Research Of Thick-walled Q235/1Cr18Ni9Ti Dissimilar Steel Welding With UNGW

With the wide application of welded structures of thick-walled austenitic stainless steel/carbon steel dissimilar materials in the mechanical,chemical,electric power and nuclear industries,higher requirements are put forward in the welding efficiency,welding cost and welded joints of thick-walled dissimilar steels under different working condition.However,there are usually two problems in the process of welding the thick-wall austenitic stainless steel and carbon steel dissimilar materials.On the one hand,thick-walled dissimilar steels are prone to defects in poor sidewall fusion when narrow-gap gas shielded welding is used.On the other hand,in the case of dissimilar steel welding,a martensite layer is formed in the region where the Ni content is less than 5% to 6% in the fusion transition zone,a type II boundary is easily formed on the weld side of the fusion transition boundary,and high local stress is formed at high temperature due to the difference of linear expansion coefficients between the two sides of the dissimilar interface area.The arc is chosen as the heat source for ultra-narrow gap welding,and the "I" type groove is adopted,of which the gap width is not more than 6mm and obviously smaller than the gap width(>9mm)used for narrow gap welding.What's more,ultra-narrow gap welding is single pass welding for each layer.Less welding deformation and residual stress are obtained compared with conventional narrow gap welding,and the softening problem of HAZ can be effectively solved owing to the minimum linear energy,which is only 0.5 kJ/mm.In addition,the problem that the root of the narrow gap welding sidewall is not easily heated by the arc is fundamentally solved due to the higher energy density and side wall penetration ability accomplished.Apparently,the ultra-narrow gap welding method for thick-wall dissimilar steel welding show the advantages of higher welding efficiency,lower welding cost and excellent joint mechanical properties than narrow-gap welding.In this thesis,ultra-narrow gap welding of 1Cr18Ni9Ti/Q235 dissimilar steel was carried out by DC and pulse arc constrained by fine particle flux,and the microstructure and mechanical properties of the joint were tested and analyzed.In addition,the workpieces were preheated,and the ultra-narrow gap pulse welding of 1Cr18Ni9Ti/Q235 dissimilar steel was carried out under the preheating conditions at room temperature(18 ?),113 °C,196 °C,274 °C and 336 °C,respectively.What's more,the microstructure of the joint obtained was analyzed.The conclusion of this experiment is as follows:(1)The solidification mode of DC bottom welding was FA mode with the solidified structure of fine equiaxed crystal and the matrix was austenite on which dendritic ferrite was distributed,while the solidification mode of the molten pool near the fusion line of pulse bottom welding was AF mode and the solidification structure was fine austenite cell crystal and intergranular ferrite but the solidification mode of the molten pool away from the fusion line was FA mode and the solidification structure was Austenitic equiaxed crystals with dendritic ferrite distributed on the austenite matrix.The solidification mode of the molten pool near the fusion line of DC/pulse fill welding was AF mode with the solidified structure of austenite cell crystal,while the solidification mode of the weld center was FA mode and the solidification structure was columnar dendrite.The solidification mode of the molten pool near the fusion line of DC cover welding was AF mode with the solidification structure of austenite cell crystal,while the solidification mode of the weld center was FA mode and the solidification structure was columnar dendrites.The solidification mode of the molten pool of pulse cover welding was AF mode,and the solidification structure was austenite cell crystal and intercellular ferrite.(2)The tensile strength of DC and pulsed UNGW joints was larger than that of Q235 base metal,exhibiting excellent bending performance of the joint.The toughness of fusion zone at room temperature was better than that of weld center and heat-affected zone,meanwhile the impact energy of pulsed UNGW weld zone was better than DC.In addition,the impact energy of the pulsed UNGW weld zone was about 30% higher than that of the DCUNG joint.(3)With the increase of preheating temperature,the cooling rate of the molten pool gradually slowed down,with the FA mode of solidification of the weld center,and the solidification structure changed from columnar dendrites to equiaxed crystals.The weld microstructure near the side of the carbon steel fusion line changed from cell crystal to columnar dendrite,and the solidification mode of the molten pool exchanged from AF mode to FA mode.The cooling rate of the fusion transition boundary is slowed down,the time of carbon steel in the solid austenite phase increased when the cooling rate of the fusion transition boundary slowed down and the migration distance of the type II boundary increased rapidly.When the temperature reached 274 °C,the migration distance of the type II boundary increased slowly.Martensite structure could be clearly observed near the carbon steel side fusion line.When the preheating temperature was increased from room temperature to 196 °C,the thickness of the martensite layer gradually decreased.As the preheating temperature continued to increase,the thickness of the martensite layer slowly increased.