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In this study, using a C(T) specimen for 12Cr steel used for a steam turbine rotor, crack growth tests under creep-fatigue conditions with and without overloading were conducted and the fundamental behaviours of deformation and crack growth were investigated experimentally. As a result, load line displacement was found to be suppressed under overload condition regardless of temperature and stress holding time. Crack initiation life under high temperature and low stress condition with overloading was delayed as compared with no overload condition. However, under low temperature and high stress condition with short stress holding time, crack initiation was accelerated remarkably. Furthermore, although crack growth life under creep-fatigue conditions with overloading was found to be shortened as compared with no overloading, the effect of overloading on crack growth life was decreased with an increase in stress holding time. [ABSTRACT FROM AUTHOR]

• Crack growth tests under creep and initial overload creep conditions were conducted for 12Cr steel. • The initial overload was found to be suppressed the crack initiation. • The effect of initial overload does not significantly affect the crack growth rate. • The dual value behavior of C* parameter was dominated by crack initiation. 12Cr steel is used as one of the materials for steam turbine rotor. Steam turbine rotor is an important component and high reliability is required. In actual turbine rotor which has central hole, thermal stress induced by inflow of steam is generated at start-up. At this time, an overload greater than the load level during steady-state operation is exerted on the stress concentration area near the central hole. Therefore, it is important to evaluate the effect of overload on the creep crack initiation and growth behavior. In this study, using a C(T) specimen for 12Cr steel used for steam turbine rotor, crack growth tests under creep and initial overload creep conditions were conducted and behaviors of creep deformation, crack initiation and growth were investigated. In addition, these results were evaluated by various fracture mechanics parameters. As a result, it was found that the initial overload suppressed creep deformation and delayed creep crack initiation. Especially, this tendency was more pronounced at the low temperature and high stress condition. However, the initial overload does not significantly affect the creep crack growth rate. [ABSTRACT FROM AUTHOR]

Threshold stress intensity factor ( ΔK), one of the important parameters in plant design, is the value below which fatigue crack growth becomes unrecognizably slow. Although studies on the each effect of various factors such as temperature, frequency, stress ratio, oxidation and grain size on the ΔK have been reported widely, only few research works were reported on the complex effect of the plural factors acting simultaneously. In this study, the analysis of the complex effect of temperature and oxidation on ΔK of 12Cr steel, the turbine rotor material of the USC (ultra super critical) power plant, has been carried out. Two types of fatigue tests were performed in the air and in the nitrogen gas conditions. Experimental results showed that ΔK value increases with temperature increase and formation of oxidation scale. However it was observed that the magnitude of each effect is reduced when two factors are introduced simultaneously. [ABSTRACT FROM AUTHOR]