The Influence of Metallurgical and Mechanical Pre-Damage on Creep-Crack and Fatigue-Crack Growth Rates of CrMoV Steel

Abstract

The effects of mechanical degradation on the creep- and fatigue-crack growth rates in power plants operated long-term were studied. Creep-crack growth tests and fatigue-crack growth tests were performed using creep-pre-strained and repetitive-strained CrMoV rotor-steel specimens. It was found that the creep-crack growth rates, da/dt, of the creep-pre-damaged specimens were larger than those of virgin specimens under constant load. It was also found that under the same stress intensity factor K, da/dt in the case of about 10%-crept and creep void induced specimens was increased five times, while in the case of 0.8%- and 2.8%-crept specimens, it only increased a little. However, all the data fell in a narrow scatter band in the relationship between C* and da/dt. The effect of long-term operating degradation appeared on the creep- and fatigue-damage under the Slow-Fast waveform. Also the crack density was larger in the damaged specimens compared with the virgin specimens. Other results showed that the fatigue-crack growth rates, da/dN, of creep- and fatigue-pre-damaged specimens were larger than those of virgin specimens. Under the same stress intensity factor range ΔK, da/dN in a specimen with approximately 10%-creep was increased 20 times. Moreover da/dN in fatigue-pre-damaged specimens was accelerated 10 times. However, the results of the strain-controlled crack growth test fell in a narrow scatter band in the relationship between J integral range ΔJ and da/dN. These results suggest that the creep remaining lives and fatigue remaining lives of mechanically damaged CrMoV steel can be estimated using the C* & ΔJ values considering the creep rate and the deformation rate of the pre-damaged materials and the da/dt and da/dN values of the virgin material.