Evaluation of Stability of Precipitates under Irradiation in 316FR Steel Used as Fast Reactor Structural Material

As one of the options to achieve carbon neutrality to combat global warming, a sodium-cooled fast reactor (SFR) is being developed in Japan as a new energy source that can reduce carbon dioxide emissions. In addition to high safety and economic efficiency, the SFR has a Generation IV nuclear energy system that reduces the volume and hazardousness of radioactive waste and makes efficient use of uranium resources. The SFR is a fast neutron irradiation environment, with a maximum operating temperature of approximately 550 °C. Therefore, the structural materials of the SFR are exposed to a severe environment. To withstand the severe environment, 316FR austenitic stainless steel, a modification of 316 austenitic stainless steel, will be used for the structural material of the SFR.

316FR steel is an austenitic stainless steel based on 316 series stainless steel, and its development began about 40 years ago as a structural material for fast demonstration reactors [1]. Generally, when heat-resistant steel is used at high temperatures for long periods of time, microstructural changes such as precipitation occur, resulting in deterioration of mechanical properties such as reduced strength and ductility. The chemical composition of 316FR steel is controlled to improve creep strength, which is standardized in the Japan Society of Mechanical Engineers (JSME) code, Rules on the Design and Construction of Nuclear Power Plants (JSME fast reactor code). The concentration of C in 316FR steel is controlled to prevent the precipitation of coarse carbides at the grain boundaries, which reduces the grain boundary strength and creep strength [2-4]. Nitrogen is added to supplement solid solution strengthening by carbon [2,3,5,6], and the lower limit for phosphorus concentration is set for creep rupture strength and ductility [2,3,7]. At 550 °C, the temperature at which 316FR steel is exposed in the SFR reactor, the Laves phase precipitates at the grain boundaries mainly during prolonged creep in 316FR steel due to its chemical composition [8]. In 316FR steel, the precipitation of M23C6 carbide at grain boundaries is suppressed due to low carbon concentration and the effects of phosphorus and molybdenum. Since carbon is an element that constitutes carbides, reducing carbon concentration suppresses carbide precipitation. Since 316FR steel is a low carbon steel, adding molybdenum causes M23C6-type carbides to compete with Laves phase precipitation and reduces the reaction ratio of chromium and carbon, thereby suppressing carbide precipitation [5]. Since phosphorus easily combines with vacancies [9,10], it is thought that the formation and growth of carbides are suppressed due to the reduced diffusion rate of the vacancies Hosoi et al. found that increasing the amount of phosphorus added to austenitic stainless steels has the effect of increasing creep strength by promoting the formation of the Laves phase [11]. On the other hand, an increase in phosphorus concentration can cause embrittlement and cracking, so a phosphorus concentration of 0.026% is selected, which is considered an appropriate value. In addition, the Laves phase precipitated at grain boundaries is less coarse than M23C6- type carbides and G phase precipitated at the grain boundaries of other austenitic stainless steels such as 304 steel, and is therefore resistant to grain boundary embrittlement as compared to M23C6 carbide and others [4,6,7]. As shown in the conceptual diagram in Figure 1; [7,12,13], controlling the precipitates that precipitate during creep improves the creep rupture time at 550 °C in 316FR steel compared to 316 and 304 austenitic stainless steels [4,5,8]. For example, under the conditions of 550 °C and 200MPa, 304 and 316 steels will fracture at about 5,600h and 58,000h, respectively, in the creep rupture formula defined in the JSME fast reactor code, while the rupture time of 316FR steel is about 431,000h. Thus, 316FR steel is a material with less degradation of material properties in high-temperature service than the 304 steel used as a structural material in the prototype fast breeder reactor “Monju” or the conventional 316 steel. In order to verify that 316FR steel, which has excellent material properties at high temperatures, can maintain its properties even under high-temperature irradiation conditions, in this study, in-situ observations under electron beam irradiation at high temperatures were conducted to investigate in detail the irradiation effects on precipitates, which are a characteristic feature of 316FR steel.

Figure 1:Conceptual diagram showing microstructural changes due to precipitates in 304 and 316FR steels subjected to prolonged thermal aging during creep at 550 °C. In 304 steel, carbides precipitate in the short term, and in the long term, increased precipitation and coarsening of carbides and precipitation of coarse G, α, and σ phases occur, which easily cause grain boundary embrittlement [12]. On the other hand, 316FR steel precipitates carbides in the short term, but the number is lower than that of 304 steel because of the suppressed C content. In the long term, G phase and Laves phase precipitate, but the main precipitate is Laves phase [7,12,13]. The Laves phase precipitated on the grain boundary of 316FR steel does not coarsen easily, which suppresses grain boundary embrittlement, and so 316FR steel has higher creep strength than 304 steel.

Microstructural observations of precipitates in 316FR steel after irradiation have been performed in the past using Transmission Electron Microscopy (TEM) and other methods, and in addition to the Laves phase, precipitates of M₂₃C₆, phosphides, and M₆C have been confirmed [14,15]. In a previous study, these precipitates prevented intergranular embrittlement by trapping helium produced by neutron irradiation, and the 316FR steel exhibited less creep strength reduction under irradiation than 304 steel. Meanwhile, Wakai et al. [16] reported a more detailed evaluation of the irradiation effects of displacement damage and helium production on tensile, creep, and microstructures of 316FR steel. The evaluation of irradiation effects is very important for evaluating the material properties of structural materials comprising the SFR. Since creep strength is greatly affected by irradiation damage caused by fast neutrons and helium produced by thermal neutrons, it is necessary to evaluate creep strength appropriately based on these irradiation damage mechanisms.

For 316FR steel, which is used as a structural material in lowirradiation regions such as reactor vessels, the main irradiation effects that are thought to affect material properties are displacement damage and helium production. This study investigated how one of these effects, displacement damage, affects the microstructure of this material. Specifically, we examined changes in irradiationinduced changes in grain boundary precipitates that affect creep properties.

In this study, we observed in-situ the microstructure at grain boundaries in grains of 316FR steel under high-energy electron beam irradiation at 1.25MeV, in which the atoms that form the material are damaged by displacement damage. 316FR steel of heat number B11 was used as the specimen material. Type 304 steel of heat number A7 was also used for comparison. The chemical composition and heat treatment conditions of the specimens are shown in Tables 1 & 2, respectively. Samples for observation were cut, punched, and mechanically polished from the material, and then electropolished to produce discs approximately 0.1mm thick and 3mm in diameter.

Table 1:Chemical compositions of test materials.

Table 2:Heat treatment conditions of test materials.

In the observation of the growth behavior of grain boundary precipitates by irradiation, the grain boundaries and precipitates of 316FR and 304 steels were irradiated with electron beams with 1.25MeV at 550 °C, and microstructural changes were observed in-situ. Assuming the amount of irradiation to which structural materials are subjected when the SFR is used for long periods of time, the irradiation dose was set to about 1dpa. Boundary components such as the reactor vessel are located somewhat far from the core, and the irradiation dose is assumed to be less than 1dpa even after 60 years (500,000 hours) of operation. Therefore, irradiation damage exceeding that to which the structural materials of the SFR will be exposed for 500,000 hours can be evaluated by irradiating at 1dpa in a simulated irradiation experiment. An ultrahigh voltage electron microscope (JEM-ARM1300) at Hokkaido University was used for in-situ observation of electron beam irradiation. Observation conditions are shown in Table 3.

Table 3:Observation conditions for in-situ observation of electron beam irradiation.

Electron beam irradiations of 316FR steel (heat B11) and 304 steel (heat A7) were performed on each material at 550 °C up to about 1dpa in the regions containing the grain boundaries. Figure 2 shows microstructural photographs at an irradiation dose of about 1dpa. In 304 steel, granular precipitates that were thought to be carbides precipitated throughout the grain boundaries after electron beam irradiation. This precipitate grew particularly large at the triple points of the grain boundaries. Based on previous studies [17], TTP curves, and the EDS results, which chromium concentrations increased in the precipitates, measured in this study and shown in Figure 3, this precipitate is considered to be M₂₃C₆ carbide [18]. The average size of the carbide precipitated at the grain boundaries of 304 steel was 33nm. On the other hand, as shown in Figure 2, a small amount of carbides that are considered to be M₂₃C₆ based on the TTP curve [19] were observed at the grain boundaries of 316FR steel, but their number and size were smaller than those of 304 steel, with an average size of 11nm.

Figure 2:Precipitates formed at grain boundaries of 316FR and 304 steels irradiated at 550 °C to 1dpa (left: 316FR steel, right: 304 steel). Precipitates were observed in both 316FR and 304 steels, but the precipitates in 316FR steel were smaller in number and size than those in 304 steel, while those in 304 steel were coarser at the grain boundaries.

Figure 3:EDS analysis results of (a)grain boundary and (b)matrix phase of 304 steel.

To confirm the time dependence of precipitation due to irradiation, the size of precipitates was measured from microstructure of 316FR steel and 304 steel at approximately 0.5dpa irradiation. Microstructure at 0.5dpa irradiation are shown in Figure 4. Due to the influence of grain boundary strain, it was difficult to measure the size of precipitates at 0.5dpa irradiation in the same area as the micrograph shown in Figure 2. Therefore, Figure 4 shows micrographs of other areas. In the micrograph of 304 steel shown in Figure 4, large precipitates present at the grain boundary triple points were already present before irradiation. In 316FR steel, no precipitates were observed at approximately 0.5dpa irradiation. On the other hand, in 304 steel, precipitates of approximately 20nm were observed at the grain boundaries at approximately 0.5dpa irradiation. Since the average sizes of precipitates in 316FR steel and 304 steel after 1dpa irradiation were 11nm and 33nm, respectively, it can be concluded that 316FR steel exhibits a delayed onset of precipitation and slower growth rate of precipitates compared to 304 steel under irradiation. In this study, the electron beam irradiation temperature was 550 °C and the irradiation time was about 0.5h. While precipitates precipitated at the grain boundaries of both 316FR and 304 steels under the electron-beam irradiation conditions of this study, the precipitates found at the grain boundaries of 316FR steel were smaller in number and size than those of 304 steel, indicating that irradiation of 316FR steel did not promote the precipitation of grain boundary precipitates as much as 304 steel. Accordingly, 316FR steel can preserve its characteristics in the high-temperature and irradiation environment of the SFR, indicating that the irradiation resistance of 316FR steel is superior to that of 304 steel.

Figure 4:Microstructure of 316FR and 304 steels irradiated at 550 °C to 0.5dpa (left: 316FR steel, right: 304 steel). Precipitates were observed at the grain boundaries in the 304 steel, but not in the 316FR steel. The size of the grain boundary precipitates observed in the 304 steel was approximately 20nm.

Since grain boundary precipitates are one of the important factors for creep strength, the stability of grain boundary precipitates has an important influence on creep properties. Compared to type 304 stainless steel, the grain boundary microstructure of 316FR steel is characterized by higher stability against heat and irradiation at about 550 °C, which is the operating temperature of SFR reactors. Therefore, it is important to systematically analyze and evaluate the intergranular microstructure of 316FR steel in detail because it is considered to contribute to the reduction of creep strength under irradiation. In previous studies by Miyaji et al. on 316FR steel [13,14], the mechanism that suppresses the reduction of creep strength under irradiation was examined based on the results of microstructural observations. Miyaji et al. observed the microstructure of postirradiation creep specimens irradiated at around 550 ℃ at 0.21 dpa and 0.58dpa and confirmed that grain boundary precipitates (carbides) were stable in the 316FR steel irradiated at 0.21dpa and that the grain boundary precipitates grew slightly after 0.58dpa. In addition to these research results, the growth behavior of grain boundary precipitates in 316FR steel was investigated in detail by in-situ observation under irradiation up to 1dpa in this study, and it was found that the precipitates (carbides) on the grain boundary were more stable in 316FR steel compared to the growth of grain boundary precipitates in 304 steel. These results indicate a smaller rate of decrease in creep strength under irradiation for the 316FR steel, which is one reason for its superior long-term creep strength. In other words, irradiation may reduce grain boundary strength by promoting or inducing precipitation of precipitates and generating helium, a nuclear transmutation product. However, 316FR steel with suppressed carbon concentration has a greater effect of suppressing precipitation of precipitates, resulting in improved creep properties under irradiation compared to 304 steel.

In this study, microstructural observations around grain boundaries under irradiation were conducted to verify that the microstructural characteristics of 316FR steel can be preserved even under a high-temperature irradiation environment. As a result, it was found that although precipitates precipitated at grain boundaries due to electron beam irradiation at 550 ℃ to 1dpa, the number and size of precipitates were smaller than those of 304 steel, and that the characteristics of 316FR steel, in which carbide precipitation is controlled by reducing carbon concentration, were preserved even under irradiation. As an industrial application example, 316FR steel exhibits superior creep properties compared to 304 steel and 316 steel and can be fabricated using manufacturing methods similar to those of 304 and 316 steels, making it effective for general industrial use and expected to contribute to global warming countermeasures. In particular, it was revealed that 316FR steel is suitable as a fast reactor structural material from the perspective of grain boundary precipitates that precipitate under irradiation. In the future, in order to study the creep mechanism of 316FR steel under irradiation in more detail, the authors will continue their research from perspectives other than grain boundary precipitates, such as observing grain interior precipitates and calculating the activation energy of atomic vacancies and interstitial atom migration, in order to make the analysis of the phenomenon more quantitative.

This research was partially supported by a Grant-in-Aid for Scientific Research (A) (General) (21H04668) and was conducted using the facilities of the Advanced Research Infrastructure for Materials and Nanotechnology in Japan (ARIM). We would like to sincerely thank Prof. T. Shibayama, Mr. K. Okubo, and Mr. T. Tanioka of Hokkaido University for their great help in obtaining the results of this research.

The authors declare no conflict of interest.

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