| At present,most large high-temperature,high-pressure and pro-hydrogen equipment use Cr-Mo steel as raw material,but the same kind of material is difficult to meet the requirements in practical application.Taking advantage of materials with different performance characteristics,it can achieve materials saving and manufacturing costs reducing while meeting requirements in the different service condition.However,due to the differences in the microstructure and elemental composition of dissimilar steels,a series of problems such as weld dilution,elemental segregation and carbon diffusion will occur during welding process,which will jeopardize the reliability of the welding joint.Currently,for large and super-large equipments,dissimilar steel welding generally uses SMAW,GTAW and other methods,but these welding methods often have a low degree of mechanization,resulting in low welding efficiency,reliability and other problems,so it is difficult to meet the increasingly high requirements of modern industry.Powder Pool Coupled Activating TIG Welding(PPCA-TIG)is a new and efficient welding method that enables one-side welding with back formation of 8 mm plates without beveling,which can significantly improve welding efficiency and achieve mechanical automation.Therefore,it is important to extend Powder Pool Coupled Activating TIG welding to the welding of dissimilar steels.In this thesis,12Cr2Mo1 R low-alloy heat-resistant steel with a plate thickness of8 mm is self-melting welded with 00Cr19Ni10 austenitic stainless steel using PPCATIG welding,and partial joints heat treatment of 690°C×8 h is performed after welding.ER309 L and ERNi Cr-3 welding wires were used for overlay welding on heat-resistant steel side,and 690℃×8 h heat treatment was performed after overlay welding.By comparing the microstructure,diffusion and migration of carbon elements,mechanical properties and corrosion resistance of joints under different processes,it is analyzed whether the quality of joints under different PPCA-TIG welding processes meets the engineering standards.The microstructure distribution of welded joints of each group shows that the weld microstructure is lath martensite and a small amount of ferrite under the self-melting welding process.The weld microstructure of ER309 L overlay process is austenite and ferrite.The weld microstructure of ERNi Cr-3 overlay process is austenite.Under different processes,carbon diffusion and migration occurred at the interface between heat-resistant steel and weld bead,and finally decarburized layer and carburized layer were formed.Among them,the ERNi Cr-3 overlay process has the best inhibition effect on carbon diffusion and migration,and can significantly reduce the width of the carburizing layer.EDS line scanning results of HAZ and weld interface of the two materials show that there are different concentration gradients between the weld and the base metal,but there is no segregation phenomenon.The joint hardness test results of each group show that the joint weld hardness distribution is relatively uniform under the welding process of each group,but there is a high hardness zone between the 12Cr2Mo1 R heat-resistant steel HAZ and the weld interface.Affected by heat treatment,the hardness of heat-resistant steel base metal,HAZ and weld generally decreases.The high temperature environment causes the HAZ hardness of 00Cr19Ni10 stainless steel to increase,but it has no effect on the hardness of stainless steel base metal.Tensile test results show that: under the process of selfmelting welding and ER309 L,ERNi Cr-3 overlay welding without heat treatment,the tensile strength of the joints performed well,which fractured in the heat-resistant steel base metal location,but after the heat treatment process,self-fusion welding and ER309 L overlay welding joints’ tensile strength and elongation drop obviously,while ERNi Cr-3 overlay welding joint’s tensile strength and elongation drop inconspicuously.The low temperature impact test results show that the HAZ impact performance of12Cr2Mo1 R heat-resistant steel is poor in the self-melting welded joint,and its fracture is a combination of ductile fracture and cleavage fracture.After the overall heat treatment,the impact value of weld and HAZ of heat-resistant steel decreases significantly with typical cleavage fracture morphology.The weld of the joint with overlay process and the heat-resistant steel HAZ have good low temperature impact toughness.After heat treatment,the low-temperature impact toughness does not decrease significantly,and can meet the relevant industry standards.The impact value of 00Cr19Ni10 HAZ zone of each joint has no significant decrease compared with stainless steel base metal.The corrosion test results show that the weld corrosion resistance of 00Cr19Ni10 stainless steel base metal and four groups of welding process is excellent,and the use of transition layer effectively ensures the weld intergranular corrosion resistance.Under the same welding process parameters,the corrosion resistance of the joint mainly depends on its microstructure and element composition.The pitting corrosion resistance of the welds obtained under the four processes is excellent,in which the weld corrosion resistance of ER309 L overlay process is better than the ERNi Cr-3overlay’s.To sum up: the use of PPCA-TIG welding can achieve one-side welding with back formation of 8 mm thick 12Cr2Mo1 R heat-resistant steel and 00Cr19Ni10 stainless steel in the case of unbeveled,greatly improving the efficiency of welding.The mechanical properties of the self-fusing welded joints can meet the relevant standards when they are not subjected to heat treatment process.Among the joints of the overlay welding process,the comprehensive mechanical properties and corrosion resistance of the joints of ER309 L overlay welding process are more excellent.The joints of ERNi Cr-3 overlay process can effectively inhibit the diffusion and migration of carbon elements. |
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