The effects of reversion heat treatment on the recovery of thermal aging embrittlement of CF8M cast stainless steels

• Reversion heat treatment (R-HT) was used to recover thermal aging of CF8M.
• During the R-HT, Cr-rich phases dissolved but Mo-rich phases precipitated in ferrites.
• After the R-HT, the rate of re-embrittlement was similar to that of previous aging.
• Pre-application of the R-HT reduced the degree of aging embrittlement of CF8M.

The thermal aging embrittlement of cast stainless steels (CASSs) is one of the key material property degradation that would limit the long-term operation nuclear power plants. In this study, we investigated the recovery behaviors of thermal aging embrittlement of cast stainless steels (CASSs) by the reversion heat treatment. Two heats of CF8M with different ferrite contents were used and the degree of aging embrittlement was measured by the micro-hardness of ferrite phases. It was found that the micro-hardness values of ferrites in the aged CF8M were significantly reduced after the reversion heat treatment at 550 °C for 30 min. Meanwhile, those of the un-aged CF8M were slightly increased by the reversion heat treatment. Also, the re-aging embrittlement behaviors of the recovered CF8M were similar to the initial aging behaviors. Finally, if the reversion heat treatment was applied to the un-aged CF8M, the degree of the embrittlement was reduced during the thermal aging heat treatment. These changes in the micro-hardness values were explained in view of the dissolution of the Cr-rich region formed during spinodal decomposition and the formation of Mo-rich precipitates in the ferrite phases during the reversion heat treatment.