CRACK INITIATION BEHAVIOR OF COLD-WORKED ALLOY 690 IN SIMULATED PWR PRIMARY WATER – ROLE OF STARTING MICROSTRUCTURE, APPLIED STRESS AND COLD WORK ON PRECURSOR DAMAGE EVOLUTION

摘要

Long-term stress corrosion crack (SCC) initiationbehavior in Alloy 690 is being evaluated by constant loadtensile testing in simulated pressurized water reactorprimary water. Seven commercial Alloy 690 heats withdifferent starting microstructures and cold-work levelsare being tested at either 90% or 100% of the materialyield stress. Previous studies have revealed grainboundary (GB) cavity formation after 7,110 to 9,220hours of exposure in certain thermally treated (TT) Alloy690 heats in highly cold-worked (CW) conditions, servingas precursors to intergranular cracks. This papersummarizes the new characterization results for theevolution of GB cavities in four Alloy 690TT heats in 21%and 31%CW conditions loaded at yield stress after 18,400hours of exposure. Link-up of GB cavities has led toformation of long internal creep cracks, resulting in crackinitiation detected by direct current potential drop in ahighly CW Alloy 690TT heat with uniform, semicontinuousdistribution of small GB carbides. Incomparison, highly CW materials featuring larger andmore widely spaced GB carbides did not show asignificant increase in GB cavity density or size.Quantitative assessments were performed on GB cavityevolution in highly CW specimens for two Alloy 690TTheats demonstrating the strong dependence of GB cavityevolution on applied stress and starting GBmicrostructure.