PSI - Issue 68

Akihiko Fukunaga et al. / Procedia Structural Integrity 68 (2025) 1059–1065 Akihiko Fukunaga / Structural Integrity Procedia 00 (2025) 000–000

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2. Experimental 2.1. Materials

A286 is known as a high-strength, heat-resistant iron-based superalloy with more than 24% nickel content. The chemical composition of A286 is shown in Table 1. SSRT tests specimens of A286 were cut from the longitudinal direction of 9/16 inch-pipe which has a stable austenite phase and is isotropically oriented as shown in invers pole figure by EBSD in Fig. 1(a). The Kernel average misorientation (KAM) map exhibits little strain as shown in Fig. 1(b).

Table 1. Chemical composition of A286 specimen (mass %). C Si Mn P S

Ni

Cr

Mo

Cu

N

0.037

0.11

0.38

0.029

<0.001

24.01

13.64

1.08

0.16

0.0043

Fig.1. (a) Inverse pole figure, and (b) KAM map of A286 by EBSD.

2.2. Hydrogen charge and hydrogen content measurement The hydrogen charging process was carried out as follows. The test specimens were placed in a pressure vessel, and after being replaced three times with nitrogen gas (6N: 99.9999%), it was replaced three times with hydrogen gas (7N: 99.99999%). After that, it was exposed to a hydrogen pressure of 50 ± 0.5 MPa at 270 ± 2 °C for 300 hours to ensure that the hydrogen content inside the test specimen was uniform. After exposure, the temperature was lowered to room temperature, and after depressurization, the specimens were exposed to the atmosphere after being replaced with nitrogen three times. To avoid the release of hydrogen from the surface, the specimens were stored in a freezer at -70 °C [10]. To measure the hydrogen content, gas chromatography (ULVAC ST-200P special model) was used to heat the specimen from room temperature to 800 °C at a rate of 10 °C/min. As a result of the measurement, no clear emission peak was observed in the hydrogen-uncharged specimen, and it was confirmed that the hydrogen content was almost non-existent at 0.16 mass ppm. On the other hand, the hydrogen-charged specimen had an emission peak at around 500 ℃ , and the hydrogen content was 56 mass ppm This value is almost equal to the equilibrium hydrogen content (hydrogen content at the outer surface) of A286 specimen in 70 MPa gaseous hydrogen [11]. 2.3. SSRT tests and fracture surface observation SSRT tests specimens with 2.5 mm in diameter and 20 mm in parallel portion which was mirror-finished by α alumina were prepared. The strain rates of SSRT testing in gaseous hydrogen are specified in ASME BPVC Section VIII Division 3 Article KD-10, and ASTM G142-98 as 5 × 10 -5 and 7 × 10 -5 s -1 , respectively. In this study three strain rates, 5 × 10 -5 , 7.5 × 10 -6 and 1.25 × 10 -6 s -1 were employed to investigate strain rate dependance of SSRT tests. The SSRT test environment was 70 MPa gaseous hydrogen or air, and the temperature was 150 °C, which was the operating environment around the compressor. For comparison, SSRT tests were conducted for 56 ppm hydrogen charged