PSI - Issue 71

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

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These precipitates are speculated to be (Cr,Mn) carbides and (Mo,Si) carbides in 4340 and 300M, respectively. These mixed metal carbides are commonly formed during the tempering process of high strength steels (Peral et al., 2019). Notwithstanding, (Cr,Mn) carbides in 4340 have are coarser in size than (Mo,Si) carbides in 300M. 3.2. Hydrogen diffusivity in 4340 and 300M The electrochemical permeation results revealed significant differences in the hydrogen diffusivity between the two steels. Figure 3 (a), shows the differences in diffusivity, revealing faster hydrogen diffusion kinetics and higher permeation capability for 4340 than 300M. For the three charging conditions, the permeation rates of 4340 tend to increase with current density. Figure 3 (b) shows an expanded portion at the onset of permeation, showing 4340 takes a shorter time to diffuse through the steel compared to 300M. Consequently, a higher apparent diffusivity is obtained for 4340 (8.83 × 10 -11 m 2 /s) than 300M (3.53 × 10 -11 m 2 /s).

Fig. 3. (a) Permeation curves of 4340 and 300M steels (b) Enlarged portion of the permeation transients in (a) showing the early stage of hydrogen charging (marked with red color) This difference in diffusivity can be attributed to (Mo,Si) carbides that can strongly trap hydrogen and reduce its mobility in 300M compared to (Cr,Mn) carbides in 4340. Other researchers showed that mixed metal carbides containing Mo can trap significant amounts of hydrogen (Nagao et al., 2014). This finding was further supported by (Lui et al., 2024) using atom probe tomography (APT), however the trapping nature of these precipitates needs to be investigated further. SPT curves (Figure 4 (a)) shows that the mechanical properties of 4340 and 300M degrade with the entry of hydrogen during the permeation test. The shear stress reduces with increase in the current density. However, a sharp drop in shear stress is observed for 4340 steel which indicates more hydrogen entry, resulting in greater loss of mechanical properties compared to 300M for the same charging conditions. For the ease of identification, the hydrogen concentration in 4340 and 300M samples for each charging condition is labelled in Figure 4. Correspondingly, a progressive loss in fracture strength with increasing hydrogen content is observed for both steels, but this is more significant for 4340 compared to 300M as indicated in Figure 4 (b). The superior performance of 300M is attributed to the reduction in hydrogen diffusivity which can be attributed to the strong trapping capacity of (Mo,Si) carbides. Strong hydrogen trapping reduces the deleterious effect of mobile hydrogen moving into critical areas such as crack and voids during the fracture process and therefore enhanced the HE resistance in 300M than 4340. Moreover, some studies reported that the increased addition of molybdenum to high strength steels significantly improves the resistance to HE (Yoo et al., 2021). However, more studies will be carried out to confirm the current observations. 3.3. Hydrogen content effects on the mechanical properties of 4340 and 300M