PSI - Issue 42

Simon Vander Vennet et al. / Procedia Structural Integrity 42 (2022) 813–820 S. Vander Vennet / Structural Integrity Procedia 00 (2019) 000–000

818

6

10 -3

10 -3

0% YS 100% YS 125% YS

Uncharged 0% YS 100% YS 125% YS

2

2

1.5

1.5

1

1

0.5

0.5

Hydrogen flux (wppm/s)

Hydrogen flux (wppm/s)

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0

400

500

600

700

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400 500 600 700 800 Temperature (°C)

Temperature (°C)

(a)

(b)

Fig. 5: TDS spectra for selected conditions after electrochemical hydrogen permeation for (a) Q&P and (b) TRIP. (Note: the uncharged condition in (b) was not subjected to any hydrogen charging.)

of DP, Van den Eeckhout et al. (2020) reasoned that the formation of dislocations already started before reaching the YS due to microplasticity. However, in their comparison of DP and TRIP steel, Wasilkowska et al. (2006) found that the presence of a significantly harder phase, i.e. martensite, in DP steel causes significant strain hardening already at low strains with dislocation generation in the ferrite grains. Whereas in TRIP steels, the strain hardening exponent gradually increases with increasing strain due to the continuous formation of martensite and dislocations are generated at a later stage. Due to this vastly di ff erent deformation mechanism, it is reasonable to assume that the dislocation generation in TRIP below the YS, i.e. corresponding to small strains, is negligible compared to the expansion of the ferrite lattice, which is said to be responsible for the apparent di ff usivity increase (Zhao et al. (2016)). The same reasoning does not apply for Q&P, however, since the maximum apparent di ff usivity already occurs in the unstressed state. Using the same reasoning in the elastic regime, i.e. for the 50% and 75% YS condition, the plastic deformation should be insu ffi cient to significantly slow down hydrogen di ff usion and instead, the apparent di ff usivity should increase. Therefore, it can be reasoned that an additional trapping site is activated upon applying the constant load. In order to verify this, TDS was performed on the specimens after reaching a steady state during electrochemical hydrogen permeation. The part of the spectrum between 400 ◦ C and 800 ◦ C is presented for selected testing conditions for both Q&P and TRIP in Figure 5 since this part of the spectrum is expected to contain information regarding trapping at RA (Claeys et al. (2020); Depover et al. (2020)). Comparing Figures 5a and 5b, several di ff erences can be noted. First of all, it is apparent that the spectrum for TRIP tested in the unloaded condition (0 %YS) in Figure 5b already contains a high temperature peak, whereas this is not the case for Q&P. However, by comparing this result with the uncharged condition, i.e. without charging the specimen with hydrogen, it can be seen that the measured hydrogen peak at high temperatures is already present before charging the sample with hydrogen. Instead, the hydrogen is already present in the sample before any charging is applied. Similar to Escobar et al. (2012), the hydrogen is assumed to be already present in the RA due to industrial processing. Furthermore, seeing that the high temperature peak in Figure 5b remains relatively unchanged, it is reasoned that no additional hydrogen is trapped during electrochemical hydrogen permeation, regardless of whether a constant load is applied during the test or not. The opposite observation can be made for Q&P, where the high temperature peak appears only in the specimens which were tested at a constant applied load. Therefore, the trapping of hydrogen at RA in Q&P seems only possible when applying a constant load during charging. Comparing this to literature, some authors observed embrittlement related to RA in Q&P and TRIP-assisted steels without observing hydrogen trapping, whereas other authors did report trapping by RA during electrochemical charging (Zhou et al. (2018); Zhu et al. (2014)). Considering