The Heavy-Section Steel Technology (HSST) Program at Oak Ridge National Laboratory (ORNL) includes a task to investigate the effects of constraint on the cleavage initiation fracture toughness of reactor pressure vessel (RPV) steels in the lower transition temperature region using relatively large cruciform fracture toughness specimens under varying degrees of biaxial loading. One of the materials used for the project was a plate of A533 grade B steel (HSST Plate 14A) which was specially heat treated to result in a yield strength comparable to that of a radiation-sensitive RPV steel near the end of design life. During the testing phase to characterize the fracture toughness behavior of the plate with uniaxial three-point bend specimens, some relatively low fracture toughness values were observed. Subsequent metallography revealed the presence of varying degrees of dark bands in the microstructure. These observations prompted an investigation of the relationship between the experimentally determined fracture toughness results and the microstructure of the plate steel used for the biaxial-loading effects project, especially with regard to the results obtained from the biaxial test specimens. The primary issue in the investigation is whether the fracture toughness results obtained from the biaxially loaded specimens were influenced by the steel microstructure in a biased manner, i.e., were the observation regarding effects of biaxial loading on fracture toughness significantly affected by the microstructural segregation in heat treated HSST Plate 14A. A secondary issue is whether segregated microstructures are common in steels used for RPV construction and if the current procedures for evaluating fracture toughness of RPV steels adequately account for such microstructures. Various metallurgical tools, including metallography, microhardness testing, scanning electron fractography, electron microprobe analysis, and analytical electron microscopy were used to characterize the nature of the bands and evaluate the potential effects on the fracture toughness results.

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