13th International Conference on Fracture June 16–21, 2013, Beijing, China -1- On Modeling of Thermal Embrittlement in PWR Steels using the Local Approach to Fracture Antoine Andrieu1, André Pineau1*, Pierre Joly2, François Roch2 1 Centre des Matériaux, Mines ParisTech, UMR CNRS 7633, BP 87, 91003 Evry Cedex, France 2 AREVA, Tour Areva, 1 Place Jean Millier, 92084 Paris La Défense * Corresponding author: andre.pineau@ensmp.fr Abstract Experiments on Charpy and CT specimens were carried out on one heat of A533B steel under two conditions: (i) as received, and (ii) thermally aged (450°C – 5000h). A shift of the ductile to brittle transition temperature (DBTT) was measured after aging. In both cases SEM observations showed that fracture occurred both by intergranular and transgranular cleavage fracture modes when the materials were tested at sufficiently low temperature. Detailed examinations revealed that intergranular fracture was associated with micro-segregated zones, enriched in carbon and phosphorus. A recent model developed by the authors for predicting the fracture toughness of inhomogeneous materials was applied to describe the large scatter related to the bimodal failure modes observed in both conditions and the DBTT shift after aging. It is shown that thermal aging produces a slight decrease of the critical cleavage stress (due to the crossing of grain boundaries embrittled by phosphorus segregation) and a larger decrease of the critical intergranular fracture stress. The McLean-Guttmann- Militzer model is used to predict the kinetics of segregation during aging. An attempt is made to show how these results can be used to model DBBT variations under in-service conditions. Keywords Ductile to brittle transition, intergranular, phosphorus segregation 1. Introduction It is often noticed the presence of segregated zones (S.Z) in large components used for the fabrication of pressurized water reactors (PWR) (see e.g. [1, 2]. These SZs are linked to the solidification process of large ingots. They are enriched in the alloying elements (Mn, Ni, Mo, C, etc.) present in the steel. They are also enriched in impurities, such as P which may segregate at grain boundaries during aging. These local microstructural modifications can generate a bimodal fracture process (cleavage and intergranular fracture), as shown elsewhere [2]. Brittle (cleavage and intergranular) fracture in homogeneous ferritic (or bainitic) steels has been investigated by many authors, for a review see e.g. [3]. The seminal work by Beremin [4] has shown that in a homogeneous material the fracture toughness, KIC obeys a size effect given by 4 IC K B= constant, where B is the thickness of the specimen, while the scatter in test results can be described by a Weibull law. The probability of failure can be expressed as: 4 4 , 1 exp m IC o m n r m o u K B C P V − ⎡ ⎤ σ = − ⎢− ⎥ σ ⎣ ⎦ (1) Where σo is the yield strength of the material, m is the Weibull shape factor, Cm,n is a parameter depending both on material work-hardening exponent, n, and the Weibull shape factor, m (see [5] for detailed values of Cmn.) Vo is a representative volume and σu is the Weibull scale factor. Recently it has been shown how Eq.1 must be modified in the presence of inhomogeneities leading to local intergranular fracture [2]. The main aim of the present study is the application of these recent modifications of the original
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