Microstructure evolution and toughness degeneration of 9Cr martensitic steel after aging at 550 °C for 20000 h

Abstract

The China Low Activation Martensitic (CLAM) steel is one of 9Cr martensitic steels, and it has been selected as the primary candidate structural material for future fusion reactors in China. It is essential to understand the thermal stability of CLAM steel to ensure the operation safety during exposures at operating temperatures. In this paper, the thermal aging behavior of CLAM steel at 550 °C for 20000 h was studied. Microstructure evolution and mechanical property change were investigated. The results showed that Laves phase was mainly precipitated along the martensitic lath boundaries and grain boundaries in CLAM samples after aging for more than 4000 h, and its size increased faster than that of M23C6. The size of TaC had no obvious changed but its number increased after aging. The microstructure evolution resulted to degradation of the mechanical property, especially the ductile-to-brittle transition temperature (DBTT), which was increased from − 92 to − 43 °C after aging for 20000 h. The possible mechanism of DBTT increasing had been analyzed based on the interaction between precipitations and grain boundaries.