Applicability of Miniature-C(T) Specimen for the Master Curve Evaluation of RPV Weld Metal and Heat Affected Zone

Abstract

The Master Curve approach is a powerful tool to evaluate material-specific fracture toughness of ferritic steels, such as RPV steels, using a limited number of specimens. However, preparing a sufficient number of standard fracture toughness test specimens is difficult for irradiated RPV steels of existing surveillance programs. Utilization of miniature specimens that can be machined from broken halves of standard Charpy specimens is a possible solution to address this issue. CRIEPI has been working on the test technique utilizing a miniature C(T) (Mini-C(T)) specimens, whose dimensions are 4 × 10 × 9.6 mm (0.16 inch thickness specimen). The basic applicability of the Mini-C(T) Master Curve approach has been confirmed [1] for the base metals of typical Japanese RPV steels. International round robin tests confirmed the reproducibility of fracture toughness data obtained by Mini-C(T) specimens [2–4]. Ensuring the applicability of the Mini-C(T) Master Curve approach to weld metals and heat affected zone materials is of great importance to meet the future demand from the RPV surveillance programs for over 40 or 60 years’ reactor operation. For a weld metal deposit, we verified that valid reference temperature, To, can be estimated using the Mini-C(T) specimens and the statistics of the fracture toughness data [5] show good conformity to the assumption of the Master Curve method [6]. In the present paper, fracture toughness of a weld joint, which consists of two different heats of RPV plate material was examined. Five sets of Mini-C(T) specimens taken from two base metals, their heat affected zones (HAZ) and weld metal deposit, were subjected to the fracture toughness test. 0.5T-C(T) specimens taken from similar locations were also subjected to the fracture toughness tests to investigate specimen size effect. All the Mini-C(T) data sets taken from base metal, HAZ and weld metal were eligible for the determination of valid To with each 12 to 16 Mini-C(T) specimens. The relevance of the specimen size correction in the Master Curve method was confirmed for two base metals and a weld metal. The fracture toughness data for HAZ materials gave a reasonable agreement with the specific Weibull distribution assumed in the Master Curve method. Nevertheless, To values of four data sets of HAZ materials, including two Mini-C(T) datasets and two 0.5T-C(T) datasets, showed larger variation than that of the base metals or the weld metal. The crack initiation sites of HAZ specimens were all within so-called fine grain HAZ. However the HAZ width near the crack initiation site was dependent on the individual specimens. Higher fracture toughness tended to be gained from the specimens with narrower HAZ width. The resulting To values for HAZ material were close to or lower than that for base metals. The results suggest that the HAZ material gives equivalent or higher fracture toughness than in base metals.