| With the continuous improvement of human living standards,people's demand for energy has increased substantially,and the huge consumption of traditional energy has continued to increase the pressure on the environment.Therefore,to vigorously develop clean energy-nuclear energy has become one of China's energy strategies.China has introduced the third generation of nuclear power plant-AP1000 nuclear power plant in order to achieve technical innovation,which uses shielded motor.While nuclear power plants are mostly built near the coast,the metal parts would be corroded by small chloride particles and salt mist in the atmosphere.Nuclear power system must have high corrosion resistance to ensure the safety of nuclear power.Therefore,a large number of materials with good corrosion resistance,such as Hastelloy and stainless steel,are used in the manufacturing of nuclear power key parts.The stator shield sleeve of AP1000 nuclear main pump is made of Hastelloy C-276,and the end cover connected to it is made of 304 stainless steel.Welding would be used to product the motor main pump parts.Traditional fusion welding methods such as TIG welding with high heat input are used for C-276 alloy and 304 stainless steel,and wide heat affected zone and coarse grain of weld were got,and harmful phase is formed to make the corrosion resistance of welding joint poor,which has become the weakest area affecting the service life of materials.Therefore,it is necessary to seek for more advanced welding methods.Laser welding is a new welding method due to the characteristics of high energy concentration,high energy density and low heat input,it has shown great advantages in the welding of nickel base alloy,stainless steel and other materials,and has attracted more and more attention of researchers,and there are appearance of a lot of researches on the microstructure and mechanical properties of laser welded joints of nickel base alloy and stainless steel,but systematic research of the corrosion resistance of the joints is lacking.In this paper,the well formed C-276 / C-276 and C-276 / 304 pulse laser welded joints were prepared.The microstructure and element distribution of laser welded joints were observed and analyzed by metallographic observation,scanning electron microscopy(SEM)and energy dispersive spectroscopy(EDS).The corrosion resistance of the welded joints was studied in three aspects,and the corrosion mechanism was analyzed.The results showed that:(1)The C-276 / C-276 pulsed laser welded joint with welding parameters of 78 A,7.5ms,9Hz and 190 mm / min is well formed and has no obvious heat affected zone.The weld structure includes columnar crystal,cellular crystal and equiaxed crystal,and the grain size is obviously refined.Compared with the base metal grain size of 20-30 ?m,the minimum grain size of the weld is 2-3 ?m.It is found that there is slight element segregation and the content of Mo element in the grain boundary in the weld is slightly reduced.(2)The results of intergranular corrosion test show that there is an obvious tendency of intergranular corrosion of the weld,mainly due to the phenomenon of Mo deficiency at the weld boundary.The electrochemical corrosion test shows that the corrosion resistance of C-276 / C-276 in neutral,acid and alkaline solution is better than that of the base metal,which is mainly related to the refinement of weld grain.In the neutral solution,the weld of C-276 alloy and C-276 / C-276 has selective corrosion,with obvious scratch corrosion and shallow corrosion in the rest part;in the acid solution,intergranular corrosion occurs between C-276 alloy and the weld,which is caused by Mo segregation;in the alkaline solution,C-276 alloy and weld happened pitting corrosion,which is related to lattice defects.The results of slow strain rate tensile test of C-276 / C-276 laser welded joint show that there is no fracture in the air and corrosion solution,and the mechanical properties of the joint are good.The elongation and shrinkage of the joint in the corrosion solution decrease obviously in the solution,which shows that there is an impact on the mechanical properties of the material.(3)C-276 alloy and 304 stainless steel were welded under the parameters of 75 A,7ms,8Hz and 180mm/min by pulse laser welding.The welding joint was well formed,and the weld showed vortex shape.The alloy elements are redistributed,and there is element segregation,but no precipitate is formed in the weld.(4)The results of intergranular corrosion test of C-276 / 304 showed that the corrosion resistance of C-276 alloy is the best,followed by weld,and the worst is 304 stainless steel.The results of electrochemical corrosion showed that the order corrosion resistance of C-276 alloy,304 stainless steel and the welds in neutral and acid solutions is C-276 alloy > weld > 304 stainless steel,and that is weld > C-276 alloy > 304 stainless steel in alkaline solutions.304 stainless steel happened local corrosion,and the welds were uniformly corroded in the neutral solution;304 stainless steel and the weld occurred intergranular corrosion in the acid solution,which is related to the element segregation;and 304 stainless steel and the welds surface have large and deep corrosion pits in the alkaline solution,and the corrosion pits of the weld are mainly concentrated in the bottom side,which related to element distribution.The results of slow rate stress test showed that the fracture time of C-276 / 304 joints in air and solution is 64 h and 50 h respectively.The elongation and area shrinkage of the sample in solution are lower than those in air.The solution has a certain influence on the stress corrosion sensitivity of the joint.The fracture types are mixed fracture of dimple fracture and intergranular fracture. |
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