| At present,the welding structure of low-alloy heat-resistant steel and stainless steel can meet the requirements in different working conditions,reduce production costs,and has been widely used in the production of high-pressure vessels.However,there are many differences between low-alloy heat-resistant steel and stainless steel in the microstructure and chemical composition.It is easy to induce some defects such as metal dilution,alloy element segregation,carbon migration and other damage to the quality of the joint.Moreover,traditional welding methods such as manual tungsten argon arc welding is usually used for welding,which is inefficient,and it is difficult to meet the needs of the rapid development of modern industry.Arc-assisted active TIG welding(AA-TIG)is an efficient welding method with deep penetration and easy to implement automation.It can realize single side welding and double side forming without groove for 3-12 mm thick materials,with high welding efficiency and automation.Therefore,the application of AA-TIG to the welding of dissimilar steels has important practical significance for improving the welding efficiency and the quality of welded joints.In this thesis,8 mm thick 12Cr2Mo1 R low-alloy heat-resistant steel and 06Cr18Ni11 Ti austenitic stainless steel are welded in a butt configuration.After welding,a heat treatment is performed for the whole weld plate at 690℃×8 hours.Another welding process: After the transition layer,heat treatment is performed for 12Cr2Mo1 R at 690℃×8 hours,then,AA-TIG weld dissimilar steel was carried out.The microstructure,carbon migration,diffusion,mechanical properties,and corrosion resistance of the joints were compared and analyzed.According to the test results,analyze whether the joint quality of different AA-TIG welding processes meets the actual engineering requirements.The microstructure analysis of the welded joint shows that the weld structure of self-fusion welding is the lath martensite and a small amount of ferrite.The weld of ERNiCr-3 surfacing process is a single-phase austenite.The weld seam microstructure of ER309 L surfacing process are austenite and ferrite.T he micro-cracks are generated in the weld area of self-welding after heat treatment,which will reduce the mechanical properties of the joint.There is a certain degree of carbon migration and diffusion at the interface of 12Cr2Mo1R/weld.ERNiCr-3 pre-edge surfacing can reduce the width of carburized layer compared with self fusion welding and ER309 L pre-edge surfacing.Scanning of interface elements from the both sides of the heat-affected zone(HAZ)to the weld shows that there is a certain concentration gradient of alloy elements such as Fe,Ni,Cr between the weld and the base metal on both sides.Mechanical testing of welded joints show that the hardness of the joints is related to microstructure and heat treatment.The hardness of ERNiCr-3 and ER309 L pre-edge overlay welding is significantly lower than that of self-fusion welding.Heat treatment reduces the hardness of the 12Cr2Mo1 R HAZ hardness of the self-fusion welding and ER309 L pre-edge overlay welding processes.For the ERNiCr-3 pre-edge overlay welding process,the heat treatment increases the hardness of the 12Cr2Mo1 R heat-affected zone.The hardness distribution of the 06Cr18Ni11 Ti side of different process joints is basically the same.The tensile strength and elongation of the self fusion welded joint are 654.35 MPa and 10.91% respectively,which are broken in the base metal of 12Cr2Mo1 R side.The ER309 L pre-edge overlay welding process has the highest tensile strength and elongation of the joint in the welded state,and it breaks on the 12Cr2Mo1 R side base material.The tensile strength of the ERNiCr-3 pre-edge overlay welding process is lower than that of the ER309 L pre-edge overlay welding process.Heat treatment reduces the tensile strength and elongation of the joints of the self-fusion welding.The tensile strength and elongation of joints heat-treated after ER309 L pre-edge surfacing have decreased.but the tensile strength and elongation of joints heat-treated with ERNiCr-3 pre-edge overlay welding were increased by 7.52% and 8.11%,respectively.At-20℃,the impact value of the self-fusion weld and the 12Cr2Mo1 R HAZ after heat treatment significantly decreased,and they were brittle cleavage fractures.However,the impact value of the pre-surfacing weld and the 12Cr2Mo1 R HAZ were higher th an the standard requirements.The fracture morphology is ductile fracture;the 06Cr18Ni11 Ti HAZ impact value of the six joints is above 120 J,which is not significantly lower than that of the base material.Corrosion resistance tests on the joints of ERNi Cr-3 and ER309 L pre-edge overlay welding process show that the 06Cr18Ni11 Ti base metal and joints have good resistance to intergranular corrosion,pitting corrosion and electrochemical corrosion.In summary,AA-TIG welding can realize single side welding and double-sided forming of 8 mm thick 12Cr2Mo1R/06Cr18Ni11 Ti dissimilar steel.When the self-fusion joint is not heat treated,the mechanical properties of the joint meet the industrial standards.The mechanical properties and corrosion resistance of the welded joint with ER309 L pre edge surfacing process are the best.The heat treatment can be adopted after the ERNiCr-3 pre-edge surfacing process to improve the joint strength. |
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