Study On Application Of Welding Process For Safety End Of Nuclear Island Main Equipment

The electricity demand is growing rapidly thereupon with the rapid growth of the national economy.Currently,the market is given priority to thermal power generation.The thermal power generation is based on coal as an energy source,which has its irreversibility as a non-renewable energy resource.As a new type of clean energy,the nuclear energy power generation is mainly characterized by high efficiency and less pollution,and has aroused enormous interests all over the world.At present,in the nuclear power market,due to the higher degree of participation by the design unit,all design types of reactor and any differences between the design requirements,it is necessary to take any measures for study of manufacturing process for nuclear island main equipment,in order to comply with the development trend of the market,improve the level of technological development of nuclear power equipment manufacturing industry,and enhance the market competitiveness.In the pressurized water reactor(PWR)nuclear island system,the main equipment will be connected to the different equipment by pipelines to form a loop system,so that there are a lot of welded joints between the connecting pipe and the safety end of nuclear island main equipment.The operating experience and history of nuclear power plant show that,the dissimilar metal welded joint between the connecting pipe and the safety end shall be considered as the weak link in the entire primary side loop system.Any failure of this critical joint might result in any leakage of radioactive water medium,water loss of nuclear reactor pressure vessel,overheated melting of reactor core and other major accidents,or great impact on the environment and safety operation of nuclear power plant.This paper focuses its first study on the structural characteristics of safety ends of second-generation improved CPR1000,new one-generation(three-generation)AP1000 and EPR PWR nuclear island main equipment.This paper carries out the systematic study on the welding performance and physical properties of low alloy steel,stainless steel and nickel-based alloy materials in the RCC-M and ASME nuclear power standard systems.This paper carries out the study on the application of welding process for different types of safety-end joints.By analyzing the weldability of SA508Gr3Cl2,16MND5 and 18MND5 low-alloy steels for nuclear island main equipment,this paper analyzes the cold cracking susceptibility of such low-alloy steels by means of the carbon equivalent and cold cracking susceptibility coefficient methods,respectively;this paper analyzes the hot cracking susceptibility of suchlow-alloy steels by means of the hot cracking susceptibility coefficient method;this paper analyzes the lamellar tearing susceptibility of such low-alloy steels by means of the lamellar tearing susceptibility index method.The research results show that,among the SA508Gr3Cl2,16MND5 and 18MND5 low-alloy steels,the cold cracking tendency and hot cracking tendency of SA508Gr3Cl2 low-alloy steel are the largest;the cold cracking tendency and hot cracking tendency of 16MND5 and 18MND5 low-alloy steels are equivalent.The higher the content of C,S,P and Si in the alloy,the greater the hot cracking susceptibility.The higher the content of Mn in the alloy,the smaller the hot cracking susceptibility.There is a certain lamellar tearing susceptibility of SA508Gr3Cl2,16MND5 and 18MND5 low-alloy steels.By analyzing the weldability of Z2CN18.12N2 and SA182F316 LN low-alloy steels for nuclear island main equipment,this paper analyzes the chemical composition of such low-alloy steels to confirm that the above two materials belong to the 18-12 type austenitic stainless steels.Generally,the alloying extent of such materials is relatively low.If such materials contain a small amount of δFe(about 5%),the resistance to hot cracking of welded joint can be greatly improved.By means of the method for converting the chemical composition into the nickel equivalent and chromium equivalent,this paper analyzes the composition of such materials by using the Schaeffler and Delong graphs.The research results indicate that,the Z2CN18.12N2 and SA182F316 LN low-alloy steels belong to the austenitic steels containing a small amount of ferrite structures,which have a good resistance to hot cracking susceptibility.The main reason for the intergranular corrosion of stainless steel is that,the chromium element is combined with the carbon element on the grain boundary combine to form the carbide precipitation,which leads to the rapid reduction of chromium around the crystal to 12.5% or less to reduce the corrosion resistance.The tendency of intergranular corrosion of stainless steel is related to the carbon content in steel.When the carbon content is 0.02%-0.03% lower than the solubility of austenitic steel at room temperature,it is possible to avoid the occurrence of intergranular corrosion.By analyzing the chemical composition of stainless steel,it can be found that the stainless steel materials for nuclear island main equipment are the ultra-low carbon stainless steel materials,which will further reduce the possibility of intergranular corrosion of nuclear power materials.The 690 nickel-based materials for nuclear power are the nickel-chromium-iron-based corrosion-resistant alloys.Such alloys are solidified into austenite,leading to the strong segregation of alloying elements and impurity elements.Such segregation may affect the solidification cracking susceptibility,and can promote the embrittlement in the PWHT process.Such alloys are also susceptible to the liquefaction cracking of HAZ and weld metal,due to the grain boundary segregation in HAZ and the residual solidification segregation in the reheated weld metal.There is a difference between the physical properties of the materials for nuclear island main equipment.The data analysis shows that,compared to the low-alloy steel,the nickel-based alloy and stainless steel should be mainly characterized by: Larger coefficient of linear expansion,smaller thermal conductivity,higher melting point and higher density,lower strength but higher toughness.Regardless of chemical composition or physical properties or mechanical properties,there is a large difference between the low alloy steel and the nickel-based alloy & stainless steel.According to the structural characteristics of the three safety ends,this paper analyzes and studies the adaptability of shielded metal arc welding,automatic submerged arc welding and automatic tungsten argon arc welding to the safety end welding of nuclear island main equipment.It carries out the systematic analysis and study of the analysis related method for factors to welding process,technological characteristics,technological process and other aspects.And the parameters of automatic tungsten argon arc build-up welding on the isolated layer of safety end,automatic submerged arc butt welding at the safety end and automatic tungsten argon arc butt welding are developed.By test for simulation of dissimilar metal welding at the safety end,this paper carries out a series of tests and studies according to the nuclear power design and manufacturing standards,e.g.chemical composition analysis,tensile,bending,impact and other mechanical property tests,intergranular corrosion test and macro & micro metallographic analysis,etc.The results show that,the chemical component and structure of welded joints at the safety end as obtained through the three welding processes are normal,and that the mechanical properties can meet design requirement of the power operation.