UNDERSTANDING THE EFFECTS OF DEFORMATION ON CORROSION FATIGUE OF TYPE 304 AUSTENITIC STAINLESS STEELS IN HIGH TEMPERATURE WATER

UoM administered thesis: Phd

  • Authors:
  • Hanxiao Wang

Abstract

Austenitic stainless steels find a widespread application in pressurised water reactors (PWRs) because of their good corrosion resistance at both room and high temperatures. However, they are susceptible to environmentally-assisted cracking when exposed to elevated temperature water. In the present work, two heats of low sulphur Type 304 austenitic stainless steels that demonstrated distinct corrosion fatigue behaviours in simulated PWR primary coolant environment have been interrogated. Both of them underwent enhanced crack growth initially, with the extent of enhancement increasing with decreasing load frequency (or increasing rise time). Only one sample remained fully enhanced throughout the test, whereas the second sample retarded to rates close to or even below the ASME air line when rise time was gradually increased to 510 s or 1500 s. This study aims to understand the origin of their distinct corrosion fatigue resistance by evaluating their deformation behaviours. The deformation of both heats when subjected to monotonic tension was examined via ex-situ high resolution digital image correlation (HRDIC) as a function of temperature (298 K vs 573 K) or macroscopic elongation. The technique is capable of quantifying the amount of plastic strain and sampling a statistically meaningful region with sub-micron spatial resolution. Extensive attention has been focused on two components that could determine the effective crack driving force, namely: the degree of slip localisation and alternate-slip. The latter was found to be in line with planar cross-slip regarding both crystallography and morphology. The hypothesis is that it might act as an active crack deflection mechanism, as suggested by a possible correlation between cross-slip-induced deformation structures and a tortuous crack path identified at a sub-grain scale. HRDIC analysis confirmed that localised plasticity was more pronounced on the sample that retarded, regardless of temperatures and macroscopic strains. A straightforward correlation between slip localisation and corrosion fatigue properties could not be established. However, the retarded sample displayed a notably higher cross-slip propensity in contrast to the other that was always enhanced, especially at elevated temperature. It is therefore proposed that the enhanced cross-slip on the former possibly led to a more tortuous crack path by inducing angled and periodic deviations. This could diminish the effective driving force for crack propagation, which agrees with its significant retardation. It is also worth pointing out that the beneficial effects of enhanced crack-tip oxidation cannot be ruled out, although both heats had the same nominal sulphur content.

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Original languageEnglish
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Award date1 Aug 2021