Biaxial loading effects on fracture toughness of reactor pressure vessel steel

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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Fracture Toughness Evaluation of Heat-Affected Zone Under Weld Overlay Cladding in Reactor Pressure Vessel Steel

Volume 1A: Codes and Standards ◽

10.1115/pvp2018-84535 ◽

2018 ◽

Author(s):

Yoosung Ha ◽

Tohru Tobita ◽

Hisashi Takamizawa ◽

Satoshi Hanawa ◽

Yutaka Nishiyama

Keyword(s):

Fracture Toughness ◽

Base Metal ◽

Heat Affected Zone ◽

Pressure Vessel ◽

Reactor Pressure Vessel ◽

Pressure Vessel Steel ◽

Vessel Steel ◽

Weld Overlay ◽

Reactor Pressure Vessel Steel ◽

Reactor Pressure

An evaluation of the fracture toughness of the heat-affected zone (HAZ), which is located under the weld overlay cladding of a reactor pressure vessel (RPV), was performed. Considering inhomogeneous microstructures of the HAZ, 0.4T-C(T) specimens were manufactured from the cladding strips locations, and Mini-C(T) specimens were fabricated from the distanced location as well as under the cladding. The reference temperature (To) of specimens that were aligned with the middle section of a cladding strip (HAZMCS) was ∼12°C higher than that of specimens that were aligned with cladding strips at the overlap (HAZOCS). To values of partial area in the HAZ were obtained using Mini-C(T) specimen. The To values obtained near the side of the cladding were ∼13°C higher than those away from the cladding. To values of HAZ for both 0.4T-C(T) and Mini-C(T) specimens were significantly lower than that of the base metal at a quarter thickness by 40°C–60°C. Compared to the literature data that indicated fracture toughness at the surface without overlay cladding and base metal of a quarter thickness in a pressure vessel plate, this study concluded that the welding thermal history showed no significant effect on the fracture toughness of the inner surface of RPV steel.

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