Microstructural changes around grain boundary after long-term creep in low-alloy steels

Abstract

Microstructural changes after long-term creep were investigated in low-alloy Cr–Mo steels, focusing on the precipitation-free zone (PFZ) around the grain boundary. The grip and gauge portions of creep-ruptured samples obtained through the NIMS Creep Data Sheet Project were observed by SEM. The PFZ was clearly observed around the grain boundary after long-term creep in 1Cr-0.5Mo (JIS STBA22) and 1.25Cr-0.5Mo–Si (JIS STBA23) steels. Large precipitates were formed on the grain boundaries near where the PFZ was observed. The width of the PFZ increased with increasing creep time at 550 °C–650 °C. The increase in width was remarkable at higher temperatures. The clear relationship between the PFZ width and the Larson-Miller parameter (LMP) has already been reported for 2.25Cr–1Mo steel. This relationship provides a method for predicting the metal temperatures of serviced components in power plants. A good relationship between the PFZ width and the LMP was confirmed for 1Cr-0.5Mo and 1.25Cr-0.5Mo–Si steels. However, the relationship was not consistent with that for 2.25Cr–1Mo steel. The increase in the PFZ width was faster in 1.25Cr-0.5Mo–Si steel than in 1Cr-0.5Mo steel.