Abstract
The effect of microstructure on the impact toughness and the temper embrittlement of a SA508Gr.4N steel was investigated. Martensitic and bainitic structures formed in this material were examined via scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and Auger electron spectroscopy (AES) analysis. The martensitic structure had a positive effect on both the strength and toughness. Compared with the bainitic structure, this structure consisted of smaller blocks and more high-angle grain boundaries (HAGBs). Changes in the ultimate tensile strength and toughness of the martensitic structure were attributed to an increase in the crack propagation path. This increase resulted from an increased number of HAGBs and refinement of the sub-structure (block). The AES results revealed that sulfur segregation is higher in the martensitic structure than in the bainitic structure. Therefore, the martensitic structure is more susceptible to temper embrittlement than the bainitic structure.
Highlights
Reactor pressure vessels (RPV) are generally considered to be the most critical components in the nuclear plant beside the reactor core[1,2]
This microstructure consists of two types of M-A islands, namely: elongated islands distributed in the bainitic ferrite matrix, and blocky islands located primarily at the interface between the matrix and the prior austenite grain boundaries
The impact toughness of the martensitic structure is better than that of the bainitic structure, and the ductile-to-brittle transition temperature (DBTT) values differ by 60 °C
Summary
Reactor pressure vessels (RPV) are generally considered to be the most critical components in the nuclear plant beside the reactor core[1,2]. The forged RPVs are quenched and tempered, as result, the through-thickness microstructure vary significantly because of the differential cooling rates during quenching. These differing microstructures will lead to heterogeneous mechanical properties along the thickness direction of the RPV. The results revealed that, compared with the fully bainitic microstructures, the martensitic microstructures are more susceptible to non-hardening embrittlement. The concentration of P on the grain boundaries increases with increasing duration of the embrittlement heat treatment and, in turn, will accelerate temper embrittlement These studies have elucidated the influence of the initial microstructure on the mechanical properties of Ni–Cr–Mo steels, the relationship among the microstructure, impact toughness, and temper embrittlement of SA508Gr.4N steels remains unclear
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