Investigation On The Effect Of Micro-region Mechanical Heterogeneity Of Welded Joints On Fracture Behavior In Nuclear Thick-walled Steel

The ‘14th Five-Year Plan' has clearly defined the status and role of nuclear power in China's clean,low-carbon,safe and efficient energy system,and has given an important impetus to the high-quality development of China's nuclear power industry.Nuclear power large-thickness welded structures are usually manufactured using multilayer multi-pass narrow gap welding technology,so welding defects and large gradient mechanical property heterogeneities will inevitably exist in the welded joints,and thus the welded joints often become the weakest part of the entire structure.Therefore,the systematic study of the impact of mechanical heterogeneity of the welded joint microregion on the fracture behavior is of great importance to accurately evaluate the rotor operating safety and extend its service life.The micro-region mechanical properties of narrow-gap welded joints were tested using the micro-region shear method,which reflects the micro-region mechanical hetergeneity of welded joints through micro-region shear strength,stress-strain curve and failure strain.On this basis,the effect of micro-region mechanical properties on the quasi-static expansion of cracks in the three-point bending experiment of welded joints was further investigated.It was found that the simulated CTOD values matched the experimental results,and the P-V curves under the room temperature simulation did not show a sudden point of expansion without instability.The fracture toughness simulations of the cover welding layer,intermediate welding layer and TIG base welding layer showed that the CTOD values were decreasing and the fracture toughness was decreasing sequentially.For the cover layer,the fracture toughness of the heat affected zone is better than the fracture toughness of the weld.Tensile tests were simulated on compact tensile specimens,and the effect of mechanical property heterogeneities on crack extension was investigated considering material nonlinearity,yield strength,and strain effects.It was found that the maximum equivalent plastic strain occurs at the crack tip,and high yield strength and failure strain prevent crack traversal.The initial crack extension depends mainly on the magnitude of the yield strength,and the instantaneous crack extension depends mainly on the magnitude of the failure strain.When the yield strength of the micro-region material located in the notch is less than the material on both sides,the crack expands forward along the intermediate path.When the yield strength of the micro-region material located in the notch is in the middle value,the crack expands to the side of the material with lower yield strength.When the yield strength of the micro-region material located in the notch is higher than the material on both sides,the crack expands in the direction of the large yield strength gradient.Numerical simulation of nuclear power welded rotor steel welded joints mechanical properties of micro-region heterogeneity on the impact of high cycle fatigue life,low cycle fatigue life.It was found that the trend of the equivalent force distribution and fatigue life distribution trend into a mirror image distribution.For high cycle fatigue,the micro-region equivalent force affects the fatigue life.For low cycle fatigue,the fatigue life is lowest in the micro-region with high yield strength or high yield strength gradient,especially at the junction of weld micro-region.