Creep behaviors and microstructure analysis of CNS-2 steel at elevated temperatures and stresses

Creep behaviors of CNS-2 steel have been investigated under different conditions at 600-650 °C and 105-180 MPa, and the microstructures and precipitations of crept and aged specimens have been characterized. It is showed that the CNS-2 steels have remarkable creep resistance at 600°C/105MPa until at least 1500 h, and the accelerated creep occurs at the elevated temperature of 650 °C and applied stress of 180MPa. The CNS-2 steels are toughened after creep without hardening and embrittlement. Their grain microstructures are stable but the sub-structures change during the creep. The subgrains coarsen with creep strain and the density of dislocations decreases due to the recovery at elevated temperature. Three kinds of precipitations including M23C6, MX-type carbonitride and Laves phase are identified in the crept steels by TEM, EDS and SEAD analysis. It is also found that the applied stress promotes the precipitations formed at the grain or subgrain boundary.