Effect of Thermal Aging on Impact Toughness of 20Cr25NiNb Stainless Steel

In view of the degradation of alloy mechanical properties under prolonged high temperature exposure, the thermal aging study of candidate cladding materials for supercritical gas-cooled reactor was carried out. The prototype 20Cr25NiNb stainless steel for advanced gas-cooled reactor (AGR) reactor and improved alloys with different elements were subjected to thermal aging test at 650°C for 3000 h. The results of microstructure and properties showed that the impact absorbed energy (KV₂) of all alloys decreased as the thermal aging proceeded. This plasticity decrease was closely related to the evolution of second phases at high temperature. The precipitation of M₂₃C₆ and G phases along grain boundaries led to a decrease and then a slow increase in the impact toughness of the prototype alloy. The addition of W and Mo elements brought the precipitation of Laves and σ phases at grain boundaries, causing a faster decrease in KV₂; B element refined σ phase and made the impact toughness decrease less than that of the alloy without B. After adding Al, a large number of Laves and NiAl phases precipitated in the matrix, while the σ phase at grain boundaries coarsened rapidly, leading to severe material embrittlement.