PSI - Issue 68

Birhan Sefer et al. / Procedia Structural Integrity 68 (2025) 1129–1139 Author name / Structural Integrity Procedia 00 (2025) 000–000

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Figure 1. Microstructure of investigated materials a) SiMo51; b) CGI and c) NCF3015.

SSRT was performed according to ISO 7039:2024 using hollow specimens internally pressurized with 200 bar H 2 compared to inert gas (Ar) as reference. The specimens have a hole diameter of 2 mm and outer diameter of 8 mm and parallel length of 16 mm in the gauge section. The hole was produced through drilling with controlled parameters to achieve an inner surface roughness of Ra 0.2 μm. The testing was carried out using an MTS servo-hydraulic testing rig. Prior to testing, each specimen was flushed with Ar 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 and held constant for 10 minutes before testing began. For tests with 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 and maintained for one hour prior to starting the test. The tests were conducted at both room and high temperatures. The high temperatures were selected based on the maximum service temperature for the respective material and were as follows: 800 °C for SiMo51, 300 °C for CGI and 600 °C for NCF3015. For the high temperature testing, the specimens were heated by induction, and thermocouples were spot-welded onto the parallel section of the specimen at a distance corresponding to the gauge length of the contact extensometer. This ensured a uniform temperature over the area where strain was monitored and confirmed that the correct temperature was achieved before proceeding. 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 tests were defined as the point where a pressure drop occurred, indicating leakage from 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 Ar and H 2 performed at both room and elevated temperatures was examined. 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 content in the hollow specimens after the performed SSRT in H 2 . To prevent loss of hydrogen all SSRT specimens were immediately stored in liquid nitrogen after executing the SSRT. Only the gauge section of the 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. The following TDMS parameters were used to analyse the hydrogen content and its distribution: for SiMo51 and CGI the test specimens were heated from room temperature to 800 °C using heating rate of 1 °C/s while for the NCF3015 material the test specimens were heated for the same temperature range using 0.25 °C/s.