PSI - Issue 79

Giuseppe Macoretta et al. / Procedia Structural Integrity 79 (2026) 508–516

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The uniformity of the mechanical properties along the sheet thickness was investigated via microhardness tests. Patterns of Vickers indentations were carried out by using a load of 0.1 kg and a dwell time of 10s, employing a Struers DuraVista-40A hardness tester (Struers ApS, Ballerup, Denmark). A pitch of 0.08 mm between the indentations was used, in compliance with the ASTM E384 standard, resulting in 14 indentations per specimen. Microhardness investigations were carried out on samples perpendicular to the sheet plane, polished and etched by employing the same procedure adopted for microstructural investigations.

2.2. Permeation and TPD tests

To provide a basic characterization of H diffusion and trapping in the tested materials, a permeation test and a TPD test were performed. For the permeation test, a standard Devanathan Stachurski double setup was employed, with NaOH 0.1 M solution in both anodic and cathodic cells. Square specimens of 50x50x1.2 mm size were cut from the metal sheet. Specimen preparation and setup were the same as described in [14]. The specimen outlet face on the anodic cell was electrodeposited with a thin Pd layer and polarized at +300 mV against Ag/AgCl reference electrode. After the background current reached values below 0.1 μ A/cm 2 , the cathodic cell was filled with electrolyte, and permeation testing began. A buildup and decay transients between a steady state of -1 mA/cm 2 and -0.5 mA/cm 2 and back to -1 mA/cm 2 cathodic current were considered. TPD tests were carried out by using the Hiden TPD Workstation (Hiden Analytical, UK), which is equipped with a quadrupole mass spectrometer that continuously measures the H flux escaping from the sample as it is heated. Specimens of 10 x 10 x 1.2 mm were cut and prepared following the same procedure as the permeation specimens. Hydrogen charging was carried out by using a NaCl 35 g/L + NH4SCN 2 g/L solution and a cathodic current of 1 mA/cm 2 and charged for 48h. Linear heating ramps up to 1000 °C were adopted, following a rate of 10, 2.5, and 1 °C/min, while the vacuum level in the measuring chamber kept a value of 1 10 -8 mbar. Three samples were investigated for each heating rate. The influence of H on the static mechanical properties of QP1180 was examined through tensile SSRT, followed by H measurement and fractographic analyses. A batch of 14 smooth specimens, featuring a gauge length of 24mm and a ligament width of 6mm, was extracted by laser cutting from the same sheet. The specimen longitudinal axis was parallel to the sheet rolling direction. The specimen geometry is reported in Fig. 1. Hydrogen was introduced into the specimens via electrochemical charging before testing. Samples were charged at room temperature in an aqueous solution with 3.5% NaCl and ammonium thiocyanate, after the zinc coating had been removed. An MTS 634.31F-25 extensometer, having a gauge length equal to 10 mm, was used to measure the average strain in the central region of the specimen during the test. The adopted crosshead speed led to a strain rate of 1.5 10 -5 s -1 . Immediately after the mechanical test, the hydrogen content was measured via the hot extraction method on one half of the specimen using the LECO DH603 H analyzer (LECO, St. Joseph, MI). The effective hydrogen concentration in the critical region of each specimen was thus determined, enabling a correlation between mechanical properties and hydrogen content. Fractographic analyses were carried out via stereomicroscopy and SEM, by using a FEI QUANTA 450 ESEM FEG at the “Centro per la Integrazione della Strumentazione della Università di Pisa (CISUP)”. 2.3. Tensile SSRT and fractographic analysis

Fig. 1. Geometry of the SSRT specimen. Dimensions in millimetres.