| Dissimilar metal welded joints(DMWJs)are widely used to joint the ferritic steel pipe nozzles of the pressure vessels to the austenitic stainless steel safe-end pipes in primary water systems of pressurized water reactors(PWRs).The running history of PWRs has shown that the interface regions in DMWJs are weak components in the primary water systems.When the crack flaws occur in interface zones during manufacturing process and in service,the mismatches in mechanical properties including strength mismatch,work hardening mismatch and intrinsic toughness mismatch between two sides of cracks can influence crack-tip stress field and plastic deformation development,thus further influence the fracture resistance behavior and crack growth paths.In order to evaluate the effect of the interface region cracks on structural integrity of DMWJ accurately,the effects of mechanical property mismatches and material constraint on fracture resistance behavior of cracks in interface regions should be investigated and understood.In this paper,the local fracture behavior of interface regions in Alloy52M DMWJs in the new generation of PWR has been studied by numerical simulations,experiments and theoretical analyses.The main work and conclusions are as follows:(1)The finite element method(FEM)based on GTN damage mechanics model was used to investigate the effects of work hardening mismatch and material intrinsic toughness mismatch on fracture resistance behavior of bi-material interface regions.The results show that material constraint effects caused by work hardening mismatch is detrimental for fracture resistance of interface cracks.This detrimental effect increases,with increasing the mismatch degree in work hardening.The effects of toughness mismatch on fracture resistance behavior of bi-material interface are related to strength mismatch.For the interface cracks with higher strength mismatch and lower toughness mismatch,the crack growth paths and fracture resistance behavior are mainly controlled by strength mismatch.While for the interface cracks with lower strength mismatch and higher toughness mismatch,the crack growth paths and fracture resistance behavior are dominated by toughness mismatch.(2)The interaction effects of geometry and material constraints on fracture resistance behavior of bi-material interfaces have been investigated.The results show that the material constraint effects on fracture resistance of bi-material interfaces is related to geometry constraint.For lower geometry constraint,the material constraint effect on fracture resistance is insignificant.Under the condition of middle geometry constraint,the material constraint effects on fracture resistance is the most significant.With further increasing geometry constraint,the fracture resistance behavior of the interfaces is gradually dominated by the higher geometry constraint,and material constraint effect becomes weaken.(3)The local failure behavior of interface region with different crack locations in a DMWJ has been examined by FEM numerical analyses.The results show that for the cracks with smaller difference of intrinsic toughness between their two sides,the crack growth path and fracture resistance are mainly controlled by the local strength and work hardening mismatches,and cracks always deviate to the material regions with lower strength and work hardening properties.For the cracks with significant difference of intrinsic toughness between their two sides,the crack growth path and fracture resistance are determined by the interaction between local mechanical(strength and work hardening)mismatches and intrinsic toughness,and the cracks tend to deviate into the material regions with lower strength and intrinsic toughness.(4)The constraint parameter Ap was calculated by finite element analyses for the cracks located at different locations in two interface regions in a DMWJ.The capabilities of the parameter Ap for characterizing material constraint have been examined.The results show that the parameter Ap can characterize material constraint effect caused by material mismatch and initial crack positions in the interface regions.Based on the Ap,the correlation lines and formula of material constraint with fracture toughness of the interface region cracks in the DMWJ can be established,and they may be used for obtaining material constraint-dependent fracture toughness for the interface region cracks.(5)The local mechanical properties and fracture resistance of cracks with different initial locations in two DMWJs and their patched welded joints(PWJs)with different manufacturing processes were measured by the mini-sized flat specimen tensile tests and three-point bending specimen fracture tests.The results show that there are small differences of macro mechanical property,plasticity and fracture toughness among the base metals and weld metals obtained by different safe-end joint designs,welding manufacturing and patch welding processes.This indicates that the manufacturing process of DMWJ without buttering layer and patch welding process have small influence on the mechanical property and fracture toughness of DMWJ,and they are feasible in actual manufacture.(6)Based on the GTN ductile damage model,the load-carrying capacity and fracture resistance behavior of two dissimilar metal welded structure(DMWS)with and without buttering layer for joining pipe-nozzle with safe end in nuclear power plants were investigated by FEM.The results show that the plastic collapse loads of the two DMWSs are principally controlled by the global yield of 316L steel with the lowest yield strength,and are not affected by initial crack positions.When the initial cracks are located in the material regions with higher yield strength in the DMWSs,the macroscopic material strength mismatch has beneficial influence for improving crack growth resistance and load-carrying capacity.While the initial cracks are located in the material regions with lower yield strength in the DMWSs,the opposite happens.The strength of the two DMWSs is essentially the same,and the 316L safe end and its welding zone with lowering yield strength in the two DMWSs are confirmed to be the weakest regions for mechanical and fracture resistance.The results also show that for the DMWS without buttering layer,the weld width mainly influences load-carrying capacity and fracture resistance behavior of cracks in the weld metal,and has no influence on those of the weakest Alloy52Mw/316L interface crack. |
PDF Full Text Request