Figure 9 – uploaded by Milos Djukic

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Fig. 9. SEM micrograph of the Charpy specimens: (a) specimen S3 (HELP > HEDE); (b) specimen S4 (HEDE > HELP) — Figure 9 SEM micrograph of the Charpy specimens: (a) specimen S3 (HELP > HEDE); (b) specimen S4 (HEDE > HELP)

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Chemical composition of St.20 steel.

Fig. 1. Boiler tube, hydrogen damage: (a) the thick wall, “window” type fracture, metallographic specimens - ME1 and ME2; (b) position and designation of the “Roman tile” Charpy specimens and hardness measurements positions in the vicinity of the fracture; (c) hardness measurements positions along a tube, x-distance from the fracture edge.

The corrosion products on the tube surface inner side, in the vicinity of the “window” type fracture, have been identified. The corrosion products formed at various locations in the area with an extreme corrosion gouging (see Fig. 2b) were characterized by energy-dispersive X-ray spectroscopy (EDS) and their morphology was also observed by scanning electron microscopy (SEM). The atomic absorption spectroscopy (AAS) procedure was also used for the chemical elements’ quantitative determination, as well as the Zimmermann-Reinhardt (Z-R) method to determine iron in corrosion products in the area with a thick deposit (see Fig. 2b). Fig. 4a shows the SEM micrograph of the secondary corrosion pit (cavity) in the area with a corrosion gouging, near the fracture surface. The results of chemical elements distribution (elemental mapping) in the corrosion products deposited within the Fig. 2. Fractured boiler tube: (a) schematic representation of the “window” type fracture; (b) tube inner surface in the vicinity of the fracture

Fig. 3. Measurements of the tube wall thickness: (a) schematic representation of the measurement positions; (b) thickness distribution along the tube fracture surface.

Fig. 4. SEM/EDS of the secondary corrosion pit (cavity) in the area with a corrosion gouging: (a) micrograph; (b) measurement positions - line profile along the pit; (c) distribution of chemical elements.

AAS and Z-R analysis of deposit, shown in Fig. 2b. Table 2

Mechanical properties of specimens. Table 3

Fig. 5. Optical micrograph of specimens: (a) specimen ME1: ferrite—pearlite, elongated MnS inclusion; (b) specimen ME2: complete decarburization, ferrite, micro- fissures and IG micro cracks.

Mean hardness of the Charpy specimens. Table 4

Fig. 6. Variation of the Vickers hardness (HV5) as a function of the distance (x) from the edge of the fracture (see Fig. 1c).

Fig. 7. Variation of the impact strength (KCVyor.) and its crack propagation component (KCVp) and crack initiation component (KCV;) of the Charpy specimens (S1-S6) as a function of the specimen hardness (hydrogen concentration).

Fig. 8. SEM micrograph of the Charpy specimen S5: (a) magnification 450x; (b) detail, magnification 3000x.

Fig. 10. SEM micrograph of the Charpy specimen S2 (HEDE + HTHA). The transition from ductile to brittle failure in steels due to hydrogen remains a phenomenon that is still not sufficiently ex- plored and clarified either. In the world literature, there is disagreement and polarization of opinions about the inherent nature, trigger mechanism and hydrogen embrittlement mechanisms responsible for the transgranular, microvoid coalescence, cleavage