Modification Of Microstructures And Properties Of T91Steel Towards ADS

Accelerator Driven Sub-critical System (ADS) is one of the most promising fission type reactors, and also one of the hotspots in the new nuclear energy system studies. The load bearing structures in the ADS, for instance, shell and spallation target work in a high temperature and strong radiation environment, and meanwhile, are subjected to liquid alloy erosion, radiation damage as well as cyclic thermal fatigue. Therefore, it is necessary that the related structural materials possess excellent physical and chemical properties to endure high temperature, anti radiation, resist corrosion. It has been known that T91steel with a ferrite/martensite dual phase structure exhibits good high temperature strength and oxidation resistance. Since it has been widely used in thermal power systems as a high temperature structural material, it is believed to be one of the most prospective candidates in the ADS. In order to further improve the microstructures and properties of T91steel, particularly the high temperature fatigue and liquid Pb-Bi alloy erosion resistant properties, the present study is carried with the purpose of tailoring the microstructures and properties of T91steel through dispersion strengthening, surface passivation and crystal orientation optimizing etc to give theoretical and technology bases for the applications in the ADS. The main contents and results are as follows.1. Oxides dispersion strengthening (ODS)The ODS methods were investigated by use of micron sized Y2O3particulates as the enhancement and T91steel as the matrix through direct casting and intermediate alloy casting technologies.In the former technology, the micron sized Y2O3particulates were directly added in the molten T91steel and uniformly distributed in the molten alloys with the help of electro-magnetic stirring in the induction surface. It is shown that there were a number of oxides containing Y at the Y2O3particulates-matrix interfaces. The diameter of oxide particles was varied from2μm to5μm. These particles made both the strength and plasticity increased by the Orowan strengthening mechanism, giving rise to uniform dimple fracture surface. When the addition of Y2O3particulates was separately0.2wt.%and0.4wt%, the tensile strength of T91steel after high temperature normalizing and tempering was increased11.6%and17.7%, respectively, compared with that of original T91steel. In addition, addition of0.4wt.%Y2O3particulates in a9Cr low activation steel also made the tensile strength increased to14%. These results demonstrate that direct casting technology does be an effective method to realize ODS of T91steel.In the latter technology, an intermediate alloy was firstly prepared by milling pure Fe and Y2O3powders in a high energy ball mill to make the Y2O3powder refined to nano dimension and uniformly distributed in the Fe powder, then sintering the mixed powder to form bulk composite. The bulk composite was added in the induction furnace and melted together with T91steel. After the alloy was poured and solidified, a nano Y2O3enhanced T91steel was produced, which was verified by TEM observation. The tensile tests shown that, in comparison of original T91steel, the strength of nano Y2O3enhanced T91steel increased9.6%and the elongation almost kept unchanged. Accordingly, this technology would be useful to fabricate nano Y2O3particulates enhanced T91steel and result in strengthening effect.2. Surface passivationThe study on the surface passivation of T91steel was conducted to improve the resistance to molten metal erosion. A Fe-Al intermetallic layer was formed on the surface of T91steel through hot-dip Al technique. It was proved by SEM observation and XRD analysis that the aluminized layer had a thickness of around60μm containing Al, FeAl3and Fe2Al5. After heat treatment at1050℃for0.5h, all the intermetallics were transformed to single Fe3Al phase, and the thickness of aluminized layer increased to about100μm. It was shown by EDX analysis that about8.9%Al atoms were substituted by Cr atoms in the Fe3Al phase, thus the real composition of Fe3Al phase should be Fe3(Al, Cr). The microhardness of aluminized layer was closed to that of T91steel matrix, suggesting that the both have similar mechanical properties. After immersed in a molten Pb-Bi alloy for200h, no obvious corrosion can be observed on the surface of samples, and the O content there was also very low, demonstrating that the aluminized layer had a very good corrosive resistance.3. Grain morphology tailoringThe load bearing structures in the ADS are subjected not only to radiation and liquid metal erosion, but also to cyclic thermal stress because they are used in a high temperature environment and contacted with liquid Pb-Bi alloy. There will be fatigue cracks arising in the T91steel along the flow direction of liquid Pb-Bi alloy. Therefore, it is very important to improve the thermal fatigue resistance of T91steel for prolonging its service life. In the present study, a directional solidification method was utilized to produce T91steel with controllable grain orientation and morphologies. The solidification process and grain morphology were examined and correlated with temperature gradient and solidification rate. It is found that, through phase diagram analysis and microstructure observation, the directionally solidified structures contained block and plate ferities, retained austenite, multi-directionally grew martensite as well as carbides, in which the plate ferities grains were arrayed in parrlell to the growth direction of grains. At the early stage of solidification, there appeared a number of orthogonal martensites, and as the solidification proceeded, the temperature gradient and the growth of columnar a ferrite grains tended to stable. At the late stage of solidification, there were large plate martensites and ferities formed near the end of columnar a ferrite grains. Moreover, there were5～15°differences in the orientation of columnar grains due to the difference in transverse heat dissipation among them. The difference in the grain orientations tended to increase with increasing the solidification rate. It has been also found that, close to the outsides of solidification samples, there existed columnar grains growing in the opposite directions due to the effect of varied direction of thermal flow.The measurements show that the modulus of elasticity in the longitudinal direction of columnar grains increased as the solidification rate increased, being in the range of124GPa to216GPa. The relationship between the modulus of elasticity and grain structure can be summarized as Edendritite＜Ecell＜Emartensite≈Eequiaxed＜Edual-direction <Eover30°orientation difference.In addition, the modulus of elasticity of directionally solidified structures was dependent on the orientation of grains, and it can be varied in the following turn E0°<E5°<E15°<Eequiaxed<E90°<E30°><E45°.