![]() ![]() The decrease of dislocation density was due to the formation and arrangement of dislocation cells in martensitic laths at the initial stage, and pinning of lath boundaries by carbide precipitates at the following stage 16, 17. The different cyclic softening degree during LCF process was related to the formation of subgrain boundary 12, 13, 14 and the interaction between dislocations and carbide precipitates 15. The process of cyclic softening contains two stages: the rapid softening in the early stage and stabilization in the following stage 9, 10, 11. ![]() The steel used for steam turbine rotors (e.g., CrNiMoV steels) would soften during LCF process, especially at elevated temperatures, which was characterized by the decrease of dislocation density and the increase of subgrain size 7, 8. It appears that LCF and SCC may have interacted with each other, an issue that needs focusing and clarifying for underlying mechanisms, design, manufacturing and maintance techniques. Stress corrosion cracking (SCC) is another potential failure mode for nuclear steam turbine rotors operating in a wet steam environment 4, and the SCC behavior of steam turbine rotors has been widely studied 5, 6. Several notched areas such as the heat-relieved grooves of the glands at the inlet end of the rotor, fillet radii at the base of disks, balance holes in the disks and blade grooving in reaction-type rotors 3 will undergo severe LCF damage. Low cycle fatigue (LCF) is a common loading mode for steam turbine rotors and blades during start-up and shut-down due to variations of centrifugal and thermal stress together with system resonance 2. Fatigue failure is induced by a cumulative form of damage under cyclic loading associated with micro-crack initiation and propagation 1. ![]()
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