원전 압력용기용 이종금속용접부의 미세조직과 기계적 성질에 대한 장시간 시효 영향

Abstract

The purpose of this study is to investigate the effects of the thermal aging on the microstructure and mechanical properties of dissimilar metal welds for reactor pressurized vessels in the primary system of nuclear power plants. The dissimilar metal welds composed of SA 508 Cl.3 low alloy steel and AISI 316L stainless steel are prepared after buttering with alloy 82 on the SA 508 side by the gas tungsten arc welding process using Inconel 82 welding consumable. The test specimens are heat-treated at 600℃ for 10000 h at each predetermined aging time to simulate the degraded microstructure of dissimilar metal welds subjected to high temperature and pressure. The long-term aging tests are interrupted at various stages to obtain the different level of degraded specimens. AISI316 austenite stainless steel showed polygonal-shaped grains with many annealing twins that gradually tended to become more circular-shaped grains during long-term aging. In the initial material degradation, the twins were distributed uniformly within most grains, but they all recovered and disappeared after prolonged 10000 hours aging time. Delta ferrite along austenitic grain boundaries transformed to sigma phases and Cr23C6 precipitates during long-term aging. Elongation and toughness decreased continuously, demonstrating the material embrittlement during long-term aging. SA508 steel showed a typical bainitic phase consisted of well developed lath substructure before aging and most fine precipitates lay along the lath boundaries. As the aging time increased, precipitates were redissolved into matrix and the lath substructures were recovered. The microstructure of SA508 steel finally changed into ferritic microstructure after 5000 hours aging due to dissolution of fine precipitates and lath migration. Hardness and tensile strength decreased during long-term aging due to bainite decomposition. The microstructural changes in base metals and weld metal have been evaluated by the optical and electron microscope in relation with twins, grains, precipitates, and phase transformation. The residual stress and mechanical softening were also discussed in terms of microstructural changes during long-term aging.