Effect of nickel content on creep strength of Fe-15Cr-Ni-2.5Mo-0.3Ti austenitic steels for fast breeder reactor fuel cladding tube

Abstract

The fuel cladding tubes of fast breeder reactors (FBR) are subjected to internal pressure caused by fission product gases at temperatures above 923 K under fast neutron irradiation. Therefore, resistance to swelling and creep strength are the principle requisites for cladding materials. Austenitic steels such as type 316 steel have been selected for cladding material. The resistance to swelling of austenitic steels is known to be suppressed by cold working [1] and increasing nickel content [2]. However, the effect of nickel content on the creep strength of austenitic steels is not well understood. This letter describes the effect of nickel content ranging from 15 to 35 wt % on the creep strength of cold-worked Fe-15Cr-Ni-2.5Mo -0.3Ti austenitic steels strengthened by the precipitation of MC type (TIC) carbides. The chemical compositions of the test materials are given in Table I. Only the nickel content was varied, ranging from 15 to 35 wt %, and the other compositions, including molybdenum, titanium, phosphorus and boron (elements that have been found to be effective in improving the creep strength), were held constant. The test materials were melted in a vacuum-induction furnace and cast into 10 kg ingots. Th%ingots were hot forged and hot rolled to sheets of 6 mm thickness. After solution treating at 1373K and cold rolling to 2.5ram thickness, they were final solution treated at 1373 K or 1423 K. The sheets were cold rolled for 20% before creep-rupture testing. Creep-rupture tests were performed at 973 K using plate specimens 2 mm thick and with gauge section 6 mm wide and 25 mm long. The microstructures of the specimens were examined by transmission electron microscopy (TEM).