PSI - Issue 68

Eduard Navalles et al. / Procedia Structural Integrity 68 (2025) 1105–1114 Eduard Navalles / Structural Integrity Procedia 00 (2025) 000–000

1107

3

Table 1. Chemical composition of investigated steels.

Element wt. % Ferritic-pearlitic

C

Mn

P

Si

S

Cr

Ni

Mo

Cu

N

Ti

V

Al

Fe

0.18 1.10-1.70

0.02

0.5

0.005 0.30 0.80 0.08 0.30 0.02 0.03 0.10 0.02 Bal.

Bainitic

0.24 0.70-1.35 0.025 0.15-0.50 0.025 0.25 0.25 0.08 0.35 - - - - Bal.

Figure 1. SEM secondary electron images extracted from the middle of the plates longitudinal to the rolling direction of a) ferritic-pearlitic steel and b) bainitic steel.

Figure 2. a) Drawing of the hollow specimen. b) Example of hollow specimen tested at RT.

Slow strain rate tensile testing with strain rate of 10 -6 s -1 was used to evaluate the effect of hydrogen on the tensile properties of both steels. Five specimens of each steel type were tested, two in argon and three using pure hydrogen environment. All tests were performed at room temperature using 200 bar gaseous pressure. The low cycle fatigue testing was performed according to ASTM and ISO standards, (ASTM E606/E606M-21, 2021; ISO 12106:2017, 2017), using a triangular waveform with strain-controlled deformation at total strain amplitudes of 0.3%, 0.6% and 1.2%, fixed frequency of 0.5 Hz and a strain ratio of R = 0 was applied. For each steel, a total of twelve specimens were tested: for 1.2 % and 0.6 % total strain amplitudes, two were in argon and three in hydrogen. For the 0.3 % total strain amplitude, one specimen was tested in argon and one in hydrogen. Prior each test, the specimen was flushed several times with the pertinent gas (Ar or H 2 ) to ensure a pure gaseous environment with no oxygen. After flushing, a dwell time at 200 bar pressure of 15 minutes was used for the argon tested specimens and 1 hour for the hydrogen one to ensure no gas leakage and pressure loss. All mechanical tests were completed to a pressure drop of below 5 bar, meaning that the tests were not carried out until failure, but until leakage, i.e. pressure drop. After the completion of SSRT or LCF, the specimens for fractography investigations were pulled apart and the specimens for thermal desorption spectroscopy with mass spectrometry (TDMS) analysis were submerged in liquid nitrogen. An analysis of distribution and quantification of the total hydrogen content, being the sum of diffusible and trapped hydrogen were carried out using a thermal desorption spectroscopy (TDS) instrument (Bruker, model Galileo G8) coupled to an InProcess Instruments mass spectrometer (MS) with a quartz tube infrared furnace (Bruker, model IR07). The calibration of the TDMS instrument was performed using three defined volumes of pure hydrogen gas, repeating the procedure three times to ensure accuracy of the calibration curve. Prior to calibration and measurements, all molecular sieve filters were replaced. The hydrogen content was measured by heating the specimens from room temperature to 800 °C using a heating rate of 1 °C/s with additional hold for 900 seconds at 800 °C to ensure desorption of all hydrogen present in the specimen. 3. Results and Discussion 3.1. Influence of hydrogen gas on tensile properties Figure 3 shows stress-strain curves obtained for both steels tested in Ar (black curves) and H 2 environment (red curves) at 200 bar and room temperature, up to leakage. Both steels exhibit similar behaviour, with no degradation in yield strength and ultimate tensile strength (UTS). Therefore, it may be corroborated that for these steels, hydrogen does not influence their performance up to the UTS. However, a noticeable reduction in elongation can be seen, indicating a detrimental effect of hydrogen on the ductility and steel toughness.