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

819

7

the ambiguity in these studies (Lovicu et al. (2013); Park et al. (2002); Ronevich et al. (2012); Zhou et al. (2018); Zhu et al. (2014); Escobar et al. (2012)), there seems to be no generally accepted concept of hydrogen trapping by RA. Despite the fact that a conclusive explanation has not yet been found, it is still remarkable that the trapping behaviour of RA at a constant applied load di ff ers between TRIP and Q&P. This discrepancy can be explained, at least partially, by the presence of internal stresses. As is observed in the XRD spectrum if Figure 3b, the RA in Q&P is in a higher stressed state than in TRIP. This is likely caused by the presence of internal stresses originating from the presence of martensite in the microstructure of Q&P (Allain et al. (2018); Maeder et al. (1981)). Not only does the initial martensite transformation introduce a hydrostatic compressive stress, the di ff erence in thermal expansion coe ffi cient between austenite and the surrounding martensite additionally induces thermal strains during the partitioning heat treatment and final cooling. It can therefore be reasoned that the constant applied tensile load partially relieves the compressive hydrostatic stress, possibly changing the local chemistry of the interface. A possible explanation could be that defects which were previously occupied by carbon are freed upon ap plying the load and hence sites in the austenite lattice become available for hydrogen trapping. Alternatively, the simultaneous loading and hydrogen charging may be able to facilitate the generation of defects, as proposed by the hydrogen-enhanced strain-induced vacancy (HESIV) concept (Nagumo (2004)). These newly generated defects could be able to accommodate hydrogen. Nevertheless, further research on the properties of the RA phase and the associated martensite / austenite interface in Q&P is necessary to fully explain this observation. This work studied the influence of a constant applied load on the hydrogen di ff usion in DP, TRIP and Q&P steel. The apparent hydrogen di ff usivity was characterised using electrochemical hydrogen permeation and subsequent ther mal desorption spectroscopy in order to verify the presence of hydrogen in retained austenite. • Both DP and TRIP steel showed the same trend in apparent hydrogen di ff usivity with respect to applied constant load, with the di ff usivity increasing in the elastic regime and decreasing in the plastic regime. • The threshold stress level for maximum apparent di ff usivity in TRIP was higher than in DP since dislocation in the latter are formed at lower strains due to a higher strain hardening exponent, compared to the more gradual increase in strain hardening and associated dislocation generation in TRIP. • TDS characterisation of the permeation specimens of Q&P steel immediately after permeation revealed high temperature trapping sites, which were associated with trapping at RA, thus explaining the gradual decrease in di ff usivity upon loading for Q&P. TRIP, on the other hand, did not contain any additional hydrogen trapped at RA. 5. Conclusions

Acknowledgements

The authors gratefully acknowledge the financial support from the Ghent university - Special Research Fund under the grant numbers BOF15 / BAS / 062, BOF19 / GOA / 026 and BOF20 / BAS / 121.

References

Allain, S.Y.P., Gaudez, S., Geandier, G., Hell, J.C., Goune´, M., Danoix, F., Soler, M., Aoued, S., Poulon-Quintin, A., 2018. Internal stresses and carbon enrichment in austenite of quenching and partitioning steels from high energy x-ray di ff raction experiments. Materials Science and Engineering: A 710, 245–250. Archer, M.D., Grant, N.C., 1984. Achievable boundary conditions in potentiostatic and galvanostatic hydrogen permeation through palladium and nickel foils. Proc. R. Soc. Lond. A 395, 165–183. Caskey, G.R., Sisson, R., 1981. Hydrogen solubility in austenitic stainless steels. Scripta Metallurgica 15, 1187–1190. Claeys, L., Cnockaert, V., Depover, T., De Graeve, I., Verbeken, K., 2020. Critical assessment of the evaluation of thermal desorption spectroscopy data for duplex stainless steels: A combined experimental and numerical approach. Acta Materialia 186, 190–198.