| With the increasingly fierce market competition,the petrochemical industry's demand for large-scale equipment has increased sharply.The large-scale equipment makes the wall of the cylinder thicker and the diameter becomes larger,and the types of steel to be connected are increasing.In addition,the pressure vessel equipment is mostly applied in the high temperature,high pressure,large load,strong corrosion,hydrogen and other complex working environment.The above requirements put forward severe challenges to the manufacturing of large pressure vessels.Using different steels in different sections can maximize the performance of materials and reduce production costs,so the welding technology of different steels has become a very important production process.The welded joints are prone to uneven composition,unstable microstructure and even cracks since the difference of composition,physicochemical properties and microstructure for different kind steels.In view of the above technical problems in dissimilar steel welding,it is urgent to put forward an advanced welding process,study the microstructure and mechanical properties,corrosion resistance and related laws of dissimilar steel welding joints,reveal the related phenomena and mechanisms,this topic will play an important theoretical guiding role in the practical application of dissimilar steel welding technology.Aiming at the basic scientific and technical problems of S30408 stainless steel and 12Cr2Mo1R low-alloy heat-resistant steel plate welding,such as carbon migration and carbide precipitation.This paper puts forward a series of complete process flow including prefabrication of surfacing layer,backing and filling.First,E309L,ENiCrFe-3 and ENiCrMo-3 electrodes were used to build up a transition layer on the heat-resistant steel side,then the arc assisted active TIG welding(AA-TIG)method was used for back joing,thirdly the submerged arc welding was used to fill the groove,and finally heat treatment of welded joints for different times.The microstructure,carbon migration and carbide precipitation of the joints with different surfacing transition layers are studied.The effects of microstructure on the tensile properties,microhardness and impact properties of the joints after welding,and the effects of post weld heat treatment on the microstructure and properties of the joints are analyzed.The electrochemical corrosion properties of the joints under different processes are investigated.Finally,based on the finite element theory of crystal plasticity,a numerical model of carbide precipitation at grain boundary is established,and the influence mechanism of carbide precipitation on micro stress-strain is discussed.The main conclusions drawn from the experiment as follow:It is found that the content of ferrite in the weld is obviously higher than without surfacing transition layer.The existence of pre-edge surfacing transition layer can effectively hinder the diffusion of elements in the fusion line area,especially the diffusion of carbon.The content of ferrite in the weld decreases in welding processes of E309L,ENiCrMo-3 and ENiCrFe-3 transition layers.It is found that the thickness of martensite layer in E309L,ENiCrMo-3 and ENiCrFe-3 pre-edge surfacing transition layer decreases in turn.Through line scanning,it was found that the transition layer had a significant inhibition effect on the element dilution ratio of the weld alloy.Based on the diffusion theory,the width of diffusion layer was defined.The numerical analysis results were close to the experimental results.It is found that the content of Cr and Ni in carbide precipitated phase is lower than that of Fe.In order to clarify the influence of the surfacing layer on the mechanical properties,the mechanical properties of joints with different pre-edge surfacing transition layer and different post weld heat treatment were studied.E309L has the highest microhardness and releases a small amount of residual stress after heat treatment.When ENiCrMo-3 is used as pre-edge surfacing material,because the weld has more austenite structure,and the ferrite is evenly distributed on the austenite grain boundary,and can effectively reduce the formation of carbides,the joint with better tensile properties can be obtained.Comparing impact toughness under different welding processes,it is found that the impact toughness of welded joint obtained by pre-edge surfacing transition layer process is better,the impact toughness after heat treatment is not obviously decreased,and the dimple distribution is uniform.The electrochemical corrosion behavior of 12Cr2Mo 1R/S30408 dissimilar steel joints with different pre-edge surfacing process,heat treatment and welding speed was studied.The results show that Due to the high Cr content in E309L electrode,the corrosion resistance of E309L pre-edge surfacing joint is higher than that of ENiCrFe-3 surfacing joint.However,ENiCrMo-3 electrode has a high content of Mo element,and forms a passivation film of molybdenum oxide with strong corrosion resistance in the weld,and the activity of Mo element is better than that of Ni element and Cr element,so the corrosion resistance of pre-edge surfacing joint of ENiCrMo-3 electrode is better than that of ENiCrFe-3 and E309L electrode.The ferrite content in weld decreases with the increase of heat treatment time,and the weld shows stronger pitting corrosion resistance compared with heat treatment.The increase of welding speed leads to the decrease of Cr element in the weld,the corrosion resistance of the weld is strong when the welding speed is small.Finally,in order to explain the essential reason of the effect of carbide precipitation on the mechanical properties of the joint,based on the crystal plasticity theory,a multi grain plastic model of the stress-strain field around the carbide precipitation phase was established.The effect of the number of carbide precipitates on the stress and strain of the multi grain model was studied.The existence of carbide precipitates will cause the stress concentration in the surrounding area,the increase of the carbide precipitates makes the stress distribution in the grain more complex,and has no obvious effect on the stress distribution in the precipitated phase region,but causes the strain to increase.With the increase of carbide content,the stress concentration in local area increases,and the high stress is mainly distributed at the intersection of three grains. |
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