Microstructural Evolution of Super304H Steel upon Long-Term Aging

The Nb-stabilized and Cu-strengthened austenitic stainless steel Super304H was aged at 600~700°C up to 20,000 hrs. Scanning electron microscope (SEM), and transmission electron microscope (TEM) were used to study the effect of aging on the microstructural evolution of the specimen, with focus on the precipitation behavior in relation to the temperature and time. Only NbCs in size range 2 μm ~ 50 nm were observed to be scarcely distributed in matrix in the as-received specimen. Upon aging, precipitation of σ-phase (~5 μm) and Cr-rich M23C6 (~1 μm) along grain boundary, and nanosized Cu precipitates (~65 nm) in the grain interior were formed. The size and fraction of the σ-phase and M23C6 increased with the increase of aging temperature and/or aging time, with higher sensitivity to the temperature. The size of Cu precipitates was relatively stable, while the fraction and number density increased with the aging temperature/time. The microhardness upon aging increased with increase of aging temperature/time, due to the precipitation of the nanosized Cu precipitates.