PSI - Issue 68

Renata Latypova et al. / Procedia Structural Integrity 68 (2025) 1115–1120 Author name / Structural Integrity Procedia 00 (2025) 000–000

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Figure 1. (a) Utilized DSC settings and (b) DSC curves with highlighted RA decomposition region.

Table 1. Mechanical properties and RA concentrations of the materials.

Material YS (MPa)

TS (MPa) Hardness (HBW) RA (%)

DQ T50

1310 (± 20) 1520 (± 12) 1370 (± 27) 1570 (± 17) 1480 (± 16) 1580 (± 9) 1420 (± 12) 1450 (± 8)

440 (± 9) 440 (± 6) 400 (± 21) 410 (± 5)

< 1 < 1 < 1

T150 T250

0

Galvanostatic H-charging was performed with 8x6x1 mm specimens at a current density of -1.5 mA/cm² (0.1 M H₂SO₄ + 1 g/L thiourea) for a duration of up to 4 hours followed by melt extraction using the GALILEO G8 ON/H (Bruker) device to determine the H concentrations of the materials. For each charging time, 3 repetitions were conducted. Furthermore, the H diffusion coefficients were assessed through EP tests. The H-charging side of the specimens was finished up to 600 grit, while the detection side was mirror polished and electroplated with a 1 µm layer of palladium. Each specimen had a thickness of 0.83 ± 0.01 mm. First, the detection side was polarized at 0.3 V Hg/HgO , and H-charging was initiated when the detection current density dropped below 0.1 µA/cm². During H-charging, specimens were pre-charged (-1.2 V Hg/HgO ) for 12h, followed by four alternating decay and build-up transients (-1.1 and -1.2 V Hg/HgO ). The charging process was concluded with a complete decay at 0.3 V Hg/HgO . The four partial transients were then analysed using a refined successive transient method to calculate H diffusion coefficients (D) (Liu et al., 2016). During EP, both compartments were filled with 0.1 M NaOH solution and were deaerated with N 2 throughout the test. Additional information about the test setup and procedure is thoroughly described in (Latypova et al., 2023b). Novel tuning-fork tests (TFT) were performed using an incremental step loading technique to determine the threshold stress levels of the materials (ASTM F1624-12, 2018; Latypova et al., 2023a) in H-charging environment. For each material, 2 – 3 tuning-fork specimens were stressed by bending so that the initial stress concentration at the notch was 25% TS. The specimens were then in situ charged with H (0.1M H 2 SO 4 + 1 g/l thiourea, -1.2V Hg/HgSO4 ) with a 5% hourly increase of the tensile load until the fracture of the specimen. 3. Results and discussion H concentration is highest for DQ, and it decreases progressively with increasing tempering temperature as shown in Figure 2. Since there are no transition carbides or their fraction is negligible in the tempered materials, the only occurring change in the LTT microstructures is C segregation and transformation of RA to cementite in T250. Therefore, the reduction in H concentration is attributed to C diffusion and the formation of Cottrell atmospheres, which reduce the available spots for H-trapping at dislocations (Sakamoto and Mantani, 1976; Makoto et al., 2021).