Microstructure and Mechanical Properties of a Nitride-Strengthened Reduced Activation Ferritic/Martensitic Steel
- PDF / 1,079,275 Bytes
- 9 Pages / 593.972 x 792 pts Page_size
- 20 Downloads / 199 Views
DUCTION
THE structural materials in future fusion reactor need to withstand high temperature under long-term loading and irradiation.[1] Currently, reduced activation ferritic/martensitic steels (RAFM) are the leading candidate structural materials for fusion reactors due to the high thermal properties and superior swelling resistance compared with austenitic stainless steel.[2] The typical microstructure of the RAFM steels is the tempered martensite with a large number of precipitates.[3] There are two main types of precipitates, M23C6 (M is for Cr, Fe, W, etc.) and MX (M is for V, Ta, etc. and X is for C, N) in RAFM steels. However, it has been proved that the coarsening rates of these two kinds of precipitates are different during creep and the coarsening rate of M23C6 carbide is much higher than that of the MX-type nitride.[4] It has been recognized that one of the effective methods of enhancing creep strength is to achieve good thermal microstructure stability by using thermally stable precipitates.[5] Taneike et al.[6] reported that when the carbon content was decreased to a very low level, the time before rupture was significantly increased in the heat-resistant steel for power plant. It could be attributed to the elimination of thermally unstable M23C6 carbides. Therefore, strengthening the RAFM steels with only nitrides that have excellent thermally stability can extend the creep life.
In the microstructure and composition design of nitridestrengthened RAFM steels, instead of Co or W, Mn was employed to suppress the d-ferrite that was more prone to form due to the carbon elimination, taking into account of the reduced activation property and the formation of Laves phase Fe2W. A fully martensitic microstructure could be obtained by 3 pct Mn addition. However, the high Mn content leads to such severe problems as the low Ac1 temperature, Mn segregation along prior grain boundaries, and many MnS inclusions. These effects are so detrimental that the nitride-strengthened RAFM steels of high Mn content show poor performance.[7] Fortunately, after decreasing Mn content to 1.4 pct, the nitridestrengthened RAFM steels now can be tempered at temperature as high as 1023 K (750 °C) and thus show excellent toughness. This article presents the microstructure and mechanical properties of the optimized nitridestrengthened RAFM steel. A more thorough review of the work published on ferritic/martensitic steels, including the low-activation variety, was given in a separate, recent paper in this journal,[8] and thus, it is not repeated here. To make a ferritic/martensitic steel ‘‘low activation,’’ both Mo and Nb are removed and replaced by W and Ta. Co should be avoided, too. The new point in the current article is the new steel composition designed. Such a steel composition has never been examined before.
II. QIANGGUO ZHOU and WENFENG ZHANG, Ph.D. Students, WEI YAN, Associate Professor, WEI WANG, Research Associate, and YIYIN SHAN and KE YANG, Professors, are with the Institute of Metal Research, Chinese Academy of Sciences, She
Data Loading...
