PSI - Issue 54

Akihiko Fukunaga et al. / Procedia Structural Integrity 54 (2024) 115–122 A.Fukunaga / Structural Integrity Procedia 00 (2023) 000 – 000

116

2

1. Introduction The use of hydrogen energy, which does not emit greenhouse gases, has attracted significant attention to mitigate issues associated with global warming. Hydrogen gas has the lowest density at room temperature and protonates easily by bonding with other elements; therefore, it must be brought to a high-pressure state or liquefied for efficient transport. Hydrogen is pressurized to more than 70 MPa at refueling stations for fuel cell vehicles [1]; therefore, the materials used in such facilities must have high strength. However, the higher the strength of a material, the higher its hydrogen sensitivity. One method of evaluating the hydrogen compatibility for metallic materials is considering the value of relative reduction in area (RRA) obtained from slow strain rate tensile (SSRT) tests as an indicator [2, 3]. The SSRT test, which is a tensile test, is conducted at a very slow-strain rate under a certain ambient temperature and atmosphere. Although the conditions are different from those of a constant-load creep test, the SSRT test has been widely used in previous studies to evaluate the hydrogen compatibility of metallic materials. It is a deformation test in which the end time to failure can be anticipated. In Japan, austenitic stainless steels with RRA values of 80% or higher obtained from the SSRT test in high pressure gaseous hydrogen are classified as materials that are less affected by hydrogen. While, materials with values lower than that are restricted for use in high pressure hydrogen refueling facilities as materials with reduced breaking elongation. In hydrogen refueling stations, high-strength materials are needed because of the high pressures involved, and the temperature around the compressor is expected to rise to about 200 °C due to the heat generated by the compressed hydrogen gas as the pressure is increased. However, the RRA value for A286 is known to decrease at higher temperatures in the SSRT test. In this study, we investigated the hydrogen embrittlement mechanism for the iron-based superalloy A286 at 150°C in the operating condition of the compressor, which is used as a high-strength material in hydrogen refueling stations and liquefied facilities. 2. Experimental 2.1. Materials A286 (UNS. S66286, AMS5732, JIS SUH 660) is a precipitation hardening type heat-resistant alloy that has superior high-temperature strength than that of austenitic stainless steels up to 700 °C. As shown in Table 1, it contains more than 24 mass % Ni, and hardenable elements such as Ti and Al are added. After solution treatment at 980 °C and aging treatment at 720 °C for 16 h, the γ' phase (Ni 3 (Al, Ti)) is precipitated resulting in an iron-based superalloy with a significantly higher strength. The microstructure of the metal shows a structure similar to that of austenitic stainless steel [4]. The strength of the alloy is above 1000 MPa at room temperature and even at 700 °C the alloy maintains a strength of more than 700 MPa [5]. The alloy is also used for low temperature applications requiring a ductile, non-magnetic high-strength material at temperatures ranging from above room temperature to -253 °C.

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

2.2. Specimens of SSRT tests and fracture surface observation SSRT tests specimens of A286 were cut from the longitudinal direction of pipe and the specimen configuration for SSRT tests as shown in Fig.1. The parallel portion on the specimen was mirror-finished with alpha-alumina. The strain rates of SSRT testing in gaseous hydrogen using smooth specimens are specified in ASME BPVC Section VIII Division 3 Article KD-10, and ASTM G142-98 as 5 × 10 -5 s -1 and 7 × 10 -5 s -1 , respectively. Many SSRT tests are conducted in this strain rate region. In this study three strain rates, 5 × 10 -5 , 7.5 × 10 -6 and 1.25 × 10 -6 s -1 were