Abstract
This study aims to characterize nondestructively the creep-fatigue damage of tempered ferritic steel by dynamic coercivity measured by reversible permeability. The creep-fatigue test was achieved under load control with a hold time of 60 s and 600 s. The dynamic coercivity based on the reversible magnetic permeability profiles was successfully obtained on top of the test specimen. Based on these results, we attempted to evaluate the creep-fatigue of tempered ferritic steel with the support of microstructural evaluation such as dislocations and precipitates. The dynamic coercivity decreases with fatigue life fraction and closely related to the number of Cr23C6 precipitates with N0.8 and the dislocation density with ρ0.4 In addition, Vickers’s hardness continuously decreased up to the failure deducing the softening of mechanical strength. In conclusion, it can nondestructively characterize the influence of the precipitates and dislocation density on the dynamic coercivity of ferritic steel during creep-fatigue.
Highlights
The ferritic 9Cr steel has been recognized as a key material for turbines, rotors, pipes, etc. in nuclear and fossil power plants
They studied magnetic coercivity of 2.25Cr steel subjected to long-term thermal aging and showed the precipitate dependence on the magnetic coercivity [20]
The dynamic coercivity was obtained from the reversible permeability profiles by the yoke type
Summary
The ferritic 9Cr steel has been recognized as a key material for turbines, rotors, pipes, etc. in nuclear and fossil power plants. In this ferritic low-carbon steel, the Cr element increases generally high-temperature creep strength, oxidation resistance, and corrosion resistance, which has superior physical and chemical properties than low Cr content steels [1,2,3]. Mechanical softening causing strength drop and embrittlement is inevitable for high-temperature structural materials because of long-term exposure to high temperature and high pressure, such as creep and fatigue. The mechanical softening is physically evolved to the microstructural variation in a generation and coarsening of the secondary phases, the recovery of dislocations, the growth of martensite lath width, and depletion of the solute atoms [4,5]. Structural facilities subjected to high temperature and high pressure for long-term exposure can be deteriorated and fractured during their operation due to mechanical softening. Since the damage and fracture are localized and accelerated without notice, the monitoring and characterization of structural components are very important and necessary
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