Papers by Keyword: Microstructural Degradation

Abstract: Super304H is austenitic steel used predominantly for boiler and turbine components in thermal power plants due to the high strength, excellent creep and oxidation resistance in high-temperature steam environments. Microstructural degradation is inevitable upon long-term high-temperature exposure. Therefore, understanding the processes of degradation is critical for the determination of residual lifetime of the parts. In this study, the hardness, tensile strength and creep strength of a Super304H steel that was in service as a reheater for ~8 years at about 600°C was investigated with corresponding microstructural analysis. Grain growth is evident, but microhardness and tensile strength did not decrease, due to the precipitation of nanosized NbX and Cu-rich particles in grain interior. However, the creep rupture life of the aged S304H steel is ~90% lower at 650°C than that of the virgin S304H, due to the coarsening of σ phase and formation of M₂₃C₆ on the grain boundary. Our measurements and analysis of the specimens in this study indicates the remaining service life longer than those in the NIMS database.

Abstract: Miniature specimen techniques viz. small-punch tests (SPT) have been carried out at room temperature in order to correlate the microstructural degradation of 2.25Cr-1Mo steel with that of SPT parameters. Microstructural degradation of this steel has been introduced as a result of thermal ageing corresponding to Larson-Miller parameters (LMP) values of 33,012, 35,402, 37,846 and 38,374. SPT parameters viz. total area and area under the region of plastic instability of the load-displacement curve have been found to decrease with an increase in LMP values. A strength parameter viz. UTS obtained using uniaxial tensile tests has also been found to decrease with an increase in LMP values. The results indicated that miniature specimen techniques viz. small-punch test could be successfully used to assess the degradation of microstructures in 2.25Cr-1Mo steel generated due to their exposure to high temperatures.

Abstract: X20CrMoV12 1 steels were exposed at three different high temperatures and various durations. Aging parameter, LMP _s, was used to describe long-term thermal history. Creep rupture, hardness and tensile tests for aged X20CrMoV12 1 steels were carried out. Yield strength was slightly decreased and hardness was rapidly decreased afterLMP _s reached 20.6. Creep strength of aged X20CrMoV12 1 steel decreased rapidly after the LMP_s reached 20.6. It was shown that the creep strength decreased due to the coarsening of acicular type M₂₃C₆ at subgrain boundaries and the agglomeration of M₂₃C₆ at prior austenite grain boundaries. After LMP_s reached 20.6, the assumed linear regression line of stress and Larson-Miller parameter,LMP_f, also moved toward a lower LMP_f region as aging parameter increased. The dependency of creep rupture life on the microstructural degradation could be represented by C_LMPs, which is LMP_f at stress of 1 kg/mm². It was found that the creep rupture life of X20CrMoV12 1 steel could be predicted by considering the material degradation during and prior to creep test.