Issue 52

A. Laureys et alii, Frattura ed Integrità Strutturale, 52 (2020) 113-127; DOI: 10.3221/IGF-ESIS.52.10 121 Figure 10: Early stage of cracking in hydrogen charged cold deformed ULC steel (5 mA/cm²- 2 days). a) SEM image, b) IQ map with HAGBs (>15°) in green and LAGBs (low angle grain boundaries) (5-15 °) in red. Reprinted with permission from Ref. [6] . Figure 11: TRIP-assisted steel hydrogen charged for 1 day at 10 mA/cm²: a) SEM image and b) Kernel average image quality (KAIQ) overlapped with an IQ map of the investigated zone (Blue: cavity (610<KAIQ<780), red: martensite (780<KAIQ<1115)). c) Hydrogen void initiation at martensite (indicated by white arrows). d) Initiating crack arrested in ferritic grain (M: martensite island, F: ferrite grain). Reprinted with permission from Ref. [40]. TRIP-assisted steel Fig. 11 shows that the martensite-martensite interface is a preferential initiation site for hydrogen induced void formation in the studied TRIP-assisted steel charged under the current conditions. A kernel average image quality map allowed to identify the martensitic phase (Fig. 11b). Small voids formed at notches between two martensitic islands [40]. This fracture behavior was also observed when studying hydrogen assisted cracking in TRIP-assisted steel where an external stress is applied on samples charged up to saturation. The authors claimed that initiation occurred by the HEIDE mechanism (Hydrogen Enhanced Interface Decohesion) [2], which is a HEDE related mechanism as decohesion occurs across the interface. Koyama et al. [49] made similar observations in hydrogen charged dual phase steel under stress. Chan [50] illustrated that a fresh martensitic structure is the one most susceptible to hydrogen embrittlement. The lath martensite has