Effect of Thermal Aging on Microstructure and Stress Corrosion Cracking Behavior of an Alloy 152 1^st Layer Butter Weldment

Authors

Abstract

Nickel-based Alloy 690 and the associated weld Alloys 52 and 152 are typically used for nozzlepenetrations in replacement heads for pressurized water reactor (PWR) vessels, because of theirexcellent overall resistance to general corrosion and environmental degradation, primarily stresscorrosion cracking (SCC). However, many of the existing PWRs are expected to operate for 40-80 years. Likewise, water-cooled small modular reactors (SMRs) will use Ni-Cr alloys and areexpected to receive initial operating licenses for 60 years. Hence, the thermal stability of Ni-Cralloys is critical for the long-term performance of both existing and advanced nuclear powerplants, and possibly spent fuel storage containers. The objective of this research is to understandthe microstructural changes occurring in high-Cr, Ni-based Alloy 152 weldments during longtime exposure to the reactor operating temperatures, and the effect of these changes on theservice performance. One area of particular concern is the potential for long range ordering(LRO), i.e. formation of the intermetallic Ni2Cr phase under prolonged exposure to reactortemperatures and/or irradiation, which can increase strength, decrease ductility, and causedimensional changes or lead to in-service embrittlement of components made with these alloys.Hence, this research focused on the microstructural evolution and the SCC response of Alloy 152under accelerated thermal aging. The materials studied involved three heats of Alloy 152 used toproduce a dissimilar metal weld (DMW) joining an Alloy 690 plate to an Alloy 533 low alloysteel (LAS) plate, thermally aged at three different temperatures (370C, 400C and 450C) fordifferent durations up to 75,000h (equivalent to 60 years of reactor service). The microstructuralcharacterization by means of synchrotron X-ray conducted in small, 0.2 mm - step line scans inthe high-deformation regions of the weld root covering areas spanning from the weld heataffected zone (HAZ) in Alloy 690 to the weld and weld butter on LAS - did not show evidenceof LRO in any of the three Alloy 152 heats aged at 370C and 450C to an equivalent of 60 yearsof service. However, the first weld butter layer has high levels of deformation and is highlysusceptible to SCC even in its non-aged condition. Nanohardness testing confirmed the extremehardening with aging (HV 100) at two locations within this weldment. In absence of LRO,hardening is suspected to be due to thermally-induced Cr carbide precipitation and coarsening. Testing in a primary water environment of the 1^st layer of Alloy 152 weld butter aged at 370Cto a 60-year service equivalent revealed a fatigue and corrosion fatigue crack growth responsessimilar to those measured on the un-aged alloys. Similarly, the SCC CGR response of the agedweld butter does not appear to show a deterioration in performance, however, the difficult-to-testweldment geometry may affect the test results.