Microstructure Analysis of Stainless Steel Cladding on Low Alloy Steel Weld in a PWR Safe End Nozzle and Corrosion in High Temperature Water

The complex microstructures of stainless steel (SS) cladding on low alloy steel (LAS) joint and the corrosion behavior in high temperature water environments were examined. Scanning electron microscopy, focused ion beam, transmission electron microscope and Raman spectroscopy were used. The heat-affected zone (HAZ) in the low alloy steel was mainly comprised of pearlite and ferrite, while the HAZ in the stainless steel was mainly comprised of austenite and ferrite. The HAZ in the low alloy steel was divided into overheated crystal region, complete recrystallization region and incompletely recrystallization region. A decarburization zone in the low alloy steel side and a carbon-enriched zone in the stainless steel side were identified. M₂₃C₆ and M₇C₃ precipitation were observed mainly in carbon-enriched zone. The surfaces of the weld after corrosion in simulated PWR primary water environment at 290 °C were significantly affected by dissolved oxygen. In aerated solution, the oxide film on A508III steel was mainly γ-Fe₂O₃, which becomes spinel oxide on the 309L/308L cladding. In deaerated solution, the oxide film was mainly Fe₃O₄ on A508III steel, which was spinel oxide on the 309L/308L cladding.