PSI - Issue 68

Birhan Sefer et al. / Procedia Structural Integrity 68 (2025) 1129–1139 Author name / Structural Integrity Procedia 00 (2025) 000–000

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Keywords: CGI; NCF3015; SiMo51; hollow specimen method; SSRT; TDMS; high pressure hydrogen gas; high temperature; hydrogen embrittlement.

1. Introduction Hydrogen gas (H 2 ) is considered to have a strong potential to be used as green fuel in the automotive sector. Combustion of H 2 results in formation of water vapor (H 2 O) and NOx emissions that will be transformed into N 2 gas in the after-treatment system with urea dosing. Hence, utilization of H 2 as fuel will tremendously reduce the CO 2 emissions from vehicles. Diesel internal combustion engines (DICE) could be used with H 2 combustion, however the materials used in DICE components needs to be evaluated in terms of risk of mechanical degradation due to hydrogen embrittlement.The operating conditions of the combustion section of DICE as well as in the exhaust would include H 2 at pressures up to 300 bar and wide range of temperatures starting from ambient temperature up to high temperatures in the range of 300-800 °C. Hence, evaluation of the mechanical performance at these conditions of the metallic materials is required. The up-to date literature regarding the influence of hydrogen on the mechanical integrity of these types of materials is scarce, particularly regarding interaction of hydrogen with metallic materials under high H 2 pressure and high temperature. Most of the available works in literature are focused on cast iron, especially ductile iron and nodular iron, while no relevant published work was found for the Fe-Ni-base alloy NCF3015. Matsunga et al. (2013) and Usuda et al. (2013) reported ductility loss due to hydrogen of ductile cast iron. By using thermal desorption spectroscopy (TDS) and microprint method the authors showed that most of the absorbed hydrogen is diffusible and segregated in the graphite or graphite/matrix interfaces as well as in the cementite of the pearlite in the matrix. Sahiluoma et al. (2017) found that hydrogen in nodular cast iron reduced the ductility and time to fracture using SSRT and constant load testing, identifying two distinct hydrogen peaks in the TDS analysis. The low temperature peak corresponded to hydrogen trapped at small cracks/voids forming at the interfaces between the graphite nodules and ferrite matrix, while the high temperature hydrogen peak indicated hydrogen trapped in the graphite nodules. In this work, it was concluded that hydrogen caused brittle cleavage fracture of the ferrite matrix. Matsuo (2017) studied the role of perlite on the hydrogen induced ductility loss of ductile cast irons and concluded that increase of the perlite fraction of the cast iron leads to higher risk to hydrogen embrittlement. Matsuno et al. (2012) proposed combination of the following three mechanisms as main possible reasons for the hydrogen induced degradation of ductile cast iron: 1) supply of hydrogen to the crack tip from the hydrogen trapped in the graphite and graphite/ferrite interfaces, 2) hydrogen enhanced perlite cracking and 3) successive release of trapped hydrogen in the graphite and additional supply of hydrogen to the crack tip. Cho and Lee (1989) studied the influence of hydrogen on the mechanical properties in cast irons with various carbon equivalent and graphite morphologies. The authors concluded that cast iron with flake graphite particles exhibited greatest tendency for interface decohesion between the ferritic matrix and the graphite, while a clear transition from ductile to brittle failure mode was observed for the vermicular cast iron and ductile iron. The most recent work by Turola et al. (2023) suggested that cast irons containing graphite can accommodate diffusible hydrogen and thereby delay the embrittlement effect. In their work they demonstrated that by controlling the quantity and morphology of the graphite particles in combination with presence of key alloying elements and phases, it is possible to mitigate the embrittlement effect. The authors demonstrated that even harsh hydrogen charging procedures, including both cathodic and gaseous hydrogen charging at elevated temperatures and hydrogen pressure have not resulted in embrittlement effect. The authors argued that the reason for such behaviour is because the material have not reached saturation with hydrogen. In this work, two conventional materials commonly used for manufacturing components in the combustion section of a DICE such as cylinder head (compacted graphite iron-CGI) and engine valves (iron-nickel base alloy- NCF3015) developed by Sato et al. (1998) as well as one material commonly used for manufacturing exhaust components (cast iron-SiMo51) were selected and assessed concerning mechanical degradation due to H 2 pressurized at 200 bar. The selection of materials was with intention to cover the operation temperature range of DICE, 300-800 °C. It should be emphasized that in theory, the exhaust should not contain any H 2 because it is expected that the H 2 will be completely combusted. However, we chose to study SiMo51 due to potential presence of small amounts of unburned H 2 that may