PSI - Issue 54

Liese Vandewalle et al. / Procedia Structural Integrity 54 (2024) 180–187 Liese Vandewalle/ Structural Integrity Procedia 00 (2019) 000 – 000

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Figure 5 shows the H contents present in the Fe-C-V samples as determined by hot extraction. The material austenitized at 1250°C contained very low amounts of H, around 0.1 wppm, in its air tempered condition but also after tempering in the H containing environment. Consequently, the precipitated vanadium carbides were not able to take up H at the elevated tempering temperature. On the other hand, the material austenitized at 950°C did show a small increase in H content upon changing the tempering environment from air to formier gas. Hence, limited trapping could be observed due to the presence of the undissolved carbides. The undissolved titanium carbides resulted in a much higher uptake (up to 7.46 wppm), from the same environment and at the same tempering conditions. However, it must be noted that due to the high stability of the titanium carbides the Fe-C-Ti alloys showed much larger and more undissolved carbides than the Fe-C-V alloys.

Figure 5: Hydrogen content as determined by hot extraction at 950°C of the Fe-C-V alloys austenitized either at 1250°C or 950°C after tempering in air or in a hydrogen containing atmosphere.

The hydrogen uptake by the undissolved titanium carbides resulted in strong hydrogen trapping, as was discussed by Vandewalle et al. (2023) and is also shown in Figure 6b, containing the TDS spectra of the two Fe-C-Ti alloys varying in carbon and hence undissolved carbide content. Clearly, the strong trapping at the undissolved titanium carbides resulted in a high temperature peak. However, for the Fe-C-V 950 alloy, the increase in H content did not correspond to the appearance of a high temperature peak in the TDS spectra, as can be seen in Figure 6a. Instead a small peak at relatively low temperatures, around 180°C, could be observed. Using the Kissinger equation (Kissinger (1957)) according to the Choo-Lee method (Choo and Lee (1982)), an E a value of approximately 27 kJ/mol was obtained. By subtraction of the E d , the value of E B can be assumed to be around 22 kJ/mol. When comparing this value to those reported in literature, it is much lower than E B values to the bulk and interface C vacancies found by the DFT calculations of Kawakami and Matsumiya (2012)) and Echeverri Restrepo et al. (2020)) as well as the experimentally obtained values for trapping at interface C-vacancies at semi-coherent vanadium carbides reported by Depover and Verbeken (2016b), Drexler et al. (2020), and Takahashi et al. (2018). However, E a values in the range of 27 to 35 kJ/mol were reported by Asahi et al. (2003), Lee et al. (2016), and Turk et al. (2018) for trapping at vanadium carbides. Both Turk et al. (2018) and Lee et al. (2016) indicated the interface of the precipitated carbides as the main trapping site. Lee et al. (2016) further suggested that the interface C vacancies at the precipitated carbides are the active trapping sites but also reported a significant contribution of the C-vacancies at the broad interface of undissolved V 6 C 5 particles. Additionally, Takahashi et al. (2018) reported E a values of around 25 kJ/mol in under-aged samples, where trapping was proposed to be related to the broad coherent (001) TiC /(001) Fe interface containing few C-vacancies. Considering that only additional H uptake was observed for the material austenitized at 950°C, containing undissolved carbides, and not for the material austenitized at 1250°C, the precipitated carbides were excluded as responsible trapping site and hence trapping was not related to coherent strain field nor to the coherent VC/matrix interface. However, it might be related to trapping by interface C vacancies of special undissolved carbides, like V 6 C 5 , in correspondence to the results of Lee et al. (2016). The discrepancy with the values obtained by DFT calculations of Kawakami and Matsumiya (2012) and Echeverri Restrepo et al. (2020) might be related to the different crystal structures resulting in a change of E B .