Research on development of high-purity iron-based alloys. Manufacture, analysis of small amount of element and property tests

The purpose of this study is to understand the material properties of manufacturable high-purity iron and high-purity iron-based alloy in present technology and to get an applicable prospect for the structural and functional material of the frontier fast reactor. Then the about 10kg high-purity iron and iron-based alloy were melted using a cold-crucible induction melting furnace under the ultra-high vacuum. Subsequent to that, the compatibility between the melted material and the high-temperature sodium environment which is a special feature of the fast reactor and tensile property at room and elevated temperatures were investigated using the melted materials. Also, the creep test using the high-purity 50%Cr-Fe alloy at 550degC in air in order to understand the high temperature creep property. In addition, the material properties such as thermal expansion coefficient, specific heat and electrical resistance were measured and to evaluate the outlook for the structural material for the fast reactor. The following results were obtained based on the property test and evaluation. (1) It was possible to melt the about 10kg high-purity ingot and high-purity 50%Cr-Fe alloy ingot using a cold-crucible induction melting furnace under the ultra-high vacuum. (2) The tensile tests of the high-purity 50%Cr-Fe alloy were performed at room and elevated temperatures in order to understand the deformation behavior. From the experimental results, it was clear that the high-purity 50% Cr-Fe alloy possesses high strength and good ductility at elevated temperatures. (3) The physical properties (the thermal expansion coefficient and specific heat etc.) were measured using the high-purity 50%Cr-Fe alloy. It was clear that the thermal expansion coefficient of high-purity 50%Cr-Fe alloy was smaller than that of SUS304. (4) From the corrosion test in liquid sodium, the ordinary-purity iron showed the weight loss after corrosion test. However the high-purity iron showed the weight gain. And the grain boundaries of the ordinary-purity iron were attacked remarkably, but the grain boundary corrosion did not occur in high-purity iron. (5) The creep tests were performed using the high-purity 50%Cr-Fe alloy at 550degC in air. From the experimental results, the creep strength of the high-purity 50%Cr-Fe alloy was higher than that of Mod. 9Cr-1Mo steel in short time range, but that of the high-purity 50%Cr-Fe alloy resemble to that of Mod. 9Cr-1Mo steel in long time range. In the other hand, the creep fracture elongation and reduction area of the high-purity 50%Cr-Fe alloy decreased slightly compared to those of Mod. 9Cr-1Mo steel. (author)