Increasing long-term strength of Zr-1%Nb alloy by diffusive hardening of near-surface layer

Abstract

Zirconium alloys are widely used as structural material in nuclear industry for fabrication of nuclear fuel claddings (NFC). The operational reliability of these structures is an important element of reliability of the entire fuel cycle, given that the structures operate at elevated temperatures. The development of finishing thermochemical treatment modes of Zr-1%Nb alloy NFC is an important and actual task in the field of materials science of zirconium and its alloys. The aim of the work is to determine the influence of thermal treatment in various controlled gas media (vacuum, oxygen-containing, nitrogen-containing) on the long-term strength of the Zr-1%Nb alloy samples at 380 °C in air. The experimental material is 3 mm ring-samples with a 0.5 mm depth V-shaped concentrator, which are cut from NFC tubes of the Zr-1%Nb alloy of Ukrainian production. In order to test long-term strength of Zr-1%Nb alloy ringsamples at T=380 °C in air a multipositional assembly was used. The assembly was designed and manufactured in the Karpenko Physico-Mechanical institute of the National Academy of Sciences of Ukraine. Special grips were developed and produced for mounting ring-samples on a long-term strength test. Thermochemical treatment was conducted in a rarefied (P=1·10–4 mm Hg) gas medium (vacuum), as well as in rarefied oxygen- and nitrogen-containing gas media at temperature T=580...650 °C and at the exposure =3...10 h. The influence of oxidation, nitration and vacuum treatment on thickness and microhardness of surface layers of zirconium ring-samples, as well as resistance to delayed failure under a static long-term load are studied experimentally. Also it has been experimentally established that the treatment of zirconium ring-samples of the Zr-1%Nb alloy in oxygen-containing and nitrogen-containing gas media with respect to vacuum annealing raises the destructive stresses with a prolonged static load at a temperature of 380 °C in air. The differences in fracture surface of nearsurface layer of the Zr-1%Nb alloy ring-samples, depending on the processing mode, are shown in the paper.

Keywords

• zirconium
• thermochemical treatment
• oxygen
• nitrogen
• near-surface layer
• long-term strength