Influence of localized plasticity on oxidation behaviour of austenitic stainless steels under primary water reactor

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Abstract

The sensitivity of precipitation-strengthened A286 austenitic stainless steel to stress corrosion cracking was studied by means of slow-strain-rate tests. First, alloy cold working by low cycle fatigue (LCF) was investigated. Fatigue tests under plastic strain control were performed at different strain levels (Δεp/2 = 0.2%, 0.5%, 0.8% and 2%) to establish correlations between stress softening and the deformation microstructure resulting from the LCF tests. Deformed microstructures were identified through TEM investigations. The interaction between oxidation and localized deformation bands was also studied and it resulted that localized deformation bands are not preferential oxide growth channels. The pre-cycling of the alloy did not modify its oxidation behaviour. However, intergranular oxidation in the subsurface under the oxide layer formed after exposure to PWR primary water was shown.

Introduction

IASCC (irradiation-assisted stress corrosion cracking) is a complex phenomenon affecting core reactor components, discovered in the 1990s. It consists in degradation resulting from microscopic brittle cracks, when ductile material undergoes mechanical loading in an aggressive environment. The complexity of IASCC is actually due to the simultaneous actions of material parameters, chemical environment, loads and irradiation effects. Currently, the mechanisms underlying this phenomenon are only partially understood. Numerous reviews [1], [2], [3], [4], [5], [6] have pointed out that modifications in the material and the environment due to neutron irradiation contribute to IASCC: radiation hardening, radiation-induced segregation, water radiolysis (mainly in Boiling Water Reactor environments) and chemical species transmutations. Although all these factors are important for IASCC degradation, none appear to be the primary cause and the issue is still open. Recent works [7], [8], [9], [10], [11] identify plastic strain localization as another important contributing factor of IASCC. Onchi et al. [8] suggested that crack initiation in sensitized 304 SS may be linked to the presence of deformation bands. Jiao et al. [9] correlated different stacking fault energy (SFE) values (related to the propensity towards strain localization, in stainless steel irradiated to up to 5 dpa) to their intergranular percentage on the fracture surface in constant extension rate tensile (CERT) tests conducted under an argon atmosphere at 288 °C. It was shown that the intergranular (IG) percentage on fracture surfaces was higher for lower SFE materials i.e. where the deformation is more localized. However, it should be noted that the steel doping that leads to SFE modification also impacts radiation-induced segregation, which is a potential factor impacting IASCC. Dislocation channelling is the deformation mode responsible for plastic deformation localization in pure metals such as Cu or Al [12]. For austenitic stainless steels, localization may be characterized by channelling (high doses, high temperature and low stain rate) and twinning (low doses, low temperature and high stain rate). Onchi et al. [7], [8] also suggested that the intersection between dislocation channels and grain boundaries constitutes a high concentration stress and strain area, leading to a preferential crack initiation area. Given this, it could be useful to understand how oxide species grow on the surface of specimens with localized deformation bands, in other words if in static conditions, localized deformation bands may be considered as preferential sites for oxide growth. The study of Fournier et al. [11] on a A286 steel was along these lines. Deformation microstructure was simulated by means of low cycle fatigue (LCF), knowing that cyclic softening results in defect-free localized deformation bands. LCF tests were conducted at room temperature under 0.2% half-amplitude plastic strain. It was shown that A286 pre-strained by LCF is more sensitive to stress corrosion cracking (SCC) than the same alloy without localized deformation bands.

In order to study the impact of the degree of localization on SCC sensitivity, the same idea as that developed in [11] is explored in this paper. LCF tests at plastic strain amplitude (Δεp/2 = 0.2, 0.5, 0.8% and 2%) were performed and the resulting deformation microstructures were analyzed by transmission electron microscopy (TEM) observations. Then interactions between the oxide layer and the plasticity features were studied on specimens, with or without localized deformation bands, exposed to simulated primary water reactor (PWR). Information was obtained concerning the way localized deformation bands modify oxide growth behaviour.

Section snippets

Experimental details

The material studied was a precipitation-strengthened A286 austenitic stainless steel provided by Ugitech (France) as 30 mm diameter rod. Chemical analyses were carried out by means of GDMS (glow discharge mass spectroscopy) analysis. The chemical composition of the alloy is shown in Table 1. The alloy underwent the following heat treatment: (i) solution annealing at 930 °C for 30 min in order to remove the mechanical history of the material, and (ii) water quenching to avoid internal oxidation.

Initial microstructure

Ageing heat treatment of 50 h at 670 °C on A286 led to the formation of hardening phase: γ′ (Ni3Ti) precipitates in austenitic matrix. The interesting strengthening properties of the A286 alloy are due to this phase acting by microscopic mechanisms such as the Orowan mechanism [13] which is related to interactions between mobile dislocations and precipitates. As the contrast between γ′ precipitates and γ matrix is weak, precipitates are not easily revealed by bright field imaging. Dark-field TEM

Conclusions

This work investigated the detrimental effects of deformation localization on SCC susceptibility of austenitic steels in PWR primary water. A286 precipitation-strengthened austenitic stainless steel specimens were mechanically cycled by LCF tests (Δεp/2 = 0.2%, 0.5%, 0.8% and 2% plastic strain at RT) to simulate localization features. The microstructure of the aged (670 °C, 50 h) A286 shows an austenitic matrix, coherent with the γ′ (Ni3Ti) phase and the presence of carbides (titanium and chromium

Acknowledgments

We gratefully acknowledge L. Vincent and P. Wident from DMN/SRMA (CEA Saclay) and T. Morgeneyer (Centre des Matériaux, Mines Paristech) for conducting the mechanical tests, M.H. Mathon from Leon Brillouin laboratory (CEA Saclay) for conducting DNPA measurements, C. Guerre from DPC/SCCME (CEA Saclay) for providing facilities used for oxidation tests, Anne Maquignon and E. Herms from DPC/SCCME (CEA Saclay) for their assistance in preparing and conducting the SCC tests, and C. Armand from INSA

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