Key Takeaways & Executive Findings
- •• In-situ DIC and EBSD reveal distinct microvoid initiation mechanisms in quenched vs. tempered SA508 Gr.3 steel, linking microstructure to ductile fracture. • Tempering reduces UTS but significantly enhances ductility, with local strain reaching 191% at necking, due to carbide morphology changes. • Microvoids initiate via PAGB decohesion at M-A islands in quenched steel, but via carbide fracture and PAGB decohesion in tempered steel. • Both conditions exhibit ductile dimple fracture, confirming microvoid coalescence as the dominant failure mode, critical for nuclear pressure vessel reliability.
Abstract
Tensile deformation and microvoid formation of quenched and tempered SA508 Gr.3 steel were studied using an in-situ digital image correlation technique and in-situ electron backscatter diffraction (EBSD) measurements. The quenched steel with a mixture of upper bainite and granular bainite exhibited a high ultimate tensile strength (UTS) of ~795 MPa and an elongation of ~25%. After tempering, long-rod carbides and accumulated carbide particles were formed at the interface of bainite–ferrite subunits and prior austenite grain boundaries (PAGBs), respectively. The UTS of the tempered steel decreased to ~607 MPa, whereas the total elongation increased to 33.0% with a local strain of 191.0% at the necked area. In-situ EBSD results showed that strain localization in the bainite–ferrite produced lattice rotation and dislocation pileup, thus leading to stress concentration at the discontinuities (e.g., martensite–austenite islands and carbides). Consequently, the decohesion of PAGBs dotted with martensite–austenite islands was the dominant microvoid initiation mechanism in the quenched steel, whereas microvoids primarily initiated through the fracturing of long-rod carbides and the decohesion of PAGBs with carbides aggregation in the tempered steel. The fracture surfaces for both the quenched and tempered specimens featured dimples, indicating the ductile failure mechanism caused by microvoid coalescence.
1. Introduction
Over the past decades, numerous studies have focused on the development of new-generation steel for nuclear pressure vessels [1–2]. However, SA508 Gr.3 steel is still the most popular material for the fabrication of nuclear reactor pressure vessels [3–5]. The reliability of this steel is crucial to the safety of currently operating nuclear power plants. Therefore, various strategies are proposed to improve the initial properties of this steel [6–8].
Reportedly, two-step tempering can inhibit the formation of long rod-shaped carbides by inducing martensite–austenite (M–A) island decomposition at a low temperature, thus obtaining superior ductility and toughness [7]. Nevertheless, the role of microstructures commonly found in this steel, such as bainite, M–A islands, and carbides, in the deformation damage has rarely been reported. Thus, more studies are needed to elucidate the mechanisms of ductile fracture in this steel, especially the initiation and development of microvoids and their interactions with microstructures, which are important for the reliability of nuclear power components.
Ductile fracture of metals preceded by significant plastic deformation usually involves the initiation and development of microvoids, which is strongly related to the stress state [9–10]. Microvoids generally nucleate around stress concentrators, which are discontinuities present in the structure of materials, such as second-phase particles and crystal defects. Subsequently, the plastic strain of the surrounding matrix promotes the growth of microvoids. Second-phase particles distributed on grain boundaries or inside grains are generally considered to be the initiators of microvoids because of the localized plastic strain and the heterogeneity in the strain field around these particles.
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Zhifeng Zhang, Daming Tong, Xingyun Yang, Xiaofang Wang, Lizhan Han, Guanghua Yan, Chuanwei Li, Jianfeng Gu (2025). In-situ research on tensile deformation and microvoid formation in a nuclear pressure vessel steel. Int. Journal of Minerals, Metallurgy and Materials (矿物冶金与材料学报). https://doi.org/10.1007/s12613-024-3043-0
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Frequently Asked Questions
What is the main objective of this study?
The study aims to investigate the tensile deformation and microvoid formation mechanisms in SA508 Gr.3 nuclear pressure vessel steel using in-situ digital image correlation and EBSD techniques, correlating microstructural features with ductile fracture behavior.
How does tempering affect the mechanical properties of SA508 Gr.3 steel?
Tempering reduces the ultimate tensile strength from ~795 MPa to ~607 MPa but increases total elongation from ~25% to 33.0%, with a local strain of 191.0% at the necked area, indicating improved ductility.
What are the dominant microvoid initiation mechanisms in quenched and tempered conditions?
In quenched steel, microvoids initiate primarily by decohesion of prior austenite grain boundaries decorated with martensite-austenite islands. In tempered steel, microvoids initiate through fracturing of long-rod carbides and decohesion of PAGBs with carbide aggregation.
What is the significance of in-situ EBSD in this research?
In-situ EBSD allows real-time observation of strain localization, lattice rotation, and dislocation pileup, providing direct evidence of stress concentration at microstructural discontinuities, which is crucial for understanding microvoid nucleation.
What are the practical implications of this study for nuclear pressure vessel reliability?
By elucidating the microvoid initiation mechanisms, the study provides insights for optimizing heat treatment processes to enhance ductility and toughness, thereby improving the safety and reliability of nuclear pressure vessels.
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