High Cr concentration often increases susceptibility to aging embrittlement through the formation of Cr-rich secondary phases. The trade-off between cor­rosion resistance and aging embrittlement caused by increasing Cr content is one of the critical issues facing the developers ofhigh-Cr ODS steels. The aging effects of ODS steels with different Cr content were investigated by measuring their impact fracture energy at RT after aging at 500 °C up to 10 kh. The results are shown in Figure 24.43 The fracture energy decreases with increasing Cr content before aging. Aging, then, causes a reduction in the fracture energy. ODS steels with a Cr content >18 wt% show a significant reduc­tion in fracture energy after aging for 100 h. In contrast, 16Cr-4Al ODS steel showed a small reduction in frac­ture energy even after aging for 10 kh. Microstructure observation by TEM revealed that fine secondary phases were formed in high density after aging for 1000 h at 500 ° C. These secondary phases are consid­ered to be Cr-rich phases. In order to reduce suscepti­bility to aging embrittlement, the Cr content could be <16 wt%.