PSI - Issue 13

Evy De Bruycker et al. / Procedia Structural Integrity 13 (2018) 226–231 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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When comparing the results of T24 with these of T12, the amount of diffusible hydrogen in T12 is significantly lower than in T24 under the same charging conditions, both for the base material as for the simulated HAZ. This makes T12 far less susceptible to HIC than T24 and hence a good choice when mitigating HI-SCC. For T12 the simulated HAZ with high hardness (worst case) has a larger amount of strong hydrogen traps compared to the base material and the normal HAZ. For T24 base material the total amount of hydrogen is higher than for the simulated HAZ, due to a larger amount of strong hydrogen traps, while the amount of weak traps (diffusible H) is lower. After PWHT the amount of strong traps in the HAZ stays the same, but the amount of weak traps (diffusible H) increases. This is in correspondence with the expected microstructures. The T24 base material is a bainitic/ferritic structure containing a lot of small carbides (VC and (Ti,V)C), which are known to be strong hydrogen traps (Spencer and Duquette 1998). The T24 HAZ material has an untempered martensitic structure containing martensite laths and dislocations, which are known to be weak hydrogen traps (Pérez Escobar et al. 2012). The higher amount of weak traps after the PWHT treatment could be explained by the precipitation of coherent MC carbides during the heat treatment, which are reported to be relatively weak hydrogen traps, as opposed to the incoherent MC carbides, which are strong hydrogen traps (Wei et al. 2004). Also worth noting is that for both types of material (T24 and T12) the distribution of strong hydrogen traps inside the base material is more heterogeneous than for the simulated HAZ microstructures. 3.2. Thermal desorption spectrometry The obtained spectra are shown in Fig. 2. Good reproducibility of the measurements was demonstrated by measuring 2 T24 BM samples, which showed very similar results.

Fig. 2. TDS spectra of the 3 selected materials.

Below 300°C clear peaks corresponding to the diffusible hydrogen are visible. There is an excellent correlation between the surface under these peaks and the measurements of diffusible hydrogen. No clear peaks related to high energy trapping were observed on any of the materials. This means that the strongly trapped hydrogen is either released at temperatures above 700°C or is released gradually as a continuous signal, which cannot be separated from the background signal. 3.3. Apparent coefficient of diffusion The apparent diffusion coefficients determined from the permeation curves are listed in Table 3.The so determined coefficient of diffusion is smaller than the one corresponding to the diffusion in the iron lattice (1E-8 m²/s for ferrite at 20°C), since the diffusion process is hindered by trapping. The lower the apparent diffusion coefficient the higher the amount of hydrogen traps (dislocations, grain boundaries, voids, laths interfaces, etc.) in the material.