PSI - Issue 71

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

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requirements in standard aerospace applications (Boellinghaus et al., 2019). These structural components are usually subjected to extreme stress and environmental conditions during take-off and landing (Zheng, 2021). Therefore, there is a need for materials that can withstand high loads while maintaining the structural integrity of landing gear systems. Martensitic steels of body-centred tetragonal structure (BCT) are widely used in aerospace landing gear components (Wang et al., 2023). This is due to their exceptional mechanical properties such as high tensile strength and fracture toughness. However, landing gears operate at high impact loads and harsh corrosive environments which make them susceptible to stress corrosion cracking and degradation (Zhao et al., 2019). Therefore, external protection is typically implemented via sacrificial metal coating and plating processes to enhance the service life and durability of these components (Bellemare et al., 2020). On the other hand, electroplating processes can become sources of hydrogen entry, which can lead to HE of these components. Electroplating strategies including zinc (Zn), nickel (Ni), and cadmium (Cd) plating have been widely used to improve the corrosion resistance of landing gear components (Singh et al., 2024). Nevertheless, hydrogen produced during the plating process of these components may infiltrate into the steel substrate and result in HE (Singh et al., 2024). HE in plated steel substrates can occur as direct HE when hydrogen trapped in the metal coating slowly ingresses into the steel substrates (Figueroa and Robinson, 2010). Even after baking, residual hydrogen can still be trapped within the microstructure of steel, making them susceptible to HE (Bellemare et al., 2020). Trapped hydrogen content of a few wt. ppm can lead to a delayed failure under high stress conditions (Sun et al., 2023). In addition, re-embrittlement can still occur when coated steel corrodes in service which creates an electrochemical condition sufficient for hydrogen generation and absorption in the coated steel (Sun et al., 2023). Consequently, there is a need to develop steels having microstructures with inherent resistance to hydrogen uptake as well as less susceptibility to HE. This would minimize the complete reliance on protective coatings and reduce the risk of hydrogen-induced failures in high strength martensitic steels used in landing gear. The martensitic microstructure of high-strength aerospace steels plays an important role in their susceptibility to HE. Meanwhile, the susceptibility to HE increases as the strength of this class of steel increases (Seo et al., 2020). The microstructure of martensitic steels contributes to high hydrogen diffusivity (Venezuela et al., 2016). Mobile hydrogen diffuses and interacts with lattice defects within the microstructure such as dislocations, grain boundaries, and second phase particles (Peral et al., 2019). Hydrogen can be strongly or weakly trapped at these lattice imperfections. The nature of hydrogen trapping depends on the traps binding energy within microstructural features. Strong traps such as second phases precipitates have high binding energy whereas weak traps such as voids and dislocations have a low binding energy. Nevertheless, the movement and accumulation of weakly trapped hydrogen in critical areas such as crack, interfaces and micro voids determine the severity of hydrogen induced failure in these steels (Sun et al., 2023). Tempered and quenched AISI 4340 steels are conventionally used for aircraft landing gears due to their desirable mechanical properties (Bakhshi and Mirak, 2022). However, the need to improve the properties of AISI 4340 steel led to the development of other high strength steels such as 300M by modifying some of the alloying elements and heat treatment conditions (Kasana et al., 2022). The addition of alloying elements can also form second-phase particles such as carbides and mechanically strengthen these steels (Zhou et al., 2022). However, carbides and other second phase precipitates act as stress concentrators and hydrogen trapping sites (Sun et al., 2023). Some studies have shown that carbides, depending on their size and distribution can influence the response of steels to HE (Sun et al., 2023). Despite significant progress in understanding HE in aerospace steels, critical gaps remain in the literature, regarding the role of carbides formed by alloying elements. This research seeks to fill these gaps by investigating how 4340 and 300M perform when subjected to HE environment and the role of different carbides on the HE response in these steels. Experiments are conducted using the Devanathan-Stachurski permeation setup to study the apparent hydrogen diffusivity in these steels. Shear punch tests are also conducted to study the changes in mechanical properties in the presence of hydrogen. The study aims to provide valuable insights into the HE resistance of aerospace steels, enabling the design of more resilient materials for critical applications, where minimizing susceptibility to hydrogen is crucial for safe and reliable performance. 2. Experimental 2.1. Materials and methods The chemical compositions of the normalized 4340 and 300M steels used for the study are shown in Table 1. They were originally supplied as rods (nominal diameter: 25 mm, length: 350 mm) in standard annealed condition. Subsequently, the steel rods were reduced to 13 mm and were cut into circular samples (diameter: 13 mm, thickness: