Research On Local Mechanical Characteristics At Crack Tip And Environmentally Assisted Cracking Growth Rate Of Welded Joints In Nuclear Power Plants

Environmentally assisted cracking (EAC) in safe-end dissimilar metal welds (DMW),which represent welded components in nuclear power plants, has become a critical issueaffecting the long-term operation and safety of pressurized water reactors (PWR).Nickel-based alloys are generally pre-deposited on the low alloy steel nozzle face to form thebuttering. Then welding is carried out between the buttering and the stainless steel safe-endpipe with nickel-based alloys such as Alloy82/182. It is very difficult to directly predict theEAC growth rate of actual welded components by laboratory data because of the effects ofDMW microstructure and mechanical heterogeneity, discontinuous geometry, welding defectsand residual stress. To develop the technique of quantitative prediction on service life andpreventive maintenance of key welded components in nuclear power plants, the localmechanical characteristics at EAC crack tips of standard fracture mechanics specimens andsafe-end DMW under different loading conditions were analyzed in this dissertation. Mainworks performed are as follows:(1) The approaches to quantitatively predict EAC growth rate of key materials in nuclearpower plants were analyzed and discussed. A prediction model of EAC growth rate infull-size DMW structures with complex operating loads and residual stress inhigh-temperature water environment was proposed.(2) According to the gradient distribution of hardness in the fusion boundary (FB) regionof an Alloy182-A533B low alloy steel DMW, a heterogeneous material model adjacent to FBregion in weld metal was established by using elastic-plastic finite element method. Thediscontinuity of stress and strain is eliminated, the effect of material properties mutation oncalculation results is reduced, and more realistic welded mechanical heterogeneity is obtainedby using this model. A new method is provided to research the influence of welded mechanical heterogeneity on EAC behavior in DMW.(3) The stress-strain field and crack driving force at crack tip on different location ofDMW specimens were simulated and discussed in details. Growing behavior of crack initiatedin Alloy182weld metal near FB line was investigated. The effect of plastic strain rate oncrack growing path was concluded.(4) Based on the established heterogeneous material model, the stress-strain fieldsaround crack fronts of DMW under operating loads were simulated. The DMW crack drivingforce was also discussed. As a result of the combined effect of local plastic strain and stresstriaxiality, the crack growth direction deviates due to the influence of material yield strength.(5) The local stress-strain fields around crack fronts of safe-end DMW were simulatedand discussed under the one single overload. And the effect of one single overload ratio onDMW crack driving force was also discussed with different crack location and depth,.(6) The local stress, strain, plastic strain rate and J-integral around the inner surface axialcrack fronts of safe-end DMW were investigated under the combined effect of operating loadsand residual stress. The EAC growth rates of DMW with residual stress were quantitativelypredicted. The effect of weld residual stress on crack growing state was also analyzed.