PSI - Issue 68

Birhan Sefer et al. / Procedia Structural Integrity 68 (2025) 1121–1128 Author name / Structural Integrity Procedia 00 (2025) 000–000

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Table 1. Nominal chemical composition in wt.% of Hastelloy X.

Element

C

Si

Mn

P

S

Al

Cr

Fe

Mo

Co

W

B

Cu

Ti

Ni

Min Max

20.50 17.00

8.00

0.50 2.50

0.20 1.00

Bal. Bal.

0.10

1.00

1.00

0.040 0.030

0.50

23.00 20.00 10.00

0.010

0.50

0.15

The additively manufactured specimens of Hastelloy X were printed by the standard Laser Powder Bed Fusion (LPBF) process. They were printed horizontally, which means that the loading direction in the testing is perpendicular to the columnar grain structure aligned in the building direction. This orientation was selected since it is the orientation that is most sensitive to any grain boundary embrittling phenomena. The porosity level is typically 0.1% (max 0.3%). SSRT was performed according to ISO 7039:2024 using hollow specimens with hole diameter of 2 mm and outer diameter of 8 mm and parallel length of 16 mm in the gauge section. It was carried out using an MTS servo-hydraulic testing rig. The hole was produced through drilling. Prior to testing, each specimen was flushed with inert argon gas and evacuated by vacuum three times to ensure that any residual gases were removed. After the final evacuation, the internal pressure was raised to the testing level 200 bar and held constant for 10 minutes before testing began. For tests using H 2 , a slightly different procedure was followed. After the initial argon flushing and vacuum evacuation, H 2 was introduced, pressurized, and then evacuated five times to fully purge the specimen of other gases. Following the final H 2 evacuation, the pressure was increased to the testing level 200 bar and maintained for one hour prior to starting the test. The tests were conducted at both room temperature and 800 ° C. For the high temperature testing, the specimens were heated by induction. Thermocouples were spot-welded onto the parallel section of the specimen at a distance corresponding to the gauge length of the contact extensometer. During the heating phase, the internal gas expanded, and adjustments were made to accommodate this expansion before initiating a 10-minute dwell period for temperature stabilisation. All tests were conducted in displacement control mode, with a constant displacement rate of 5×10⁻⁵ s⁻1. The failure criterion for both argon and hydrogen testing were defined as the point where a pressure drop occurred, indicating leakage of the specimen. Scanning electron microscope (SEM) and secondary electron imaging was used to perform a fractographic analysis of the specimens after the SSRT. The fracture surface of specimens tested in argon and hydrogen performed at both room and elevated temperatures was inspected. Thermal desorption spectroscopy product of Bruker (Galileo 8) equipped with mass spectrometer (TDMS) product of IPI and external tube infra-red furnace also product of Bruker (IR07) was used to analyse the hydrogen after the performed SSRT tests in H 2 at room and high temperatures. To prevent loss of hydrogen all SSRT specimens were immediately stored in liquid nitrogen after executing the SSRT test. Only the gauge section of the hollow specimens was analysed. As reference a test piece manufactured from the as-delivered materials with similar geometry and size of the gauge section of the hollow specimens was used. For the TDMS analysis the test specimens were heated from room temperature to 800 °C using heating rate of 0.25 °C/s and the desorbed rate of hydrogen was measured as a function of temperature. The curves were integrated to determine the total hydrogen content in the specimens. 3. Results Representative stress-strain curves obtained by SSRT using hollow specimens for the Hastelloy X material tested in 200 bar H 2 and Ar are presented in Figure 1 Error! Reference source not found. for hot rolled material and in F igure 2 for AM material. The evaluated yield strength (Rp0.2 h ), ultimate tensile strength (Rm h ) and elongation to leakage (A h ) are presented in Table 2. Note that h stands for denomination of the properties according to ISO7039:2024 and using hollow test specimen. The presented property values are average values for three and two specimens tested in H 2 and Ar, respectively. The relative values for the properties are calculated as ratio between the average values obtain in H 2 and Ar. From the SSRT results it is evident that the yield strength of the hot rolled material at room temperature is significantly higher for the AM material compared to the hot rolled even though the ultimate strength is similar for both. Moreover, it is also observed that the tensile strength is much lower at 800 ° C compared to room temperature for both hot rolled and AM materials, respectively. The elongation to leakage was larger at both tested temperatures for the hot rolled compared to the AM material. Finaly, for hot rolled material the elongation appeared to be larger at 800 ° C compared to room temperature, while the AM material showed the opposite behaviour, i.e. larger