Effect of cooling rate on mechanical properties of SA508 Gr.1A steels for main steam line piping in nuclear power plants
Introduction
In nuclear power plants designed before 1983, heavy structures, such as pipe whip restraints and jet impingement shields were set up at expected points of pipe failure to minimize the dynamic effects of pipe rupture [[1], [2], [3]]. However, those heavy structures were not only expensive to set up, but they also increased the load on the pipes because of the increase in constraint stress. They also interfered with access by workers during in-service inspection and repairs, which resulted in an increase in radiation exposure.
Application of leak-before-break (LBB) design to pipes in nuclear power plants (NPPs) prevent instantaneous breaks, such as the double ended guillotine break (DEGB), by detecting leaks and taking proper action before the failure [1,2]. Load on piping can be reduced by using a LBB design because the effects of the dynamic load can be eliminated. In addition, pipe-mounted components such as pipe whip restrain or jet impingement shields do not need to be set up, which minimizes the exposure of workers to irradiation during in-service inspections. In nuclear power plants in Korea, the LBB design has been applied to most high-energy pipes with a diameter of 12 inches or more in primary systems since Hanbit NPP Units 3 and 4 [4].
As the safety of NPPs becomes more important, much effort has been made to apply the LBB design to the main steam line (MSL) piping in secondary systems. The LBB safety margin can be increased by reducing the load on pipes through design changes, reinforcing the leak detection equipment, or improving the tensile strength and fracture toughness of piping materials. However, the amount of increase in the LBB safety margin by a design change in NPPs is limited, and it is impossible to apply the LBB design to pre-existing NPPs. Therefore, in order to improve the LBB safety margin, it is necessary to improve the mechanical properties of the materials used in the pipe lines. SA106 Gr.C low alloy steel was used for the MSL piping in Korea standard nuclear power plants (KSNPs), but LBB design was not applied to MSL piping owing to an insufficient LBB safety margin [5]. SA508 Gr.1A low alloy steel, which has better fracture toughness than SA106 Gr.C steel, is being considered as the MSL piping material for next generation Korea standard nuclear power plants (KSNPs) to secure the LBB safety margin [5].
It is important to use a SA508 Gr.1A steel with high strength and J-R fracture toughness to achieve a high LBB safety margin. In previous research, a SA508 Gr.1A steel fabricated by a vacuum carbon deoxidation (VCD) steelmaking process had lower J-R fracture toughness because of a larger grain size and larger carbides compared to SA508 Gr.1A steels fabricated by VCD + Al and the VCD + Al + Si steelmaking process [6]. The advanced steel making process (VCD + Al + Si) has already become common for high quality structural materials such as NPP components. In addition to having the advanced steel making process, it is also necessary to improve the mechanical properties of SA508 Gr.1A steels through heat treatment to achieve a high LBB safety margin. The LBB margin is the most sensitive to yield strength [7]. Therefore, an increase in yield strength without a loss of toughness is the best way to increase the LBB characteristics. However, toughness generally tends to decrease as the yield strength increases, so it is difficult to improve both at the same time. The correlation between the microstructure, strength, ductility, and toughness needs to be analyzed clearly to improve the characteristic of SA508 Gr.1A steels for MSL piping. In this study, we analyzed the effects of strength, ductility, and microstructure on the J-R characteristics by varying the cooling rate of the SA508 Gr.1A steels. Based on our results, we propose a heat treatment process for improving the mechanical properties of SA508 Gr.1A steel.
Section snippets
Experiment
In this study, we used SA508 Gr.1A low alloy steel for RCS piping in KSNP. SA508 Gr.1A steel was manufactured in the form of pipes by a mandrel forging process. After the austenitizing heat treatment at 880 °C for 7 h, quenching, and tempering at 655 °C for 7 h, the final product was manufactured by rough and finish machining. The chemical composition of SA508 Gr.1A steel was analyzed using an optical emission spectrometer and the results are given in Table 1. In order to evaluate the effect of
Results
The OM and SEM images of the specimens are shown in Fig. 3. Many researchers, for example, the ISIJ Bainite Committee, Krauss, Thompson, Bhadeshia, and Lee, defined the various types of ferrite according to their morphologies and characteristics [[12], [13], [14], [15]]. The microstructure of the specimens was distinguished as ferrite, pearlite, tempered bainite (bainite), and tempered martensite (martensite) using high magnification SEM images, and the phase area fraction of the specimens was
Discussion
The grain size of the heat-treated specimens was smaller than that of AR. The austentizing time of the heat-treated specimens was 2 h, which was shorter than the 7 h for AR. Thus, the prior austenite grain size was smaller than that of AR. The prior austenite grain size is known to have a significant effect on the grain size of the final microstructure [15,16]. Microstructural defects such as grain boundaries and dislocations are known as major nucleation sites [[14], [15], [16]]. When the
Conclusions
To investigate the effect of cooling rate on microstructure formation and mechanical properties, SA508 Gr.1A steels were heat-treated by modified quenching and tempering process with different cooling rate and their tensile, Charpy impact, J-R fracture toughness properties were investigated.
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The microstructure of commercial SA508 Gr.1A steel was composed of coarse ferrite, pearilite and bainite. When the cooling rate is slower than that of commercial quenching process, ferrite and pearlite
CRediT author statement
Seokmin Hong: Investigation, Formal analysis, Investigation, Writing – original draft, Ki-Deuk Min: Conceptualization, Investigation, Se-Mi Hyun: Investigation, Jongmin Kim: Formal analysis, Writing – review & editing, Yo-Seob Lee: Resources, Project administration, Hong-Deok Kim: Funding acquisition, Project administration, Min-Chul Kim: Conceptualization, Writing – review & editing, Supervision
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIP) (2017M2A8A4015156) and by the project “Development of Technologies for Improving Mechanical Properties of Main Steam Piping” funded by KHNP.
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