PSI - Issue 71

Johnny Adukwu et al. / Procedia Structural Integrity 71 (2025) 295–301

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0.6 mm) suitable for hydrogen charging and shear punch test configurations. Batch heat treatment of the steel samples was conducted to achieve the desired martensitic structures and mechanical properties, while adhering to ASTM-F 519-13 (4340) (ASTM F-519-13, 2013) and AMS-6257 (300M) (AMS-6257, 2007) standards. 2.2. Microstructural characterization The microstructures of heat treated 4340 and 300M samples were observed via a field-emission scanning electron microscope (FE-SEM, JOEL, MA) equipped with energy dispersive X-ray spectroscopy (EDS), secondary electron (SE) and electron backscattered diffraction (EBSD) detectors. Metallographic samples were prepared and polished with 60 to 1200 - grit SiC abrasive papers and 1µm diamond suspension. Secondary electron SEM images of samples were taken in the polished and etched conditions. Martensitic lath sizes were also revealed via etching in 2% Nital solution for 4 s. Prior austenite grains (PAG) and the martensitic lath sizes of the samples were confirmed using the FE-SEM and the EBSD detector operated at 15 kV. Different precipitates and their sizes within the microstructure of the samples were further analysed using EDS.

Table 1. Chemical composition of 4340 and 300M steel rods (wt%)

C Mn Si Cu Fe 4340 0.40 0.67 0.27 1.89 0.74 0.24 <0.01 <0.05 0.07 0.06 0.28 Bal. 300M 0.42 0.79 1.67 1.87 0.80 0.40 0.001 - 0.08 0.03 0.09 Bal. Ni Cr Mo Ti Co V Al

2.3 . Hydrogen diffusivity and content measurement The Davanathan-Stachurski cell set up was used according to the method in (Peral et al., 2019) to charge steel samples with hydrogen and to correlate the change in the mechanical properties with internal hydrogen content. The hydrogen diffusivity (D app ) and hydrogen content (C app ) were quantified using equation 1 and 2, respectively (Peral et al., 2019). = 2 6. ⁄ ………………… (1) C =(J ⋅M H ⋅ L) (D a PP ⋅ F ⋅ Fe ⁄ ) ………………… (2) Where L is the thickness of the sample thickness (m), F is the Faraday constant, M H is the molar mass of hydrogen, Fe is the density of iron, t Lag is the time (s) to reach 0.63 times the steady state anodic current density J (A/m 2 ) . Shear punch tests (SPTs) were performed on hydrogen charged and uncharged, 0.60 ± 0.02 mm thick, 4340 and 300M steel samples using UTM machine with a load capacity of 10 kN. The SPT device was procured from ADMET Testing Systems (Norwood, USA) and consists of a flat end punch of 2.00 mm diameter and a die of 2.08 mm diameter. Both the dies and the punch were made of hardened M2 tool steel. A displacement probe with a travel range of 5 mm measures the deformation of the sample. The SPTs data are acquired by punching the steel samples clamped between two dies at a displacement rate of 0.2 mm/min until fracture. The shear stress ( τ ) is computed from the SPT load displacement data as shown in equation 3 (Guduru et al., 2005). τ=P 2πr avg L ⁄ ……………………. (3) 2.4. Shear punch mechanical tests