Study On Microstructure And Properties Of T22/800H Joints By TIG Welding

Increasing steam temperature is an effective way to improve power generation efficiency.In the field of nuclear power,high temperature gas cooled reactor steam generator is the the key equipment to improve steam temperature for the fourth generation of nuclear power.The core component of the steam generator is the heat exchange tube,and the heat exchange tube used in the fourth generation nuclear power high temperature gas cooled reactor is T22 heat-resistant steel and 800 H stainless steel.Due to the relevantly huge difference of physical properties and chemical composition between T22 steel and 800 H steel,high quality welding of T22 and 800 H becomes the key factor to restrict the production of the fourth-generation nuclear power plant.The study of dissimilar steel welding process between T22 and 800 H can enrich the theoretical system of dissimilar steel welding,and improve the basic data for the welding of steam generator,which has important theoretical significance and engineering application value.In this study,the pulse TIG welding process was chosen to weld T22 and 800 H tube.The welding torch was fixed,while flat tube was rotated.By a lot of experiments,the process parameters to obtain good weld were determined.The welding parameters were as the following:the peak current Ip was 80 A,and base current Ib was 40 A,and arc voltage U is 12.4 V,and pulse duty ratio N was 60%,and pulse frequency F was 1 Hz,and welding speed v was 12 cm/min,and argon flow of backside Q was 5 L/min,and the shielding gas was 70% He and 30% Ar while gas flow rate was 22 L/min.Three different materials which are A06,316 L,GH4169 were taken as the welding rings.After welding,the comprehensive application of optical microscope,SEM,EDS,X-ray diffraction analysis instrument are used to characterize and test composition,microstructure,phase structure,and the microhardness test,room temperature tensile strength test,tensile strength test of 550 ?and other means of mechanical joint hardness and tensile strength of dissimilar joint between T22 and 800H.The results show that when the melting ring is A06,the weld microstructure is austenite,and there is an obvious fusion line on both sides of the weld and the heat affected zone.The fusion line near T22 has obvious carbon migration layer.The heat affected zone contains the coarse and fine grain zone and uneven distribution zone.And the microstructure of heat affected zone involves acicular ferrite,pearlite and proeutectoid cementite with high hardness.The heat affected zonenear 800 H side is consist of coarse austenite grain,twin and precipitates.When the thickness of the melting ring was 1.3 mm,the total performance of the joint was good.The Rm at room temperature can reach 495 MPa,while the Rp0.2 was 398 MPa and the elongation was 13.69%.The Rm at 550 ? was 362 MPa,while the Rp0.2 was 190 MPa and the elongation rate was 7.16%.The results show that when the melting ring is 316 L,the weld microstructure is austenite,and there is an obvious fusion line on both sides of the weld and the heat affected zone.The fusion line near T22 has obvious carbon migration layer.The heat affected zone contains the coarse and fine grain zone and uneven distribution zone.And the microstructure of heat affected zone involves acicular ferrite,pearlite and proeutectoid cementite with high hardness.The heat affected zone near 800 H side is consist of coarse austenite grain,twin and precipitates.When the thickness of the melting ring was 1.0 mm,the total performance of the joint was good.The Rm at room temperature can reach 509 MPa,while the Rp0.2 was 210 MPa and the elongation was 17.54%.The Rm at 550 ? was 370 MPa,while the Rp0.2 was 160 MPa and the elongation rate was 8.22%.The results show that when the melting ring is GH4169,the weld microstructure is austenite,and there is an obvious fusion line on both sides of the weld and the heat affected zone.The weld alloy content is high,and is close to the 800 H base material.The weld alloy content is high,and is close to the 800 H base material.The austenite grains of the 800 H side fusion line have the indication of epitaxial growth,and the grain size of the heat affected zone is coarse.The fusion line near T22 has obvious carbon migration layer.The heat affected zone contains the coarse and fine grain zone and uneven distribution zone.And the microstructure of heat affected zone involves acicular ferrite and tempered pearlite with high hardness.When the thickness of the melting ring was 0.8 mm,the total performance of the joint was good.The Rm at room temperature can reach 519 MPa,while the Rp0.2 was 362 MPa and the elongation was 14.71%.The Rm at 550 ? was 342 MPa,while the Rp0.2 was 154 MPa and the elongation rate was 10.98%.Through the comparison of the experimental data and analysis,it is found that the microstructures and properties of the welded joints by the 316 L melting ring with thickness of1.0mm are better.