PSI - Issue 42

Lisa Claeys et al. / Procedia Structural Integrity 42 (2022) 390–397 Claeys et al./ Structural Integrity Procedia 00 (2019) 000 – 000

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tensile tested until an intermediate engineering strain of 30%. The results for the reference condition are presented in Fig. 3a and c for 304L ASS and TWIP steel, respectively . 304L ASS showed α’ -martensitic transformation, while the TWIP steel showed deformation twinning (white lines). Moreover, the engineering stress/strain curve revealed the action of dynamic strain aging (DSA) for the TWIP steel as serrations were observed starting from the onset of plastic yielding. DSA can be attributed to the short range reorientation of C-Mn complexes and their interaction with stacking faults enclosed by partial dislocations causing an increase in work hardening (Lee et al. (2011); Oh, et al. (2020)). This phenomenon was not observed in 304L ASS due to the low carbon content. Most likely, the combination of deformation twinning with DSA as well as more preferential kinetics, led to the improved mechanical behaviour of the TWIP steel. Indeed, the TRIP and TWIP effects are controlled by their formation kinetics. Too fast kinetics lead to premature failure since dislocation motion is hindered too soon. With too slow kinetics, there is insufficient twin/martensite formation to postpone necking also leading to premature failure. Upon introduction of hydrogen, both austenitic steels showed a significant reduction in ductility, cf. Fig. 2 qualitatively and Table 2 quantitatively. Moreover, the TWIP steel also showed a reduction in work hardening since the curve deviated from the reference condition starting at an engineering strain level of about 30%. The hydrogen concentration in both steels at the start of the tensile test was measured through melt extraction and is also given in Table 2. About double the amount of hydrogen was charged to 304L ASS compared to the TWIP steel. Due to the difference in charging conditions, the HE sensitivity of the two investigated steels could not be compared quantitatively. However, the effect of hydrogen on the active deformation mechanisms of interrupted tensile tested specimens can still be compared. The results are shown in Fig. 3b and d for 304L ASS and TWIP steel, respectively. The large black features on these images represent hydrogen-assisted cracks (HACs), which will be linked to the reduction in work hardening in one of the following paragraphs. For 304L ASS, hydrogen led to more planar deformation (cf. shape of α’ -martensite islands) , a higher fraction of α’ - martensite and the formation of ε -martensite. For the TWIP steel, hydrogen led to abundant ε -martensite formation accompanied by a reduction in the amount of deformation twinning compared to the reference condition. Both phenomena can be linked to the hydrogen-induced reduction of the stacking faults energy as well as hydrogen pinning edge dislocations inhibiting them from cross slipping (Pontini & Hermida (1997); Ferreira et al. (1999)).

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Fig. 2: Representative engineering stress/strain curves with and without hydrogen precharging for seven days for 304L ASS and 18Mn-0.6C TWIP steel