Precipitate evolution during the aging of Super304H steel and its influence on impact toughness

Super304H, which is an austenitic heat-resistant steel, was aged at 650 degrees C for up to 5000 h. Its microstructural evolution during aging was observed by scanning electron microscopy (SEM) equipped with electron backscatter diffraction (EBSD). The precipitate evolution in the aged specimens was analyzed by both X-ray diffraction (XRD) and transmission electron microscopy (TEM) with energy dispersive spectroscopy (EDS) and selected area electron diffraction (SAED). In addition, the precipitation kinetics of M23C6 carbide was established. Hardness and impact tests were performed on the aged specimens. Finally, the relationship between the mechanical properties and microstructural evolution was revealed. After aging, Super304H steel exhibited the same fine grains as in the as-received condition with a Cu-rich phase, secondary Nb(C,N) precipitates inside the grains, and M23C6 carbides mainly at grain boundaries. The Cu-rich phase and the secondary Nb(C,N) precipitated in specimens aged for 5000 h; this leads to a marked increase in hardness. Precipitation of the Z phase also occurred in the aged Super304H steel. M23C6 carbides precipitated rapidly during aging, with content approaching 90% of the saturated content in specimens aged for 5000 h. The reduction in impact toughness in Super304H steel was found to be associated with the precipitation of the M23C6 carbide, which is similar to that in the HR3C steel. However, the deterioration was much less significant as compared to HR3C steel because of the discontinuous distribution of carbides at grain boundaries due to the fine grain size.