Effect of titanium content on creep strength of cold-worked 15Cr-15Ni-2.5Mo austenitic steel

Abstract

The core materials, such as fuel cladding tube, in the fast breeder reactor (FBR), are required to have superior strength at elevated temperatures of around 923 K and excellent resistance to void swelling introduced by intense fast neutron irradiation to attain economic and reliable FBR performance. Cold-worked type 316 stainless steel has been used in some existing reactors. Many efforts have been made to improve the performance of candidate core materials [1, 2]. Cold working has been introduced for inhibiting the void swelling of austenitic stainless steels [3], although it accelerates structural change at elevated temperatures [4, 5]. The addition of titanium to type 316 stainless steel leads to improved swelling resistance [6]. However, data on the effect of titanium addition on the creep behaviour of cold-worked austenitic steels are lacking. Thig letter describes the effect of titanium content on the creep strength of cold-worked austenitic stainless steel similar to type 316. The base composition used was 0.07C15Cr-15Ni-2.5Mo-0.006B austenitic steel and the titanium content was varied at seven levels ranging from 0 to 1 wt %. The results of chemical analyses of the test steels are shown in Table I. They were melted in a vacuum-induction furnace and cast into 10 kg ingots. The ingots were hot-forged and hotrolled to plate of 8 mm thickness and 30 mm width. After solution-treating at 1373 K and cold rolling to produce a 60% thickness reduction, the plates were solution-treated at 1373 K and water-quenched. The grain size of the plates was approximately ASTM No.7 8. Cold working of 20% was introduced in the solution-treated plates. The plates were machined to flat specimens of 2 mm thickness and with a gauge section 6 mm wide and 25 mm long. Using the flat specimens creep-rupture tests were performed at 973 K and the microstructures were examined by transmission electron microscopy. Stress versus time to rupture curves at 973 K of the 20% cold-worked and the solution-treated specimens are shown in Figs 1 and 2, respectively. The rupture strength of both specimens increased with titanium content up to about 0.3 wt%, but decreased with increasing titanium content in the specimens containing more than 0.5 wt%Ti. The slope of stress versus time to rupture curve of the 20% cold-worked specimens containing about 0.2wt%Ti was rather gentler than that of the titanium-free specimen while, in the solution-treated specimens, the slope of the specimen with about