Microstructure Characteristic And Stress Corrosion Cracking Of The Safe-end Weld Joint Of The Nuclear Power Plant Primary Loop

Stress corrosion cracking in the safe end weld joint of the primary loop pressurized water reactors (PWRs) has threaten the safe operation of the plants and caused huge economic losses. SA508-309L/308L-316L dissimilar weld joint is used to connect the primary loop safe end nozzle of the pressure vessels in CPR1000 PWR. To date, few research has been conducted on stress corrosion cracking behavior of this weld joint. In this study, the microstructure characteristic of the weld joint was analyzed by a couple of techniques. Stress corrosion crack growth behavior of the fusion boundary region of the weld joint in high temperature high pressure water was studied following the construction of a direct current potential drop (DCPD)system. Stress corrosion cracking susceptibility of the fusion boundary region of the weld joint in high temperature high pressure water was studied by modified, creviced bent beam (CBB) experiments.The DCPD system can measure the crack length with a resolution of<2μm in high temperature high pressure water. The system also have the function of monitoring the experimental parameters such as dissolved oxygen, temperature and conductivity.Metallographic microscope, scanning electron microscope, energy dispersive spectrometer and vickers hardness tester were used to analyze the microstructure of the weld joint. The results showed that, SA508 low alloy steel base metal consists of bainite and ferrite. Grain refinement zone, coarse grain zone and poor carbon zone was found in the heat affected zone of the low alloy steel. The weld metal was characterized as austenite dendrites with 8-ferrite distributed between the austenitic grain boundary. Microstructure, hardness and chemical composition change obviously from the LAS to weld metal across the fusion boundary. Typical type-I, type-II boundaries were found near the fusion boundary in the weld metal.Austenite structure with a little of 8-ferrite on the grain boundary was found in the 316L stainless steel base metal. Al2O3 and MnS inclusions were observed in the 316L stainless steel heat affected zone. Both sides of the fusion boundary did not exhibit significant chemical composition change, not including hardness and ferrite content.Stress corrosion growth rate of the weld joint between the 316L stainless steel and 308L stainless steel was studied in the high temperature high pressure deoxidize water with boron and lithium. The weld joint showed a crack growth rate of<2x 10-9 mm/s with a trapezoidal waveform loading at a stress intensity factor of 25MPa·m1/2. Stress corrosion crack growth rate of<9×10-10mm/s was calculated at the same stress intensity factor.This study revealed the 316L/308L weld joints has an ultra low SCC growth rate in the primary water. This is in consistent with the results of the modified CBB tests which showed that SA508-309L/308L dissimilar weld joints and welding cladding had a low susceptibility to stress corrosion cracking.