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SCC of cold-worked austenitic stainless steels in PWR conditions

Raquet O., Herms E., Vaillant F., Couvant T.

Advances in Materials Science

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2007

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Vol. 7, nr 1(11)

33-46

EN

A strong susceptibility for SCC of heavily cold worked austenitic stainless steels is observed in hydrogenated primary water typical of PWRs. This susceptibility to cracking increases with the extent of cold-work and/or localization of deformation. The levels of cold-work involved in this study are very high when compared to the maximum cold-work levels required by the usual international codes relevant to PWRs (ASME and RCC-M). These high values of cold-work could be eventually encountered on the surface of same components (grinding) or in the case of improper manufacturing. The specific cold-work procedures including a compressive stage (fatigue, shot-peening) strongly favors SCC susceptibility in PWRs conditions under dynamic deformation. For a given cold-working procedure, SCC susceptibility of austenitic stainless steels materials increases with an increasing intensity of cold-work. A threshold of SCC susceptibility was identified in the ease of the shot-peening procedure of cold-working for AISI 304L stainless steels through the value of the initial surface hardness before SCC testing (CERT). SCC crack propagation is thus only observed beyond 300+-10HV on shot-peened specimens.

2

Oxide growth mechanism of Ni-base alloys in PWR primary coolant conditions

Marchetti L., Perrin S., Raquet O., Bennour I., Pellegrino S., Vaubaillon S., Miro S., Chevalier S., Heintz O., Pijola M.

Advances in Materials Science

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2007

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Vol. 7, nr 2(12)

247-253

EN

The thin passive film, formed on Ni-base alloys in Pressurized Water Reactor (PWR) primary coolant, plays a key role on different corrosion and degradation processes. In order to know how the oxide layer grows on the alloy, Iwo different kinds of experiments have been performed. The first one consisted in depositting of gold markers on the alloy surface, and corroding this sample in a corrosion loop and finally in characterizing the gold markers location in relation to the oxide layer by RBS analysis (Rutherford Backscattering Spectrometry). The second experiment consisted actually of Iwo different corrosion treatments: in an H2160 medium, then in a mixed H2 16 O /H2 18 O medium. After these two corrosion cycles, SIMS analyses gave the oxygen isotope distribution profiles. Consequently, the location of the 18 O peak in the layer allowed to locate where the formation of the oxide took place. These experiments enabled us to have a better understanding about the growth mechanism of protective oxide formed on Ni-base alloy in PWR primary conditions. The diffusion of oxygen promotes the protective oxide growth at the alloy / oxide interface. The major part of this diffusion takes place along oxide grain boundaries.

3

Hydrogen-induced intergranular embrittlementt of ni-base alloys used for steam generator tubes

Coudurier A., Odemer G., Chene J., Raquet O.

Advances in Materials Science

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2007

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Vol. 7, nr 1(11)

124-132

EN

The hydrogen-induced intergranular rupture of alloys 600, 690 and 82 has been investigated in order to improve the understanding of the possible role of hydrogen on the Stress Corrosion Cracking (SCC) sensitivity of these alloys when exposed to primary water in Pressurized Water Reactors (PWR). Small tensile specimens were hydrogenated at 150°C by cathodic charging in molten salts in order to introduce a controlled amount of hydrogen homogeneously distributed in the samples. The hydrogen concentration in the samples and its dependence on the alloys chemistry and microstructure and on additional desorption annealing was measured by using the thermal fusion method. The role of H on the strain hardening and on the tensile properties of the alloys was investigated at room temperature. SEM was used to characterize the extent of H-induced intergranular cracking. Hydrogenated samples exhibit a strong H-induced ductility lass associated with intergranular fracture. The extent of both the ductility lass and the intergranular rupture strongly depends on the chemical composition and on the microstructure of the alloys. Alloy 82, representative of the weld alloy 600, exhibits the largest HE susceptibility whereas alloy 690 is the less affected by H absorption. The tensile properties recorded after desorption annealing illustrate the reversible character of H-induced intergranular rupture and demonstrate the predominant role, in the mechanism responsible for the ductility lass, of diffusible hydrogen and its interaction with defects involved in the plastic deformation such as dislocations.