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2 Advances in Materials Science

2 2007

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SCC of cold-worked austenitic stainless steels in PWR conditions

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

Advances in Materials Science

2007

tom Vol. 7, nr 1(11)

33-46

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.

Stress corrosion cracking on cold-worked austenic stainless steels in PWR environment

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Tribouilloy L. , Vaillant F. , Olive J.-M. , Puiggali M.

Advances in Materials Science

2007

tom Vol. 7, nr 1(11)

61-69

Types 304L and 316L austenitic Stainless Steels (SS) are widely used in PWR environment. These past few years, a limited number of cases of intergranular stress corrosion cracking (IGSCC) have been detected in cold worked areas of non sensitized austenitic stainless steel components. A first study has been initiated at EDF to assess the conditions of the cracking. The main results include cold work thresholds of 240 HV 0,1 for initiation cracking, and of 310 HV 0,1 for crack propagation, and propose that a dynamic loading is necessary for SCC. The aim of the present paper is to provide a basis of a crack propaga-tion model by investigating the effcet of loading, material and cold-work. In order to try to approach a static loading, a trape-zoidal cyclic loading is applied on high cold-worked (by rolling or by tensile loading) materials. It is shown that, for the most severe loading, the rolling cold-worked (RCW) materials undergo TGSCC whereas IGSCC is observed after tensile cold-working (TCW). The ratio of loading R bas such a strong impact on the crack growth rate (CGR) that it modifies the mechanism of cracking. Moreover, we notice that CGR increases with the applied K max but this evolution depends on the R value. Therefore, [delta K] is chosen to represent the mechanical loading effects on CGRs. Finally, the CGR after a hold time of 1 hour is quite the same than for 3 hours. Additionally, to address the critical issue of the effecet of the crack tip strain rate on crack growth rate, Slow Strain Rate Tests (SSTR) are carried out on RCW specimens and provide a first relation which is not consistent to a pure anodic process. This study is going on TCW specimens.