PSI - Issue 19

Masanori Nakatani et al. / Procedia Structural Integrity 19 (2019) 312–319 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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2. Experimental methods

2.1. Material and specimens

Alloy 718 (UNS N07718) round bar and plates that had undergone two different heat treatment procedures were used in this study. Solution-treated at a low temperature, the round bar possessed a fine-grained microstructure, whereas that of the plates, which had been solution-treated at a higher temperature, was coarse-grained. For convenience, based on the individual microstructural features, the former material is hereafter denoted as FG and the latter as CG. After prior solution-treatment at 1228 K for one hour, the FG base metal was subsequently water quenched and later aged twice for eight hours at 991 and 894 K. In contrast, the CG base metal was initially solution treated at 1338 K for an hour, then cooled with argon gas and afterwards aged twice for 10 hours at 1033 and 923 K. Fig. 1 presents the backscattered electron (BSE) images of the FG and CG microstructures. Intergranular δ -phase precipitates were present in the fine-grained FG while they were not in evidence in the coarser-grained CG. Despite the differences in their microstructural properties, the measured FG and CG Vickers hardness, HV , (load 9.8 N, 30 s, 20 points average) was approximately the same at 456 and 447, respectively. Round-bar fatigue specimens were fashioned from the above materials, such that the specimen axis of each specimen is parallel with the rolling direction of each base material, each with a gauge length of 20 mm and a diameter of 7 mm at the gauge section.

Fig. 1. BSE images of the microstructure of (a) FG and (b) CG.

In the mid-gauge section of the specimens, artificial defects of varying shapes and dimensions were introduced, as illustrated in Fig. 2. In addition, some of the specimens were subjected to hydrogen-charging (H-charging) by exposure to hydrogen gas at pressures of 11 MPa or 100 MPa at 270°C for 200 hours. Prior to H-charging, the initial hydrogen content measured in FG specimens was 0.78 mass ppm, whereas CG specimens contained 0.14 mass ppm. After H charging at 11 MPa, the saturated hydrogen content, C S , in the FG and CG specimens totaled 26.3 mass ppm and 27.6 mass ppm, respectively. After 100-MPa H-charging, the C S in the FG specimens was determined to be 91.0 mass ppm.

2.2. Fatigue testing

A series of tension-compression fatigue tests was performed using a servo-hydraulic fatigue testing machine in laboratory air at room temperature. The stress ratio, R , and test frequency, f , were correspondingly set to −1 and 10 Hz. The fatigue limit was defined as the largest stress amplitude at which a specimen did not fail when the number of cycles, N , reached 10 7 cycles. Some of the tests were periodically halted for observations via the plastic replica method, in order to investigate the propagation behavior of cracks stemming from artificial defects.