PSI - Issue 77

A. Hell et al. / Procedia Structural Integrity 77 (2026) 41–48 Author name / Structural Integrity Procedia 00 (2026) 000–000

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magnitude of 0.8 ppm. Measurement and fit data further indicate that a steady hydrogen concentration is reached in the first hours of charging.

Fig. 1. (a) Total hydrogen concentration in reference sheet samples; (b) Idealized hydrogen distribution in charged reference sheet samples.

3.2. Calculation of CT50 hydrogen concentration profile The exponential fit plateau in Fig. 1a states that hydrogen diffused almost completely in the 1.4 mm thick reference samples after 8 hours of charging. In a 1D diffusion calculation, an effective diffusion coefficient of D eff =2.5*10 -11 m 2 /s fits to this observation (Fig. 1b). As a comparison, in permeation measurements of Tau and Chan (1996), the diffusion coefficient in rolled AISI 4340 steel with a ferritic-pearlitic microstructure was determined to range from 2.16*10 -11 to 7.73*10 -11 m 2 /s depending on the ferrite/pearlite-alignment and the diffusion direction with respect to the rolling axis. The aforementioned findings for rolled P355NH are therefore in the same order of magnitude, but also strongly depend on the microstructure of the specimens, and have to be considered as a first approximation for the calculation of a charging profile of CT50 samples (Fig. 2). Permeation measurements should be carried out in future works with respect to the ferrite/pearlite grain shape and distribution to narrow down hydrogen diffusivity more precisely.

Fig. 2. Calc. H-distribution in (a) crack growth and (b) in thickness direction of the CT50 specimens. Cross section indicated by red lines.