Effect of thermal aging on embrittlement of Cr–Mo–V pressure vessel steel

Abstract

Cr–Mo–V reactor pressure vessel steel (RPV) was subjected to accelerated thermal aging at 450 °C for durations up to 8400 h. This steel showed embrittlement with aging, as the impact energy and the hardness reduced monotonically with aging duration. These coincided with an increase in the transgranular (TG) brittle fracture, and a conspicuous absence of intergranular (IG) fracture. Further, absence of preferential attack at the prior austenite grain boundaries ruled out the possibility of impurity as a cause for the observed degradation. In addition, insignificant changes in grain size and misorientation indicated no major role of microstructural changes in the embrittlement. However, the coarsening of carbides and the corresponding depletion of the Cr & Mo at the carbide matrix interface region, supported by the reduced hardness, were shown to have resulted in the degradation. The carbides were identified to be predominantly of Cr-rich M7C3 along with M23C6 in the as-received low alloy steel. While M23C6 increased with aging duration, indicating a transformation of M7C3→M23C6, the primary reason for the embrittlement is attributed to the coarsening of the carbides.