625合金の750℃水蒸気環境中疲労き裂進展挙動に及ぼす負荷速度の影響

抄録

In order to clarify the crack growth behavior of Alloy 625 at elevated temperatures, the crack growth rate (CGR) was evaluated in a superheated steam environment at 750°C under cyclic loading. A compact tension specimen with a width of 50 mm and a thickness of 12.5 mm was used for the test. Three types of loading waveforms with the same cyclic period of 0.0003 Hz were applied. One was a triangular waveform in which loading and unloading rates were the same. The others were sawtooth waveforms with a fast loading rate (F-S) and with a slow loading rate (S-F). The CGR per unit cycle was 1.42 times higher for F-S and 0.87 times higher for S-F than the one obtained under the triangular waveform. It was confirmed that even at the same loading frequency, the waveform difference influences the CGR. Under the F-S waveform in which the faster CGR was exhibited, the thicker oxide film was formed around the crack, suggesting that an increase in unloading time leads to more oxide film formation. This is an acceleration factor for the crack growth rate under the same loading frequency.