| Dissimilar material welding has been widely used in transportation,power plant boiler,petrochemical industry and nuclear power plant and other industries,among which the dissimilar material welding between austenitic steel and pearlite steel is the most common.However,when the dissimilar material welding between austenitic steel and pearlite steel,the fusion transition region of pearlitic steel will form the martensite layer,lead to dissimilar steel joint plasticity and toughness reduction,and the pearlite/austenitic dissimilar metal weldments,under the environment of high temperature for a long time,easy to have the carbon migration and martensite layer formation will also provide channels for carbon diffusion,accelerate the migration of carbon atoms.Pearlitic/austenitic dissimilar steel joints normally used at high temperatures,mostly with Ni-base alloy as the filler metal,to significantly improve the Ni distribution gradient in the fusion transition region of pearlitic steel.It can not only reduce the thickness of the martensite layer and inhibit the carbon migration at high temperature,but also reduce the thermal stress caused by the different degree of expansion and contraction of dissimilar joints.However,the conventional welding process with Ni-base alloy as the filler metal,on the one hand,due to welding heat input is too high,cause the molten pool formed the massive single-phase austenitic organization,if the fusion ratio control is easy to cause weld zone during solidification cracking;on the other hand,Ni-base alloy welding material consumption,greatly improve the welding cost.For this reason,this project proposed to preset Ni-base alloy sheet and prepare the transition layer by TIG cold welding of filling wire to control of the distribution gradient of nickel in the fusion transition region of pearlitic steel,the pearlitic/austenitic dissimilar steels are then welded.Among them,the presetting Ni-base alloy sheet process is to spot weld the Ni-base alloy sheet on the surface of 15CrMo pearlite steel by resistance spot welding,and then weld between 15CrMo with Ni-base alloy sheet and 1Cr18Ni9Ti by ultra-narrow gap welding(UNGW),the process saves transition layer surfacing process,not only can significantly increase the Ni distribution gradient of the fusion boundary of 15CrMo and Ni-base alloy welding material consumption significantly reduced;prepare the transition layer by TIG cold welding of filling wire process was firstly prepared the transition layer with a structure of “high+low” Ni mass fraction by TIG cold welding of filling wire,and then there will be a transition layer of 15CrMo with 1Cr18Ni9Ti by UNGW,which to improve the Ni distribution gradient in the fusion boundary region of 15CrMo,the low Ni mass fraction cladding layer can effectively prevent the high Ni mass fraction cladding layer from melting,the solidification crack caused by Ni segregation can be effectively avoided in the filling weld.The research results of this topic come to the following conclusions:(1)Preset Ni-base alloy sheet process of the test results show that when the thickness of Ni-base alloy sheet was 0.3?0.6 mm,UNGW joint part of weld zone near the fusion line solidified with A mode and formed austenitic cellular crystals,the other zone still solidified with FA mode and was made of austenite equiaxed grains and dendrite ferrite,and that solidified crack did not form in the center of weld.However,when the thickness of the Ni-base alloy sheet is 0.9?1.2 mm,coarse austenite columnar dendrite solidified is formed with A mode in the center of weld zone,and more solutes and impurities are precipitated at the intersection of coarse austenite columnar dendrite,thus forming solidification cracks.In addition,the Ni mass fraction in the weld zone near the fusion boundary can be increased to 11.6 % by presetting the Ni-base alloy sheet with a thickness of 0.6?0.9 mm on the side of 15CrMo,which effectively increases the carbon activity coefficient in the weld zone and significantly inhibits carbon diffusion at high temperature.(2)Prepare the transition layer by TIG cold welding of filling wire process of the test results show that When the transition layer is prepared by TIG cold welding of filling wire process,if the ERNi CrMo-3 welding wire is first deposited on the surface of 15CrMo,and then the ER309 or ER385 welding wire is deposited,the TIG cold welding spot weld pool will form cracks in the subsequent solidification process,even by adjusting the process and reducing the heat input can not be avoided.If ER385 is deposited on the surface of 15CrMo and then the ER309 welding wire is deposited,no cracks will be formed in the ER309 clad layer.EDS test of transition layer samples of “ER385+ER309” shows that the average nickel mass fraction in the 15CrMo fusion boundary and its adjacent cladding layer can reach14.97 %,which is significantly higher than that of the ER309 welding wire,and can also play a more obvious role in the inhibition of carbon diffusion at high temperature than that of the ER309 cladding layer.In addition,the microstructure of dissimilar steel UNGW joint with transition layer is composed of fine austenite equiaxed grains solidified with FA mode and a small amount of dendritic ferrite,and the HAZ grains at the side of 15CrMo are finer than that of the base metal,and the mechanical properties are better. |
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