PSI - Issue 60

Ram Niwas Singh et al. / Procedia Structural Integrity 60 (2024) 411–417 RNSingh/ Structural Integrity Procedia 00 (2023) 000 – 000

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Hydride embrittlement is caused by formation of brittle hydrides and its fracture (Marchi and Somerday 2012, Bind and Singh 2021, Murty 2022, Alvarej 2004, Peterson 1992). Two forms of embrittlement have been recognized (Singh et. al., 2007). These are gross and localized. Gross hydride embrittlement is caused in hydride forming metals and alloys containing hydride volume fraction more than a critical value and it results in reduction in tensile ductility and fracture toughness. For plate shaped hydrides it is significantly influenced by hydride plate orientation with respect to tensile loading direction (Sharma 2018). In localized form of hydride embrittlement hydrogen migration takes place towards a localized spot where hydride precipitation takes place and results in stable crack growth before final failure of the components. The experimental observation of hydrides on both the fracture surfaces suggests that fracture occurs by hydride cleavage and not by debonding with the matrix (Singh 2004). Two mechanisms have been reported to explain dissolved hydrogen embrittlement (DHE) (Lee 2016). These are hydrogen enhanced decohesion (HED) and hydrogen enhanced localized plasticity (HELP). HED is caused by hydrogen induced decrease in cohesion energy across cubic cleavage planes. High hydrogen concentration is observed at trap sites and computational work supports bond weakening. During HELP, hydrogen diffusion near the crack tip takes place, promoting dislocation activity, micro-void formation and its coalescence resulting in failure. In situ TEM observations of dislocations in thin foils exposed to hydrogen gas, softening and strain localization in bulk specimens under some conditions support the occurrence of this mechanism (Lee 2016). Both hydride embrittlement and dissolved hydride embrittlement occurs at low temperatures and hence are observed during startup or shut down of a reactor. However, hydrogen attack is observed during service and is caused by reaction of H with C, N or S at elevated temperature. The fracture surface is marked with dimples. Low alloy steel piping and pressure vessels, Cu containing O are reported to be susceptible to hydrogen attack (Marchi and Somerday 2012). Susceptibility of hydrogen induced embrittlement depends to a varying extent on binding energy of hydrogen to the lattice, microstructural features like density and strength of trapping sites, applied and residual stress that may result in hydrogen redistribution and causing localized failure (Lee 2016, Marchi and Somerday 2012). Ingress of atomic hydrogen from common chemical processes such as acid pickling, electroplating, corrosive and cathodic charging has to be avoided by controlling the process parameters during these operations. The sensitivity to hydrogen attack is reported to be affected by the amount of carbon or carbide in the alloy, the hydrogen concentration, gas pressure, and temperature. It occurs typically in the temperature between 200 and 600 °C. Alloy steels with stable carbides (e.g., chromium-carbides) exhibit lower susceptibility to hydrogen attack due to the higher stability of Cr 3 C versus Fe 3 C observed in carbon steels (Lee 2016, Marchi and Somerday 2012). However, as the pressure and temperature of hydrogen environment increases, a greater amount of these alloying additions are required to reduce the severity of such attack. Thus, the factors that affect the susceptibility to hydrogen induced embrittlement are alloy chemistry, fabrication methods, microstructure, strength, surface finish, hydrogen partial pressure and temperature (Lee 2016, Marchi and Somerday 2012). Material scientists and engineers have carried out large investigations on hydrogen induced embrittlement, its failure mechanism and factors affecting susceptibility to hydrogen induced embrittlement, which have shaped the mitigation of this phenomenon so that early or premature failure of the components can be avoided (Lee 2016, Marchi and Somerday 2012). The strategy to mitigate the effects of this phenomenon has to start with selection of proper alloy chemistry, melting practice, hot and cold forming operations, heat-treatment cycle, welding procedure, pickling and surface treating operations during the manufacture of the components and control of hydrogen partial pressure and temperature during service. Baking at higher temperatures either in air or in vacuum and use of inhibitors reduces the hydrogen concentration and hence its deleterious effects (Lee 2016, Marchi and Somerday 2012). As described earlier the temperature range encountered during hydrogen economy will be from cryogenic temperature to 1000°C and the pressure could be as high as 140 MPa. The purity of hydrogen could also vary in various applications. Some of the materials used in hydrogen economy are thoroughly investigated for their susceptibility to hydrogen induced embrittlement and conditions for the same are well identified. Investigations are