PSI - Issue 17

Zizhen Zhao et al. / Procedia Structural Integrity 17 (2019) 555–561 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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Table 1 Chemical composition of 2.25Cr1MoV forged steel (wt. %) C Si Mn P S Cr Mo

Ni

V

Cu

Al

Fe

0.15

0.07

0.56

0.006

0.002

2.44

0.96

0.12

0.263

0.03

0.011

Bal.

Solid bars were firstly cut from the forged ring with its axial direction coinciding with the forging direction, and then machined into dumbbell specimens as shown in Fig. 1. The gauge section is 10mm in diameter and 27mm in length. The specimen ’ s surface was carefully polished to avoid the initiation of premature fatigue cracks in oxidative environment at elevated temperature.

Fig. 1 Geometry of the test specimen.

All tests were conducted in air on a closed-loop servo electro-hydraulic fatigue testing machine. Induction heating was employed to heat the specimen to the target temperature, which was chosen around the design temperature of most hydrogenation equipment, i.e. 455 o C. Three K type shielded thermocouples with a diameter of 0.5mm were secured in the gauge section to monitor the temperature, and a precision of ±2 o C was maintained during the test. An elevated-temperature applicable extensometer with a gauge length of 10mm was employed to measure the axial deformation. Stress controlled mode was used and three kinds of waveform were performed, i.e. ratcheting-fatigue (hereafter referred to as RF), creep-ratcheting-fatigue (CRF) tests with peak and peak/valley stress holding periods. The applied stress waveforms are shown in Fig. 2. A series of mean stresses and stress amplitudes were applied at a loading rate of 200 MPas -1 , and various holding periods were incorporated, i.e. 2s, 5s, 10s, 30s and 60s.

Fig. 2 Applied stress wave forms in: (a) RF test; (b) CRF test with peak holding period; (c) CRF test with equal peak/valley holding periods.