Doel 3 & Tihange 2 - Some Peer-reviewed Scientific Papers & Reports

Z. Que et al. / Procedia Structural Integrity 13 (2018) 926–931 Z. Que et al. / Structural Integrity Procedia 00 (2018) 000–000

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are preferred regions for hydrogen enrichment and accumulation, which further amplifies the localization of plastic deformation by hydrogen (hydrogen might result in the local shielding effect, promote the localized deformation and enhance strain-induced vacancies). 3.2. HSST material with high S content and EAC susceptibility The effects of crevice chemistry, preceding EAC crack growth and loading rates on fracture behaviour of the HSST material with high sulphur and EAC susceptibility was evaluated with EPFM tests in air, oxygenated and hydrogenated high-purity water at 288 °C, with the summary shown in Figure 1(c). Additionally, in some tests with oxygenated water, chloride or sulphate was also added. Some of these EPFM tests started with an active growing SICC crack (by cyclic loading) with an aggressive occluded high-sulphur crack crevice chemistry and thus a very high hydrogen uptake potential in the crack-tip process zone. In contrast to the low-sulphur steels with high DSA susceptibility, the high-sulphur steel HSST with moderate DSA susceptibility revealed a higher initiation toughness reduction in oxygenated than hydrogenated water at loading rates of 250 and 25  m/min in standard EPFM tests (where SICC was absent). This might be related to the sulphide enrichment in the crack crevice environment by migration due to the potential gradient in the crack mouth region in oxygenated water and the higher sulphur content. Furthermore, the MnS inclusions may act as strong H traps in the process zone at the crack-tip. Chloride addition further reduced the fracture initiation and tearing resistance in oxygenated water. Besides, a preceding SICC crack growth further reduced the toughness in oxygenated and hydrogenated water, as shown in Figure 4 (a). With chloride addition or preceding fast high-sulphur EAC crack growth, more aggressive occluded crevice environment could be reached and higher macrovoids and secondary crack fraction were found in SEM observations (Figure 4 (b)-(c)). These preliminary test results revealed a moderate, but clear reduction of the fracture initiation resistance with aggressive occluded crevice environment and with preceding fast high-sulphur EAC crack growth (favoured by, e.g., a high steel sulphur content, high ECP or sulphate/chloride impurities) in NWC.

Figure 4: (a) Environmental reduction of fracture initiation resistance JQ of HSST material in different HTW environments and test conditions at 288 °C with 250  m/min; Fracture features of HSST material tested in NWC with different test conditions: (b) EPFM, (c) EPFM with preceding SICC with addition of chloride. In HTW, the effects of steel sulfur content are synergistic with environmental variables, such as (sulfur-) anionic impurities in the bulk environment, ECP (dissolved oxygen content) and flow rate. This is believed to be due to the creation of a sulfur-rich crack-tip environment responsible for EAC, which arises from the exposure and dissolution of MnS inclusions intersected by the growing crack and by the transport of sulfur-anions by migration/diffusion/convection within the crack enclave. An aggressive occluded crevice chemistry (O 2 , S content, SO 4 2- , Cl - and preceding EAC crack growth) results in a higher reduction in the upper shelf fracture resistance. Sulfur- anions may significantly retard repassivation after oxide film rupture and therefore increase crack advance by anodic dissolution. Retarded repassivation of the film-free surface and adsorbed sulfur-anions increase the hydrogen absorption into the metal lattice. Furthermore, the dissolution of MnS is a further potential source of hydrogen and the interface between the MnS-inclusions and metal matrix in the region of maximum hydrostatic stress ahead of the