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Nuclear energy is a low-carbon, safe, efficient, and sustainable clean energy. The new generation of nuclear energy systems operate in harsher environments under higher working temperatures and irradiation doses, while traditional nuclear power materials cannot meet the requirements. The development of high-performance nuclear power materials is a key factor for promoting the development of nuclear energy. Oxide dispersion strengthened (ODS) steel contains a high number density of dispersed nano-oxides and defect sinks and exhibits excellent high temperature creep performance and irradiation swelling resistance. Therefore, ODS steel has been considered as one of the most promising candidate materials for fourth-generation nuclear fission reactor cladding tubes and nuclear fusion reactor blankets. The preparation process significantly affects the average size, number density, and distribution of nano-oxides. The main preparation method for ODS steel is the powder metallurgy process, but it has the disadvantages of low efficiency, poor stability between batches and small-size product. Therefore, it is urgent to develop a new process for preparing large-size ODS steel components. The powder metallurgy process can produce generally a few kilograms to dozens of kilograms of ODS steel. The liquid metal forming process can prepare hundreds of kilograms of ODS steel under the condition of compromised performance. Cold spray, additive manufacturing, and other hybrid processes are applicable for the preparation of small and complex ODS steel components. It is difficult to produce large-size homogenized ODS steel by these processes, which restricts the application of the materials. The Institute of Metal Research of the Chinese Academy of Sciences put forward a novel technology to manufacture heavy, high-quality forgings by additive forging, which breaks through the traditional concept that a heavy forging must be created using a large steel ingot and realizes the new manufacturing technology of “by making greatly small” and provides a new idea for the preparation of large-size ODS steel components. Doctoral student Jianqiang Wang, Prof. Mingyue Sun, Prof. Dianzhong Li from The Institute of Metal Research of the Chinese Academy of Sciences, wrote a review " Research Progress on Preparation Technology of Oxide Dispersion Strengthened Steel for Nuclear Energy " on IJEM. In this article, the authors have introduced the research background, systematically discussed the latest progress and the future outlook of preparation process for ODS steel. Figure 1 shows the classification of preparation processes for ODS steel, including the powder metallurgy process, improved powder metallurgy process, liquid metal forming process, hybrid process, and additive forging.
● The advantages and disadvantages, applicability and development direction of various ODS steel preparation processes are summarized.
● The relationship between microstructures and properties of ODS steel and preparation process is summarized and analyzed.
● Additive forging, which is promising to realize homogenized large-size ODS steel components, is proposed.
Figure 1. Classification of preparation processes toward ODS steel.
The operating temperature, radiation dose and chemical corrosion environment of the fourth-generation nuclear power system and fusion reactor will be harsher than that of the second-generation and third-generation nuclear power systems. The typical microstructures of ODS steel include: sub-micron grain size, defect sink strengths near or greater than 1016m-2, nano-oxides with average diameters <d>
3. Recent Advances
Powder metallurgy process
The main preparation method for ODS steel is the powder metallurgy process, including gas atomization, mechanical alloying, canning, consolidation forming (e.g., hot isostatic pressing, spark plasma sintering, and hot extrusion, etc.), and thermo-mechanical treatment, as shown in Figure 2. Mechanical alloying has the disadvantages of low efficiency, poor stability between batches, and susceptibility to grinding medium, container, and air pollution. In order to minimize the disadvantages of mechanical alloying, scholars have proposed improved powder metallurgy processes called cryomilling and internal oxidation. Cryomilling effectively changes the microstructure, and it produces homogeneous microstructures with narrow oxide particle grain boundary spacing and sub-micron-sized grains, as shown in Figure 3. The diffusion distance of oxygen in a matrix is limited by the oxygen partial pressure, thus the number density and the homogeneity of nano-oxides in ODS steel prepared by internal oxidation is significantly lower than mechanical alloying. In order for oxygen to diffuse fully into matrix to form the desired microstructure, some scholars proposed improved internal oxidation methods: gas atomization reaction synthesis (GARS), gas atomization reaction synthesis (GARS) and enhance oxygen diffusion capacity.
Figure 2. Schematic representation of ODS steel processing and fabrication paths by powder metallurgy process. Copyright 2019, with permission from Elsevier.
Figure 3. TEM images of 14Cr-ODS steel produced by (a) conventional mechanical alloying and (b) -150℃ cryomilling. Copyright 2016, with permission from Elsevier.
Liquid metal forming process
The liquid metal forming process is a process in which fine oxide powders are added to liquid steel and dispersed oxide particles are formed in the matrix. The liquid metal forming process has attracted sustained attention attributed to its advantages, such as short process flow, low preparation cost, and large-scale preparation in a single batch. However, the agglomeration and coarsening of oxide particles are inevitable in melting and solidification. In order to minimize the disadvantages of liquid metal forming process, scholars have carried out research on liquid metal forming technology, and proposed direct casting technology, intermediate alloy casting technology, oxygen carrier casting technology (Figure 4), pre-laying powder casting technology and in-situ nano-oxides generation technology by electromagnetic stirring (Figure 5). Liquid metal forming process is a feasible method to prepare large-size ODS steel. However, blending fine oxide particles with liquid steel and dispersing them in the matrix still present significant challenges.
Figure 4. Schematic representation of producing ODS steel by the Fe2O3 oxygen carrier casting method. Copyright 2019, with permission from Elsevier.
Figure 5. Schematic of the processing method for forming nanoparticles in melt through electromagnetic stirring. Copyright 2014, with permission from Elsevier.
The dispersed nano-oxides in ODS steel hinder the dislocation movement and significantly increase strength, which elevates the dynamic recrystallization temperature and increases the manufacturing difficulty of products with complex shapes. Cold spray, melt spinning, additive manufacturing and other hybrid processes can reduce the mechanical properties of the materials, but they are helpful for manufacturing products with complex shapes. Therefore, ODS steel prepared by hybrid processes is not expected to obtain smaller size and more homogeneously dispersed nano-oxides, achieves a compromise between the performance of the final product and the applicability and cost of industrial production.
The Additive forging breaks through the traditional concept that a heavy forging must be created using a large steel ingot and realizes the new manufacturing technology of “by making greatly small”. After surface treatment, stacking, vacuum electron beam welding, and lager plastic deformation—characterized by pressure-forging and multi-directional forging at high temperature—the homogenized heavy forgings can be obtained with fully healed interfaces, as shown in Figure 6. Owing to the large number of dispersed nano-oxides in ODS steel, a large strain gradient energy storage was formed near the bonding interface, which provided a driving force for the strain-induced grain boundary migration and led to the discontinuous dynamic recrystallization phenomenon. As the amount of deformation increases, the migration rate of the interface grain boundary increases, which promotes the long-range migration of the grain boundary and ultimately achieves metallurgical bonding, as shown in Figure 7. This method provides a new idea for the preparation of large size ODS steel components.
Figure 6. Schematic representation of the additive forging process for ODS steel
Figure 7. Microstructure evolution of bonded interfaces in 14Cr-ODS steel during metal additive forging. Copyright 2019, with permission from Elsevier.
It is very important to prepare large-size ODS steel for promoting the application of ODS steel in nuclear power. At the same time, ensuring the formation and stability of nano-oxide particles and controlling their interfacial relationship with the matrix for large-size ODS steel require further study. Additive forging provides a new idea for the preparation of large ODS steel components. Although the joint interface of additive forging could achieve metallurgical bonding by dynamic recrystallization, due to the special microstructure of ODS steel and the large deformation at high temperature during the additive forging, the size, morphology, and distribution of nano-oxides in the interface area still require further exploration. Moreover, the mechanical properties of the interface should be evaluated to ensure that the properties of the interface are equivalent to the matrix, which will lay a foundation for the application of additive forging in ODS steel. With the development of ODS steel preparation processes, homogenized high-performance ODS steel slabs are obtained by the liquid metal forming process and then homogenized large-size ODS steel components are prepared by additive forging, which promotes the engineering application of ODS steel in nuclear power and other harsher service environment fields.
5. About the Authors
Mingyue Sun is a professor and doctoral supervisor at the Advanced Iron and Steel Materials Research Department, Institute of Metal Research, Chinese Academy of Sciences. He received his Ph.D. degrees from Institute of Metal Research, Chinese Academy of Sciences in 2009 and B.S. degree from Chongqing University in 2003.
Prof. Sun’s research focuses on the homogenization and forming of large forgings, and he invents the additive forging, which solves the size effect problem in the solidification process of large ingot and create a new manufacturing direction of large-size components. The research results have been applied in nuclear power, hydropower, wind power, military industry and other fields, and have produced remarkable economic and social benefits.
Prof. Sun has given dozens of invited presentations, lectures, and tutorials. He is the recipient of many prestigious national awards, including awards from Chinese Ministry of science and technology, First Prize of Technical Inventions of Liaoning Province, Tan Kah Kee Young Scientist Award, Metallurgical Youth Science and Technology Award of China Metal Society, Young Scientist Award of Chinese Academy of Sciences, Lu Jiaxi Youth Science and Technology Award of Chinese Academy of Sciences, etc.