| With the wide application of thick-walled dissimilar steel welded structures in large industrial industries such as machinery,power,chemical industry and nuclear industry,people put forward higher requirements for the welding efficiency and the welding cost of of thick-walled dissimilar steel,performance of welded joints under different conditions,especially creep characteristics of welded joints at high temperature.How to make the welding efficiency higher and the cost lower of thick-walled dissimilar,and to further improve the elevated temperature creep properties of welded joints of thick-walled dissimilar steel has become a common concern of researchers and manufacturers.Ultra-narrow gap welding technology(UNGW)has the advantages of low heat input,high efficiency,low cost,and low residual stress,so it has a more significant advantage than conventional narrow gap welding in the welding of thick-walled workpieces.Therefore,the application of ultra-narrow gap welding to thick-walled dissimilar steels is of great practical significance in studying the structural changes and high-temperature creep properties of welded joints during thermal aging.In this paper,two kinds of welding wires,H1Cr24Ni13 and NiCrMo-3,are used to weld the ultra-narrow gap welding of 15 CrMo and 1Cr18Ni9 Ti thick-wall dissimilar steels with pulsed arc constrained by fine particle flux,and the welded joints are at different temperatures of 250 °C,450 °C and 650 °C.The thermal aging was carried out at different times.The microstructure evolution of the welded joints in different thermal aging processes was studied by means of optical microscopy,energy spectrum analysis and microhardness test.The high temperature creep properties of the welded joints were also investigated.The welded joint filled with H1Cr24Ni13 austenitic stainless steel welding wire has a fast cooling rate in the weld zone near the weld line when it is not welded(not aged),the solidification mode is AF mode,the cell crystal is developed,and the microstructure is composed of austenite and A small amount of ferrite is composed;the filled NiCrMo-3 wire has a high content of Ni element,so the solidification mode of the weld is A mode regardless of the cooling rate,which is a full austenite structure.With the increase of aging temperature,when the temperature of H1Cr24Ni13 austenitic stainless steel wire is increased to 250 °C and 450 °C,the microstructure of the weld line of the weld line is austenite and a small amount of ferrite,and the microstructure is cell crystal;One side of the line is ferrite and a small amount of pearlite.When the temperature is increased to 650 ° C,the diffusion energy of the microstructure atoms on the side of the weld line base material increases,the base material grains grow,the flaky cementite granulates,and granular cementite is produced;NiCrMo-3 is selected.When the nickel-based alloy weldingwire is used as the filling material,the heat-affected zone on the side of the fusion line is composed of ferrite and pearlite.Due to the high nickel content,the solidification mode is A mode,and the microstructure is full austenite.Compared with austenitic stainless steel welding wire,the increase of aging time has no obvious effect on the heat-affected zone structure and solidification transition layer.The energy spectrum analyzer is used to scan both sides of the fusion line of the dissimilar steel welded joint filled with H1Cr24Ni13.As the aging time increases,the atomic diffusion continues,so that the three elements of Fe,Cr and Ni are in the vicinity of the fusion line.The concentration gradient is decreasing continuously.Because of the relatively high nickel content in the nickel-based alloy,when NiCrMo-3 is used as the welding wire of the UNW welding of dissimilar steel,the elements such as Cr and Ni increase significantly,the Fe element decreases,and there are two sides of the parent metal fusion line.The more obvious element concentration gradient,which effectively inhibits the diffusion and migration of carbon,to improve the quality of welded joints.By measuring and analyzing the hardness values near the weld line of the welded joints of different filling materials under different aging temperature and time,it is found that the hardness value of the base material area is lower than the hardness of the central area of the weld;and with the increase of the aging temperature High,the peak value in the microhardness curve is decreasing,the hardness values of the base metal and the weld zone on the side of the fusion line are decreased;the hardness of the nickel-based alloy wire fusion line is also different,but with the H1Cr24Ni13 wire joint In comparison,the difference in hardness between the base metal and the weld is smaller than the hardness difference of the austenitic stainless steel wire,that is,the filler material with high nickel content can effectively improve the hardness distribution of the welded joint and reduce the fracture tendency.In this paper,the high temperature creep test is carried out on the joints under four different applied stresses.It can be seen that the applied stress is different,and the sample will undergo three different creep stages.As the applied stress decreases,the steady creep stage increases.The joint creep performance is better;when the applied stress of the sample is greater than the maximum elastic force,permanent deformation will occur;and when the external force exceeds the load peak,the sample will have a "necking" phenomenon,which will cause fracture.In summary,H1Cr24Ni13 austenitic stainless steel and NiCrMo-3 nickel-based alloy welding wire are used to weld UNGW of dissimilar steel to increase the content of Ni element,which can improve the width of the solidified transition layer in the welded joint,thereby obtaining a welded joint with good performance;According to the data obtained after the hightemperature creep experiment,it is known that the stress increase is reduced,and the fracture tendency is lowered,and an ultra-narrow gap welded joint excellent in mechanical properties can be obtained.At the same time,the research will provide theoretical basis and experimental basis for optimizing the dissimilar steel welding process,supplementing the dissimilar steel weldability theory,and finally realizing the industrial application of dissimilar steel UNGW. |
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