| Super ferritic stainless steel not only has excellent resistance to chloride stress corrosion and comprehensive corrosion resistance,but also has good thermal conductivity and low linear expansion coefficient.At present,it has been widely used in auxiliary condensers of power stations,heat exchange equipment,coastal building decoration,evaporator in diaphragm alkali system and other equipment.In the welding process,super ferritic stainless steel often has problems such as coarse joint structure and brittle phase precipitation,which affect the mechanical properties and corrosion resistance of welded joints,thus hindering its promotion and application.In order to improve the comprehensive performance of super ferritic stainless steel welded joint and expand its service field,it is particularly important to seek a kind of process parameters and wire materials that can effectively inhibit the abnormal grain growth and improve the strength and corrosion resistance of the joint.In this study,the tungsten electrode argon arc welding(TIG)method was used to prepare different welding joints by adjusting the welding heat input,Ti,Nb content and welding wire material.The scanning electron microscope,tensile testing machine,electrochemical workstation and other instruments were used to study the strong correlation between the macroscopic morphology,microstructure,mechanical properties and corrosion resistance of welded joints.It is found that the average grain size and inclusions in the weld area increase with the increase of welding heat input.During the tensile process,grain coarsening and inclusion growth induce microcrack initiation,which leads to a sharp decrease in tensile strength of the joint when the heat input is 1.143 k J/mm,and the fracture type changes from dimmpit fracture to cleavage fracture.X-ray inspection of four ferritic stainless steel welded joints with different Ti and Nb contents was carried out.The results show that the four groups of joints fuse well,and there are no defects such as porosity,collapse and bite.The microstructure shows that the main structure of the four groups of welds are columnar crystals,and the grains of the welds obtained without Ti and Nb elements are thicker.The bistabilization of base metal and wire promotes the formation of a large number of oxide and carbonitride-type inclusions in the weld area,induces an increase in the number of nonuniform nucleation,and leads to the best effect of grain refinement and increasing the proportion of equiaxed grains.But it also destroys the continuity and uniformity of the structure,so that the quality of the welded joint is affected.29Cr-4Mo stainless steel was selected as the welding wire material.It was found that the joint grain morphology obtained by S44700 welding wire was flat,on the contrary,the weld grain morphology obtained by S44735 welding wire was gradually curved,the average grain size was significantly reduced,and the inclusions in the weld were coordized.The results of mechanical tests show that the appropriate increase of Mo content in the welding wire material is beneficial to obtain a joint with a high matching degree with the base metal.Ti and Nb elements have a "double" effect on the impact toughness of Cr30Mo2 stainless steel welded joints.On the one hand,grain refinement can effectively improve the room temperature toughness of the joint;On the other hand,the generation of largesize brittle inclusions can lead to poorer low-temperature toughness of the joint.In addition,the hard brittle inclusions of the coarse-grained Al-Ti-O-N type will accelerate the corrosion rate and deteriorate the corrosion resistance.Through the optimization of alloy composition and welding process,the qualitative and quantitative analysis of grain shape,size distribution,precipitation and evolution of tensile properties of welded joints under different heat input conditions are clarified.The influence mechanism of Ti and Nb alloying on the welding performance of Cr30Mo2 super ferritic stainless steel under ultrapure condition is revealed.High Mo material is used for welding,which gives the welded joint better impact toughness and corrosion resistance.The research results open up a new technical approach and theoretical perspective for improving the welding performance of super ferritic stainless steel and broadening the application range. |
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