Recently, in order to improve thermal efficiency of a steam turbine rotors in the power plant, there is a trend increasing the unit size and operating temperature of the steam turbine rotors. As turbine rotors becoming larger in size and higher in the operating temperature and pressure, materials with high creep strength and good toughness have been required. Accordingly, 12% Cr-Mo-V martensitic stainless steel with high strength is currently beginning to be used for steam turbine rotors in place of 1Cr-Mo-V steel.
The present study investigates the high temperature low cycle fatigue behaviors and creep-fatigue interaction of 1Cr-Mo-V and 12% Cr-Mo-V steels used in turbine rotors.
Total strain controlled uniaxial fatigue tests were carried out at 298, 773 and 873K in the air atmosphere. Two different strain rates of triangular waveform tested were $4\times10^{-3}$ and $4\times10^{-5}/\sec$ selected for the strain rate effect investigation. And in order to investigate the creep damage on the high temperature fatigue life, the hold times at tensile peak strain were imposed to be 1, 10, 30 and 60min.
The experimental results show that the effect of directionality of rotor on the fatigue life was rarely observed. From the results of continuous cycling and creep-fatigue interaction in 1Cr-Mo-V steel, it is found that the fatigue life is significantly reduced at high temperature with decreasing plastic strain range and introducing tensile hold time. And under creep-fatigue interaction condition, grain boundary cavities are not formed during tensile hold time. The cause of decrease in high temperature fatigue life is considered to be the earlier crack initiation by oxidation damage.
All the results of continuous cycling in 12% Cr-Mo-V steel are shown in terms of Coffin-Manson equation. The effect of temperature on the fatigue life is known to be not existing throughout all the plastic strain range conducted. However, the test results are shown in terms of the damage function(hysteresis loop energy), it is observed that the fatigue life significantly reduced with increasing test temperature.
According to the results of strain rate effect on the fatigue life in 12% Cr-Mo-V steel, in the higher plastic strain range, the fatigue life is significantly decreased with decreasing strain rate. This is considered to be due to the damage of fatigue cavitation under slow strain rate($4\times10^{-5}/\sec$). The critical strain rate, $\varepsilon'$c, for the nucleation of fatigue cavities was found to be dependent on the strain range and the strain rate controlled. For the strain range of $\pm 2.0 \sim 1.0%$, it is found that the critical strain rate, $\varepsilon_c$, is obtained to be in the range of $1.25\times10^{-3} \sim 3.23\times10^{-4}/\sec$ for the strain rate of $4\times10^{-3}/\sec$, and in the range of $8.79\times10^{-5} \sim 2.74\times10^{-5}/\sec$ for the strain rate of $4\times10^{-5}/\sec$, respectively.
The introduction of tensile hold time during high temperature continuous cycling has been shown to decrease fatigue life compared with that of continuous cycling, and it is observed that the fatigue life is significantly reduced with decreasing plastic strain range and increasing tensile hold time. This is considered to be caused by the enhanced crack growth rate as the result of interaction between fatigue crack and creep cavities.
Under creep-fatigue interaction condition, the fracture mode is observed to be mixed (trans and intergranular). The experimental analysis indicates, that creep cavities are nucleated not only on prior austenite grain boundaries but also within the prior austenite grains boundaries but also within the prior austenite grains(lath martensite boundaries). It is suggested that the nucleation of cavity and its growth occurred around the carbides precipitated on lath martensite and grain boundaries.
From the results of tensile hold time test, the mechanism of cavity nucleation and growth in 12% Cr-Mo-V steel is considered to be vacancy clustering. From the results mentioned above, the fatigue lives under creep-fatigue interaction condition are discussed in terms of a model for life prediction developed previously by the creep cavitation damage rather than the damage of surface fatigue crack initiation and propagation. The Predicted creep-fatigue lives are in good agreement with experimentally observed ones for 12% Cr-Mo-V steel with increasing tensile hold time.
