Microstructure Characteristics And Properties Of Thick-wall Stainless Steel With Narrow Gap Of Nuclear Power Rcl Piping By TIG Welding Process

As a safe,clean and efficient resource,nuclear power is an important part of China's power industry.However,in the main pipeline of AP1000 nuclear power plant,the radioactive coolant--boron water flowing directly around the core fuel rod of pressure vessel under high temperature and pressure,as the flow medium,it is very likely to leak and cause serious harm to human health and social ecological environment,with unimaginable consequences.The thick wall of main pipeline,narrow welding space,high requirements for all-position welding and welding quality are also the most important welding parts in the installation and welding process of nuclear island mechanical equipment and pipelines in nuclear power plants,which are directly related to the operation safety of nuclear power units.Therefore,this paper adopts TIG automatic welding technology to carry out narrow gap welding for AP1000 nuclear power main pipeline,and studies and discusses the micro structure and performance of narrow gap welding seam and the method of weld repair by improving the pipe groove design and welding process parameters.Firstly,the measurement and 3D modeling of the weld groove of the main pipeline were carried out by using the laser tracker.Special processing equipment and process methods were adopted to process the inclined groove and eccentric groove of the main pipeline,so as to obtain the high-precision groove meeting the requirements of narrow gap welding group and welding of the main pipeline.By narrow gap TIG automatic welding simulation experiments,using laser measuring instrument and dial indicator on the main line of welding real-time accurate detection,dynamic tracking measurement deformation,welding contraction deformation data,master the deformation law,combining with the mai n line welding,pressure vessel installation and install evaporator,the main channel of to establish reasonable welding sequence to maximize reduce welding stress.Second,ER316 was used as the filling material,and the narrow gap cold wire TIG welding process and equipment were used to weld the 83 mm ultra-low carbon austenitic stainless steel TP316 LN specimen.The transverse tensile strength of the welded joint varied between 575mpa-607 mpa,and the fracture location was distributed at the parent metal side,the heat affected zone and the welding seam,ultimate tensile strength meet the project acceptance criteria and 60 MP a-92 MPa higher than the acceptance standard;The longitudinal yield strength changes between 505MPa-518 MPa,and the ultimate tensile strength changes between 622MPa-637 MPa.Both yield strength and ultimate tensile strength meet the engineering acceptance standards.The elongation after fracture is 30% higher than the acceptance standard.The micro structure of the tensile joint is distribu ted as tiny dimples,which is a typical ductile fracture.After the transverse lateral bending test,no cracks appeared on the surface of the weld,and the joint had a strong ability to resist inter granular corrosion.Finally,the pipe test pieces with narrow gap automatic welding,circumferential welds with excessive defects and TP316 LN material were selected for research,and the automatic welding process was selected to repair the defective welds.The location and depth of the defective welds were determ ined through ray and ultrasonic detection of the defective welds.Groove machining machine was used to remove the defects,and the causes of the defects were analyzed.By taking measures to eliminate the causes of defects,after re-welding,qualified weld.