Acceleration of Environmentally Assisted Cracking Initiation of the Type 316L Steel in High-Temperature Water and Hydrogenated-Steam Vapor Environments

Abstract

Environmentally-assisted cracking of solution-annealed Type 316L austenitic stainless steel with two different surface treatments (polished versus ground) was investigated using the constant extension rate tensile test methodology in high pressure water at 350 °C and low pressure H2-steam vapor at 350, 400, 440 and 480 °C, whilst maintaining electrochemical corrosion potentials in the NiO stability regime. Flat tapered specimens were used to indentify the threshold stress for cracking (~400 MPa) under these environmental conditions. Intensive oxidation and typical intergranular cracks were observed to initiate from the polished surface at 400 and 440 °C, whereas wavy cracks initiated on the ground surface. The results indicate that the ultrafine-grained layer formed adjacent to the ground surface effectively suppressed intergranular crack initiation under these test conditions. The slow loading under H2-steam vapor at 400 °C under the oxidizing condition (NiO) was found to be suitable high accelerated test to study early stages of the cracking.

Environmentally Assisted Cracking Initiation in High-Temperature Water

“…EAC has been observed as a root cause of some component damage in service [1,2,12]. EAC initiation has also been emulated in autoclave testing in HT water [6][7][8][9][42][43][44][45][46]; examples are shown in Figures 3 and 4. Both intergranular and transgranular EAC are observed, while the IG mode dominates cracking cases in service conditions and/or in long-term constant load testing in simulated lab conditions [12,43,44], and the TG mode is more often observed in testing accelerated by slow strain rates [12,45].…”

“…EAC initiation has also been emulated in autoclave testing in HT water [6][7][8][9][42][43][44][45][46]; examples are shown in Figures 3 and 4. Both intergranular and transgranular EAC are observed, while the IG mode dominates cracking cases in service conditions and/or in long-term constant load testing in simulated lab conditions [12,43,44], and the TG mode is more often observed in testing accelerated by slow strain rates [12,45]. Fracture surface of 08Ch18N10T AS steel after accelerated EAC testing (strain rate of 1 × 10 −7 s −1 at the tension loading up to rupture) in HT water (320 °C, KOH, <20 ppb O2, without H2 gas addition) showing the path of the crack from its initiation in the surface layer (deformed by harsh turning), where it had TG and cleavage-like character, to its propagation into depth, where the path was locally IG.…”

“…Other factors, such as intergranula oxidation depth, have to be included in the initiation mechanism. Example of EAC initiation on the surface of solution-annealed Type 316L steel strained (of 1 × 10 −6 s −1 in tension in horizontal direction) in HT water (350 °C, LiOH, <20 ppb O2, without H2 addition) [45]. The images show deformation bands, oxide particles, and cracks on the surface, together with zoomed views of boxed area of the following cracks: (a) TG cleavage-like (at local plastic strain of 0.12) and (b) IG (at 0.10).…”

“…Note the fine steps lined with fine oxide particles on the internal surfaces of opened cracks. [45]. The images show deformation bands, oxide particles, and cracks on the surface, together with zoomed views of boxed area of the following cracks: (a) TG cleavage-like (at local plastic strain of 0.12) and (b) IG (at 0.10).…”

Environmentally assisted cracking in the low pressure superheated hydrogen steam

“…Taková změna zřejmě souvisí s dostatečným časem pro vývoj korozních trhlin, které následně byly prokázané pro teploty vyšší než 400 °C. Tyto výsledky jsou v souladu s ostatními již publikovanými výsledky [3,4,26,27]. Oscilace v zatížení, které vykazují všechny křivky, jsou typické pro dynamické stárnutí, a jak bylo prokázáno dříve [26], nemají žádnou souvislost s vlivem prostředí.…”