PSI - Issue 68

Vahid Javaheri et al. / Procedia Structural Integrity 68 (2025) 1098–1104 V. Javaheri et. al, Structural Integrity Procedia 00 (2025) 000–000

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2.2. Tensile Testing Slow strain rate tensile (SSRT) tests were conducted using a Zwick/Roell 100 machine at room temperature (25°C) with a strain rate of 1.0 × 10⁻⁶ s⁻¹. Tests were performed on dog-bone shaped samples, both with and without hydrogen pre-charging. The samples were machined according to IS 16573-1:2020 standards, with a total length of 45-50 mm, a gauge length of 14.4 mm, and a diameter of 1.35 mm. Hydrogen charging was performed in a sulfuric acid (H₂SO₄) solution with a pH of 2.5, containing 0.1 mass% ammonium thiocyanate (NH₄SCN) for 24 hours. The test pieces were immersed in the solution at 25°C, using platinum as the counter electrode, with a current density of 3 mA/cm². The SSRT tests commenced within approximately 10 minutes after hydrogen charging. Under these conditions, the total hydrogen content was approximately 10 mass ppm,

as determined by the melt extraction test. 2.3. Microstructural Characterization

Microstructural analysis was conducted following standard metallographic sample preparation procedures. A JEOL JSM-7900F Field Emission Scanning Electron Microscope (FESEM) equipped with Electron Backscatter Diffraction (EBSD) was used. The analysis was performed with an accelerating voltage of 20 kV, a working distance of 15 mm,

and a step size of 50 nm. 3. Results and Discussion

Figure 1 depicts the initial microstructure of the studied steel after undergoing hot rolling followed by intercritical annealing. The microstructure predominantly consists of tempered/recrystallized martensite (α) and retained austenite, where the RA(γ) mainly exhibits a lamellar morphology, distributed within the α-matrix. Additionally, a minor fraction of fresh martensite (α') is present, formed during the final stages of cooling. Fine cementite precipitates are also observed, contributing to the overall complexity of the microstructure. Together, these phases create an almost homogeneous dual-phase microstructure, where the α-matrix and discontinuous γ dispersion coexist. Phase fraction analysis determined that the initial average RA(γ) fraction is around 40 vol.%. The mechanical properties of the samples were assessed through slow strain rate tensile (SSRT) testing, both in hydrogen-free and hydrogen-charged conditions (Figure 2). In the absence of hydrogen, the material exhibited excellent mechanical performance, characterized by high yield strength (YS), ultimate tensile strength (UTS), and significant total elongation (TEL). Additionally, the material demonstrated a moderate work-hardening response, indicative of its modest deformation capability.

Fig. 1. The initial microstructure of studied sample (after hot rolling and subsequent intercritical annealing)