Research On The Automatic Welding Process And Equipment Development Of AP1000 Nuclear Galvanized Duct

Galvanizing on the surface of the steel plate is an efficient way to enhance the corrosion resistance of the steels.Up to now,about 50% of the world’s zinc is used for surface galvanization of the steel plates.Currently,the thickness of galvanized steel used in the ventilation pipe of nuclear power plant,such as the third generation nuclear power plant AP1000,is 3 times thicker than that of the galvanized steel used in common construction industry,refrigeration industry,automobile industry,etc.Otherwise,because the life cycle of the unit is designed up to 60 years,a higher product quality is required for ventilation pipes.However,the galvanized steel is very unfavorable for welding.As a result,it is urgent to improve the construction environment of the galvanized duct welding,improving the construction efficiency and the technical level of the galvanized duct welding.Firstly,in this paper,the current situation and demand of nuclear power field wind pipe construction is investigated,and automatic gas shielded arc welding,automatic tungsten argon arc welding and automatic plasma arc welding is adopted to conduct automatic welding comparison tests for 1.6mm,2.7mm and 3.5mm respectively.The results show that GMAW can not guarantee the quality of welding automatic process of 2.7mm and 3.5mm in the whole weld galvanized plate.GTAW can obtain both front and back welds,but the evaporation and ionization of coating will be accelerated by the burning of tungsten.As filled with plasma arc welding wire with automatic fillet welding,the good welds can be obtained without removing the galvanized layer,which satisfies the technical requirements,and the plasma arc welding was considered to be suitable for automatic welding.According to the plasma arc welding process,the main points and the formation of duct welds,the automatic plasma arc welding plane prototype machine is designed and developed.The influence of different process parameters on the forming of galvanized steel were studied.The results showed that,with the increase of ion gas flow rate and welding speed,the height and width of the weld seam gradually decreased.As the wire feed speed gradually increased,the weld width gradually increased and the remaining height decreased.When the height of the welding torch was too large,the defects had appeared such as undercutsm.The weld formation was deteriorated.And,a mathematical model was established between the welding process parameters and the weld shape by adopting response surface methodology.The results showed that,when the torch height changes,the seam height in response to a change in maximum,the wire feeding speed is affected by it.The weld width is most affected by the welding speed,the wire feeding speed is influenced secondly,and the influence of height of the welding gun is the least,which showed that the weld shape prediction value and the actual measured values were coincided alsoly.The process parameters can be optimized by the established model.When the welding current is 160 A,the good weld appearance of plasma arc welding is obtained at the welding speed of 230~235mm/min,and the wire feeding speed of 230~232cm/min,as the welding torch height is 6.0mm,the shielding gas flow rate is 12L/min,and the ion gas flow rate is 3.5L/min.The mechanical properties of welded joints under optimized process parameters were tested,and the micro-structure and properties of joints were analyzed by means of modern analysis methods such as SEM,EDS and XRD.The results showed that the weld mainly consisted of ferrite and pearlite,which is mainly mainly in needle form or block form.With the increase of welding speed and wire feeding speed,the tensile strength of joints increases firstly and then decreases.The hardness of the weld zone and the heat affected zone is higher than that of the parent material.The weld surface is compacted,and there is no defects and no pores on the surface.The top surface of welds is wide and smooth,but the back of the weld is narrow.Furthermore,the surface of the weld contains only 0.74% Zn elements.Under the optimized process parameters of welding of galvanized sheet,the zinc did not obviously adverse on quality of the weld.