Effects Of Thermal Aging On Microstructure Evolution And Properties Of Austenitic Stainless Stee

Austenitic stainless steel is a candidate shell material for lead-bismuth cooled fast neutron nuclear reactors due to its excellent mechanical and corrosion properties.The long-term service of lead-bismuth fast reactor cladding materials in high-temperature liquid metals can cause thermal aging embrittlement of cladding materials,resulting in increased hardness and decreased toughness of the materials,posing a huge threat to the safe operation of nuclear reactors.Therefore,studying the changes in microstructure and properties of austenitic stainless steel cladding materials during long-term high-temperature service is of great significance for the safe operation and safety assessment of nuclear reactors.In this paper,15-15 Ti austenitic stainless steel,a candidate cladding material for lead-bismuth cooled fast neutron nuclear reactors,was studied to investigate the changes in microstructure and properties of 15-15 Ti austenitic stainless steel during long-term high-temperature service.The microstructure evolution and the impact on properties of the test steel after long-term thermal aging treatment at different temperatures were systematically studied using field emission scanning electron microscopy,transmission electron microscopy,and mechanical property testing.(1)After 20% cold deformation,a large number of deformation twins were formed inside the grain,and blocky primary phases(Ti,Mo)C were precipitated along the grain boundaries.The cold deformation organization gradually recovers during the hot aging process at 400°C-650°C,and the number of deformation twins decreases.Hot aging treatment at 800°C for 200 hours can completely degrade the deformation twins.(2)During the hot aging process at 400°C and 550°C,small secondary(Ti,Mo)C phases were precipitated.Thermodynamic simulation showed that the precipitation of secondary(Ti,Mo)C phase reached its peak at 550°C during hot aging.When the hot aging temperature was raised to650°C,a small amount of σ phase precipitated in the organization,mainly along the grain boundaries.When the hot aging temperature reached 800°C,σ phase could precipitate both at the grain boundaries and inside the grains.σ phase precipitated on the grain boundaries is easy to nucleate and grow around(Ti,Mo)C phases.(3)Thermal aging treatment caused the test steel’s microhardness to increase with increasing thermal aging time,and the change in microhardness tended to level off after 1500 hours of aging.When the thermal aging temperature reached 800℃,the average microhardness under the same time was higher than that under other experimental temperatures due to the precipitation of a large amount of harder σ phase in the grain boundaries and inside the grains.The recovery of the cold deformed test steel during thermal aging increases the plasticity.When the thermal aging temperature is higher than 650 °C,the precipitation of the hard brittle σ phase will damage the plasticity of the test steel.At the same time,the large precipitation of σ phase reduces the corrosion resistance of the test steel.The degradation of deformation twins during thermal aging leads to the decrease of ultimate tensile strength and yield strength of the test steel,but the increase of a large number of fine precipitates will increase the ultimate tensile strength.