The Local Mechanical Properties And Fracture Behavior Of Nuclear Safe-end Dissimilar Metal Welded Joint

Dissimilar metal welded joints (DMWJs) are widely used to join 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). Based on the requirements of design and use for the DMWJs, multilayers Nickel-base alloy are generally pre-deposited on the ferritic nozzle face firstly, then welding is carried out between the buttering layer and the austenitic safe-end pipe with Nickel-base alloy. The running history of PWRs shows that the DMWJs are weak components in the primary water systems. As a result, the DMWJs are regarded to be the key attention structure. In order to develop the advanced technique on integrity design and assessment for the complicated multi materials welded joint, local fracture behavior for the cracks in different position and for different depth cracks, local mechanical properties and local microstructures of the DMWJ should be investigated and understood. In this paper, Alloy52M DMWJ of the new generation PWR was studied on these issues above. The main work and conclusion were as follows:(1) The microstructures and local mechanical properties (strength, ductility, stress-strain behavior and hardness) were characterized by the microstructure observation and tensile tests of mini-sized flat specimens. The results show that in the interface regions between materials, there exist complex microstructures and drastic change in local mechanical properties. The heat flow and element migration during welding process leads to these complex microstructures and local distributions in mechanical properties. The heterogeneous mechanical distributions cause local mechanical mismatches, and which will affect the crack-tip fracture mechanics parameter, plastic deformation behavior, local fracture resistance and crack growth behavior. This needs to be considered in the integrity assessment of the DMWJs.(2) The finite element method (FEM) based on GTN model was used to investigate the ductile crack growth behavior in three-point bending specimens of a simplified DMWJ composed of four materials. Results show that the initial cracks with different locations and different depths in the DMWJ have different crack growth resistances and growth paths. When the initial crack lies in the centers of the buttering and weld, the crack-tip plastic and damage zones are symmetrical, and the crack grow path is nearly straight along the initial crack plane. While for the interface cracks between materials and near interface crack, the crack tip plastic and damage zones are asymmetric, and the crack growth path has significant deviation phenomenon. The crack growth tends to deviate into the material whose yield stress is lower between the two materials on both side of the interface. The different initial crack locations, crack depths and strength mismatches between different materials in the DMWJ affect the local stress triaxiality and plastic strain distributions in front of crack tips, and lead to different ductile crack growth resistances and growth paths.(3)The crack initiation and growth process in notched specimens made from a nuclear dissimilar metal welded joint (DMWJ) were investigated and analyzed by using in-situ tensile tests via SEM. The fracture is dominated by ductile void fracture mechanism, but there exists local brittle fracture in a few locations. The cracks in the specimens with inhomogenous material generally initiate and propagate in the materials with lower yield stress. There is great difference in the load-carrying capacity and toughness for the specimens with different initial notch locations in the DMWJ. This is caused by the local mechanical properties, the strength mismatch and crack growth resistance of the material ahead of notches.(4) An experimental investigation on the local fracture resistance and crack growth behavior in an Alloy52M DMWJ has been carried out by using three-point bending specimens. The local J-resistance curves and crack growth paths of13cracks located at various positions in the DMWJ were determined, and GTN parameters were calibrated for the13cracks. The effects of the local strength mismatch on local fracture resistance, crack growth paths and integrity assessment for the DMWJ were analyzed. The results show that the cracks always deviate to the materials with lower strength, and the crack path deviations are mainly controlled by the strength mismatch, rather than toughness mismatch. The crack extension resistance curve with larger crack path deviation only reflect the apparent fracture resistance along the crack growth region, rather than the intrinsic fracture resistance of the material at the initial crack-tip region. Without considering the local fracture resistance properties of heat affected zone (HAZ), interface and near interface zone, the use of the J-resistance curves of base metals or weld metals following present codes will unavoidably produce non-conservative (unsafe) or excessive conservative assessment results. In most cases, the assessment results will be potentially unsafe.(5) Both fracture tests and microscopic observations were carried out to investigate the fracture mechanism of different regions within the DMWJ, and the relationship between fracture mechanism in local microstructures and local fracture resistance of the DMWJ was analyzed. The results show the fracture mechanism of A508and316L base metals and heat-affected-zone (HAZ) of316L is typical ductile fracture of nucleation, growth and coalescence of voids, and different crack growth resistance is mainly related to microstructures of them. The fracture mode in A508HAZ and A508/52Mb interface region with predominant martensite microstructure is mixed brittle and ductile fracture, which leads to lower crack growth resistance. The columnar austenite crystal orientation relative to main crack growth direction apparently affects fracture mechanism and crack growth resistance of buttering Alloy52Mb and weld Alloy52Mw. The cracks in Alloy52Mb propagate across columnar crystal boundaries in a ductile mode, and higher fracture resistance is produced. While the cracks in weld Alloy52Mw propagate along the weak columnar crystal boundaries in a brittle mode, which leads to lower crack growth resistance.(6) The plastic deformation behavior, local fracture resistance, crack growth path and fracture mechanism were remarkably affected by the local mechanical properties and their mismatches. It is recommended to obtain and use local mechanical and fracture properties of all regions of the DMWJ if the complex local mismatch situation is a concern. And new integrity assessment methods based on local damage and fracture models also need to be developed for the DMWJs.