process conditions showed that the mechanical properties of the SAW joint with X groove was the worst,and the mechanical properties of the SMAW joint with V groove was the best.SMAW joints with X and V groove met the requirements,indicating that nickel-based welding materials could be used for the welding of high manganese steel for LNG storage tanks.