As the living standard of human beings continues to improve, people’s demand for energy has increased dramatically, and the pressure on the environment continues to increase due to the massive consumption of traditional energy sources, the development of nuclear energy, a clean energy source, has become one of China’s energy strategies.
In order to realize the technological innovation, China introduced the third generation of nuclear power plants – AP1000 nuclear power plants, which use shielded motors. Since most of the nuclear power plants are built near the coast, the tiny chlorine particles and salt spray in the ocean atmosphere have a corrosive effect on the surface of metal parts
In order to ensure the safety of nuclear power, nuclear power system must have high corrosion resistance, therefore, in the nuclear power key components manufacturing a large number of Hastelloy and stainless steel and other corrosion-resistant materials. AP1000 nuclear main pump stator shielding sleeve using Hastelloy Hastelloy C276, with the connected end cap part of the material is used 304 stainless steel. In the motor pump parts production will be a large number of welding methods, the Hastelloy C276 alloy and its 304 stainless steel using traditional melt welding methods such as TIG welding, due to the high heat input welding, welded joints heat-affected zone is wider, the weld grain is coarse and easy to generate harmful phases, making the weld corrosion resistance is poor, affecting the service life of the material’s weakest area, the need to seek a more advanced welding Welding method.
Laser welding is an emerging welding method, due to the concentration of energy and high density, low heat input and other characteristics, in the welding of nickel-based alloys, stainless steel and other materials show great advantages, by domestic and foreign scholars are widely concerned about the emergence of nickel-based alloys and their stainless steel laser welded joints of microstructure and its mechanical properties of a large number of studies, but the joints of the corrosion resistance of the joints is rarely mentioned, the lack of systematic research.
In this paper, we will use pulsed laser welding method to prepare well formed Hastelloy C276 alloy/Hastelloy C276 alloy and Hastelloy C276 alloy/304 stainless steel welded joints, using metallographic observation, scanning electron microscope observation (SEM) and energy spectrum analysis (EDS) and other methods of observation of laser welded joints of the microstructure and elemental distribution, analysis of its microstructural morphology and the law of elemental distribution; respectively from the Intergranular corrosion, electrochemical corrosion and stress corrosion three aspects of the corrosion resistance of welded joints, combined with microstructure analysis of the corrosion mechanism, the results show that.
(1) welding parameters for 78A, 7.5ms, 9Hz and 190mm/min Hastelloy C276/Hastelloy C276 pulsed laser welding head flat, well formed, no obvious heat-affected zone. The weld organization under the action of laser welding, including columnar crystals, cytocrystals and isometric crystals, grain size is obviously refined, compared with the base material grain size of 20-30μm, the minimum grain size of the weld is 2-3μm. spectral scanning of the weld, found that there is a slight elemental segregation, the grain boundaries at the Mo element content is slightly reduced.
(2) Intergranular corrosion test results show that there is a clear trend of intergranular corrosion at the weld, mainly Mo-poor phenomenon at the weld grain boundary. Electrochemical corrosion test shows that the neutral, acidic and alkaline solution of Hastelloy C276 alloy / Hastelloy C276 alloy weld corrosion resistance are better than the base material, mainly with the weld grain refinement. In neutral solution, Hastelloy C276 alloy weld selective corrosion, scratches corrosion is obvious, the rest of the corrosion is shallow; in acidic solution, Hastelloy C276 alloy and weld intergranular corrosion occurs, is the Mo element segregation caused by the alkaline solution, Hastelloy C276 alloy and weld corrosion are pitting corrosion, and lattice defects related to Hastelloy C276 alloy weld corrosion resistance is better than the base metal, mainly with the weld grain refinement. Hastelloy C276 alloy laser welded joints slow strain rate tensile results show that in air and corrosive solutions in the specimen did not fracture, the mechanical properties of the joints are good, but corrosive solutions in the joints of the elongation and shrinkage decreased significantly, indicating that the corrosive solution has an impact on the mechanical properties of the material.
(3) Hastelloy C276 alloy and 304 stainless steel in the pulsed laser welding parameters of 75A, 7ms, 8Hz and 180mm/min welding, welded joints are well formed, the weld presents a swirling, the organization includes columnar crystals, equiaxial crystals, grain size than the parent material is significantly refined. In the welding process, the redistribution of alloying elements, elemental segregation phenomenon, but the weld did not form precipitation phase. (4) Hastelloy C276 alloy / 304 stainless steel weld intergranular corrosion test results show that the Hastelloy C276 alloy corrosion resistance is the best, followed by the weld, the worst 304 stainless steel. Hastelloy C276 alloy, 304 stainless steel and its weld electrochemical corrosion results show that the corrosion resistance of neutral and acidic solution size of Hastelloy C276 alloy > weld > 304 stainless steel, alkaline solution for the weld > 304 stainless steel > Hastelloy C276 alloy > 304 stainless steel > weld. 304 stainless steel>Hastelloy C276 alloy. In neutral solution, 304 stainless steel local corrosion, weld uniform corrosion; in acidic solution, 304 stainless steel and weld are intergranular corrosion, which is related to elemental segregation, weld due to fine grain, the overall corrosion uniformity; in alkaline solution, 304 stainless steel and weld surface has a large and deep corrosion pits, which is related to passivation film defects, in which the weld corrosion pits are mainly concentrated in the lower The corrosion pits of the weld are mainly concentrated in the lower surface, which is related to the distribution of elements.
In the slow strain stress tensile test, air and corrosive solution in the Hastelloy C276 alloy / 304 stainless steel joint specimens fracture time is 64h and 50h, respectively, corrosive solution specimens in the elongation and section shrinkage are lower than the air, the solution has a certain effect on the stress corrosion susceptibility of the joints, the type of fracture belongs to the ligamentous fossa fracture and along the crystalline fracture mix fracture.
Post time: Aug-05-2023
