@misc{oai:ir.soken.ac.jp:00001491,
 author = {李, 芬 and リ, ヤンフェン and LI, Yanfen},
 month = {2016-02-17, 2016-02-17, 2016-02-17},
 note = {Blanket is one of the key components of fusion reactors, which provides the main heat transfer and tritium breeding systems. Currently, reduced activation ferritic/martensitic steels (RAFMs) are considered as the primary candidates for blanket structural materials because of their most matured technological base and good irradiation resistance. In the past two decades, several advanced RAFM steels have been developed with continuous improvements in the world. Among these, JLF-1 steel is a Japanese candidate, and CLAM steel is a Chinese reference.<br />　　In fusion reactor application, the structural materials will be exposed to long-term loading at high temperature, which may result in the changes of mechanical property and microstructure. The thermal changes of property and microstructure at the operation temperature are called ageing. Thermal ageing may reduce the maximum operation temperature by reduction of yield strength and/or creep strength. However, up to now, the understanding of the ageing effects is not sufficient. The objectives of this work are: (1) to investigate the thermal ageing effects on mechanical properties changes, including hardness, tensile and creep deformation, (2) to clarify mechanism of the mechanical property changes by correlation with microstructural evolution, and (3) to apply the experimental data obtained to the blanket design. <br />　　In this study, the JLF-1 (JOYO-II-HEAT) and CLAM (0603 HEAT) were used for comparison. The chemical compositions (in weight%) were 9.00 Cr, 1.98 W, 0.49 Mn, 0.20 V, 0.083 Ta, 0.09 C, and balance Fe for JLF-1, and 8.94 Cr, 1.45 W, 0.44 Mn, 0.19 V, 0.15 Ta, 0.13 C, and balance Fe for CLAM. Thus, CLAM has higher level of Ta and lower level of W than those of JLF-1. The heat treatments included the normalization (1323 K/60 minutes for JLF-1 and 1253 K/30 minutes for CLAM) and tempering (1053 K/60 minutes for JLF-1 and 1033 K/90 minutes for CLAM). By comparing the two alloys, the effects of the composition and the heat treatment were investigated.<br />　　The thermal ageing experiments were carried out at a temperature range from 823 to 973 K up to 2000 h and under high vacuum to prevent oxidation. The temperature of 823 K was chosen to simulate the typical upper limit temperature for fusion reactor, and 973 K was chosen to accelerate the thermal processes. The mechanical properties including the hardness, tensile and creep were tested. Microstructural evolution was examined by Scanning Electron Microscope (SEM) and Transmission Electron Microscope (TEM) equipped with Energy Dispersive Spectrometer (EDS).<br />　　The results showed that, the hardness increased slightly after ageing at 823 K for 100 and 2000h for the both steels, suggesting ageing-induced hardening. However, the hardness decreased at 873K and above, indicating ageing-induced softening. No significant changes were detected in both yield strength and ultimate tensile strength after ageing at 823 K for 2000 h, while the strength decreased after higher temperatures ageing. <br />　　The creep curves of the both steels, similar to other RAFM steels, exhibited three regions: the short primary creep, the secondary creep which was a linear process with a minimum creep rate, and the tertiary characterized by an increased creep rate with time. After ageing at 823 and 873 K for 2000 h, the creep property improved. However, it degraded significantly after ageing at 973K for 100 h. <br />　　JLF-1 and CLAM steels before ageing exhibited a mixture of martensitic lath and tempered martensitic structure with two types of precipitates, M<small>23</small>C<small>6</small> and TaC. Microstructural evolution revealed that, the number density of small precipitates (< 80 nm) increased significantly after ageing from 823 to 923 K up to 2000 h. However, after ageing at 973 K for 100 h, the density of small precipitates decreased, and the recovery of microstructure partially was also observed. By analysis of the chemical composition using EDS, most of the small precipitates were identified to be Ta-rich carbide (TaC), and the larger precipitates to be the Cr-rich carbide (M<small>23</small>C<small>6</small>). <br />　　A traditional dispersed obstacle model was used to correlate the hardness change and the microstructure. In this model, the dominant obstacle was assumed to be precipitates only. The typical hardening (823K/2000h) and softening (973K/100h) conditions were chosen for calculation. The results showed that the calculated hardness change almost agreed with the measured ones after ageing at 823 K for 2000 h, suggesting that the new formation of TaC was responsible for the hardening. However, there is a large difference between the calculated and measured dada after ageing at 973 K for 100 h, indicating that the loss of TaC alone cannot account for the softening. The recovery of lath structure and dislocations would contribute the major effects to the softening.<br />　　Comparing the two steels, the CLAM has higher hardness and tensile strength, smaller minimum creep rate and longer rupture time than those of JLF-1, while the susceptibility to thermal ageing of CLAM was larger than that of JLF-1. The lower normalization and tempering temperature and higher level of Ta were considered to be responsible for the finer grain and smaller martensitic lath width, thus leading to the higher hardness and strength for CLAM than those of JLF-1. However, the study showed that, because of the lower heat treatment temperature, the CLAM was more susceptible to thermal ageing, suggesting that the present heat treatment condition is not the best one. Increase in heat treatment temperature is necessary to improve the stability to thermal ageing for CLAM. <br />　　Since testing materials for the actual operation time is extremely costly and time-consuming, prediction of creep rupture performance in blanket conditions is critically important. In this work, the Larson-Miller parameter was used for prediction, which was based on the results of short-term creep experiments at higher temperatures with higher stresses. By prediction, the rupture stress at the typical blanket condition, 823 K for 100 000h, was estimated to be about 140 MPa for the both steels, and the present prior ageing influenced the rupture stress by about &plusmn; 10 MPa. This result can provide a reference for the design of fusion blanket.<br />　　In summary, this study has demonstrated the effects of thermal ageing on mechanical property and microstructural evolution for JLF-1 and CLAM steels. One of the important new findings is that the TaC is unstable, which can form newly or disappear by dissolution during the ageing. Ta is an element newly used in RAFM steels, instead of Nb in the conventional steel for the purpose of avoiding long radioactivity. Thus limited understanding of the status of Ta has been available up to now. The present study showed that the future control of TaC can enhance the thermal properties of RAFM steels. Since the Larson-Miller parameter only included partial effects of property change by ageing, the LMP correlated with pre-aged experiments in this study can provide the suggestion for developing suitable methods to predict the creep performance in blanket conditions including the overall ageing effects., application/pdf, 総研大甲第1277号},
 title = {Effects of thermal ageing on mechanical property and microstructural evolution for reduced activation ferritic/martensitic steels},
 year = {}
}