Effect of long-term aging on the mechanical properties of 316H stainless steel at 700 °C
摘要
The long-term mechanical properties of 316H stainless steel at 700 °C are of great concern for its application in the fourth-generation nuclear power plant. To this end, the effect of long-term aging on the mechanical properties is systematically analyzed. A series of hardness, tensile, and impact tests are conducted on the material aged for six different aging times at 700 °C, that is, 0 h, 1338 h, 2544 h, 3624 h, 5050 h, and 10,165 h. Combined with microstructural characterization, the evolution of microstructure and its influence on the mechanical response are elucidated. Results show that, with increasing aging time, both hardness and yield strength increase monotonically, while ultimate tensile strength decreases at 1338 h and then increases to exceed that of the unaged condition. Moreover, impact toughness declines markedly. Microstructural observations indicate that the evolution is governed by precipitation rather than by grain coarsening. M23C6 carbides preferentially grow along grain and twin boundaries and extend into the grain interior, and a scattered Laves phase appears during prolonged aging. The evolution of precipitates impedes the motion of dislocations and grain boundaries, which raises the hardness while degrading the toughness. The hardness, precipitate area fraction, and their product are found to follow a linear relationship with impact energy on the log–log scale, with the hardness performing the best, followed by the product and then by the precipitate area fraction. Accordingly, the hardness can be used uniquely for the prediction of impact energy after different aging times.