PSI - Issue 42

Lisa Claeys et al. / Procedia Structural Integrity 42 (2022) 390–397 Claeys et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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plasticity (TWIP) and transformation-induced plasticity (TRIP) effects (De Cooman et al. (2018); Sohrabi et al. (2020)). Next to the specific deformation behavior, the face centered cubic (FCC) crystal structure of austenitic steel types is also characterized by a combination of a high hydrogen solubility and a low hydrogen diffusivity. Especially the low hydrogen diffusivity has a beneficial effect on the hydrogen embrittlement (HE) sensitivity, making austenitic steels intrinsically desirable candidates for use in hydrogen-containing applications (Martin et al. (2019)). Nevertheless, accelerated fracture was also observed for austenitic steels in the presence of hydrogen. Moreover, differences in alloying strategy between austenitic steel types were shown to impact the HE resistance, e.g. (Michler & Naumann (2010); Gavriljuk et al. (2003)). This difference was most often related to the type of alternative deformation mechanism involved since hydrogen interacts with all deformation-induced features having a variable effect on the mechanical performance (Claeys et al. (2019); Claeys et al. (2021); Zhang et al. (2021)). Hydrogen was sometimes even stated to improve the mechanical performance of FCC steels at specific conditions (Yamada et al. (2016); Ogawa, et al. (2020)). The aim of this work is to compare the hydrogen interaction with two different austenitic steel types that both show excellent mechanical behavior without hydrogen, albeit triggered by other deformation mechanisms. These steels include 304L austenitic stainless steel prone to martensitic transformation and twinning-induced plasticity (TWIP) steel prone to deformation twinning. Moreover, the impact of the alloying strategy on the fracture mechanisms in the presence of hydrogen will be evaluated. 2. Experimental procedure 2.1. Materials Two austenitic steels were studied in this work. The first steel grade was AISI 304L austenitic stainless steel (ASS) received as a cold rolled and annealed plate with an initial thickness of 0.9 mm. The second austenitic steel was a TWIP steel received as a hot rolled plate with an initial thickness of 2 mm. The chemical composition of the materials as provided by the manufacturers is presented in Table 1. Different alloying strategies were used to stabilize a fully FCC structure at room temperature. 2.2. Experimental methods To introduce hydrogen in the two austenitic steels, galvanostatic electrochemical hydrogen charging was performed. The 304L ASS was charged in an electrolyte composed of 0.5M H 2 SO 4 and 1 g/L of thiourea. A constant current density of 0.8 mA/cm² was applied for seven days and charging was performed at room temperature. The TWIP steel was charged in a less corrosive environment, since the absence of corrosion-limiting alloying elements such as Cr, Ni and Mo appeared to jeopardise hydrogen charging in a sulphuric acid solution. A mixture of 1.31 mol/l sodium tetraborate decahydrate (borax, Na 2 B 4 O 7 .10H 2 O) in glycerol (C 3 H 8 O 3 ) was, therefore, used. An adequate conductivity was reached by adding 20 vol% of distilled water. The solution was used at a temperature of 50°C with a constant current density of 5 mA/cm² was applied for seven days. Melt extraction was performed to quantify the total hydrogen content in the materials after the charging procedure. A Galileo G8 was used for this purpose, equipped with an impulse furnace and a thermal conductivity detector. The specimen’s dimensions were 8x6x0.7 mm³ (Length x Width x Thickness). The surface was polished up to 1 µm. To evaluate the influence of hydrogen on the mechanical properties, constant extension rate tensile tests were performed. The specimens were tested both until fracture and until an intermediate engineering strain of 30%. As such, the fracture mechanism and the active deformation mechanisms could be investigated, respectively. Two tests were performed until fracture for every condition to verify the reproducibility of the results. Reference tests were Table 1. Chemical composition of the studied materials. Material C Cr Ni Mn Mo Si 304L ASS TWIP steel 0.025 18.05 8.05 1.81 0.32 1.54 0.6 / / 18 / /