Effect of thermal aging on mechanical properties of a bainitic forging steel for reactor pressure vessel

Abstract Control of carbon macro-segregation in the steel-making process for large steel forgings is of great importance in order to achieve the material properties and structural reliability required for the pressure vessels of nuclear power plant components. It is well known that high carbon content due to carbon macro-segregation can affect the mechanical properties of steels, leading to decreases in ductility and fracture toughness. In this study, possible effects of carbon macro-segregation have been examined using a large-scale forged steel “bottom head dome” of a reactor pressure vessel (RPV) manufactured for a recent BWR. Material testing conducted included chemical analyses, tensile tests and Charpy impact tests. In the center part of the concave disk-shaped forged material, carbon content varied slightly in the material thickness direction within the range of carbon content requirement, as expected from the relationship between the solidification and the resultant segregation process in the cast ingot material and the forging process from the ingot to the dome material. The results of each mechanical test also showed full compliance with the properties required in the code regardless of the carbon content at each of the thickness locations examined. All the tests results demonstrated that with the steel-making technology and practice employed, carbon macro-segregation is well controlled to achieve the required material properties even in large-scale forged materials used in BWRs.

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Non-hardening embrittlement mechanism of pressure vessel steel Ni-Cr-Mo-V welds during thermal aging

Advances in Mechanical Engineering ◽

10.1177/1687814020904567 ◽

2020 ◽

Vol 12 (2) ◽

pp. 168781402090456

Author(s):

Guojun Wei ◽

Chenglong Wang ◽

Xingwang Yang ◽

Zhenfeng Tong ◽

Wenwang Wu

Keyword(s):

Weld Metal ◽

Thermal Aging ◽

Pressure Vessel ◽

Impact Test ◽

Charpy Impact ◽

Charpy Impact Test ◽

Reactor Pressure Vessel ◽

Impact Fracture ◽

The Impact ◽

Reactor Pressure

The mechanical performance of reactor pressure vessel materials is an important factor in the safety and economics of the operation of a nuclear power plant. The ductile-to-brittle transition temperature tested by Charpy impact test is the key parameter for evaluating the reactor pressure vessel embrittlement. In this article, the study of thermal aging embrittlement of temperature sets of reactor pressure vessel surveillance Ni-Cr-Mo-V steel weld metal was conducted by Charpy impact test. The thermal aging effect on the impact fracture behavior was analyzed. The impact test of the three batches of weld surveillance sample indicated that the weld metal embrittled during thermal aging. The study of impact fracture and Auger electron spectroscopy indicated that the element P segregated to the grain boundaries and lowered their cohesion strength during the long-term thermal aging. Therefore, the non-hardening embrittlement of Ni-Cr-Mo-V steel welds in a reactor pressure vessel caused by segregation of impurity elements P occurs during thermal aging.

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