Recent Progress in Microstructural Control and Solid-State Welding of Reduced Activation Ferritic/Martensitic Steels
LIU Chenxi, MAO Chunliang, CUI Lei, ZHOU Xiaosheng, YU Liming, LIU Yongchang()
State Key Laboratory of Hydraulic Engineering Simulation and Safety, School of Materials Science and Engineering, Tianjin University, Tianjin 300354, China
Cite this article:
LIU Chenxi, MAO Chunliang, CUI Lei, ZHOU Xiaosheng, YU Liming, LIU Yongchang. Recent Progress in Microstructural Control and Solid-State Welding of Reduced Activation Ferritic/Martensitic Steels. Acta Metall Sin, 2021, 57(11): 1521-1538.
The International Thermonuclear Experimental Reactor (ITER) project is one of the world's largest and most ambitious international scientific research collaboration projects to date. Reduced activation ferritic/martensitic steel (RAFM steel) has been selected as the candidate material for test blanket module in ITER due to its excellent mechanical properties at high temperature, high thermal conductivity, low thermal expansion coefficient, and intense neutron irradiation swelling resistance. According to the reduced activation element selection approach, RAFM steels were created using Cr-Mo ferritic heat-resistance steels. However, RAFM steels have some disadvantages, including poor high-temperature endurance and type IV cracking in fusion-welded joints. The history of development, alloying principles, microstructural design principles, microstructure evolution and control, and solid-state joining technologies (diffusion bonding and friction stir welding) were discussed in this study. The pinning effect of nanoscale MX with excellent thermal stability on dislocations has been identified as a key factor in strengthening RAFM steel. In RAFM steel, the mechanism for a discontinuous martensitic transition during isochronal cooling has been elucidated. The microstructural formation, evolution, and failure of solid-state RAFM steel joints were shown, and its mechanical properties optimization due to thermo-mechanical treatment was realized.
Table 1 Compositions of the different reduced activation ferritic/martensitic (RAFM) steels[20-22,24,25]
Fig.1 Effects of W content on the mechanical properties of RAFM steels at different temperatures(a) yield stress and ultimate tensile strength[35](b) creep rupture life[36]
Fig.2 Schematic for microstructure characteristics of RAFM steels
Fig.3 Relationship between the lath martensite formation rate (df / dt) and temperature upon the different cooling rates in a RAFM steel[59]
Fig.4 Images of laser scanning confocal microscopy in the RAFM steel containing 0.1%C and the corresponding martensitic transformation curve with a cooling rate of 5oC/min applied
Fig.5 Bright field TEM (a) and HAADF-STEM (b) images of a RAFM steel after normalizing and tempering
Fig.6 Comparisons between the experimental and calculated yield strengths at room temperature of a RAFM steel under the different heat treatments (N&T 1—normalzing at 1000oC for 30 min and tempering at 750oC for 90 min, N&T 2—normalzing at 1050oC for 30 min and tempering at 750oC for 90 min)[86]
Fig.7 Tensile stress, yield stress, and total elongation of a RAFM steel as a function of temperature[87]
Fig.8 TEM images for dislocation distribution of a RAFM steel after plastic deformation at the different temperatures[87]
Fig.9 Summaries of yield strength-uniform elongation (a) and yield strength-elongation to failure (b) maps for T91 steel subjected to various types of processing (HT—annealing, WQ—water quenching, ECAE—equal channel angular extrusion)[100]
Fig.10 TEM images of the RAFM steel sample after rolling and tempering with the different conditions for twin-jet electropolishing samples (a-c) and extraction replica samples (d-f)
Fig.11 Schematic diagram for processing routes and microstructural design of a RAFM steel[112]
Fig.12 OM (a) and TEM (b) images of the diffusion bonded joint of a RAFM steel
Fig.13 TEM images of the diffusion bonded joints of a RAFM steel with an electrodeposited Ni interlayer before (a) and after (b) postweld heat treatment[125]
Fig.14 Process during transient liquid phase (TLP) bonding of RAFM steels[126]
Fig.15 Laves phase in intercritical heat affect zone of friction stir welding (FSW) of a RAFM steel[145]
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