Stability of the Transition Zones in a Steel–Vanadium Alloy–Steel Sandwich after Thermomechanical Treatment

Abstract

A priority in atomic power today is to develop a new material for fuel-rod casings in fast-neutron reactors. A radiation- and corrosion-resistant three-layer composite based on vanadium alloy and stainless steel has been developed. This composite potentially meets the operational requirements on fuel-rod casings in very challenging operating conditions (high temperatures, radiation, and aggressive media). The performance of this material depends on the quality of the joint between the three layers, which is determined by the preliminary deformation and heat treatment. In the present work, the influence of tempering on the chemical composition, structure, and strength of the joint between the vanadium alloy and steel in the sandwich obtained by hot pressing a three-layer pipe blank at 1100°C is studied. The components of the pipe are 20Kh13 (Russian standard) steel for the external layers and V–4Ti–4Cr vanadium alloy in the core. The structure and chemical composition at the interfaces is investigated by optical and electronic microscopy, with X-ray spectral analysis. The strength of the steel–alloy bond is assessed in compressive tests of an annular three-layer sample with a cut; acoustic-emission measurements are employed. Pressing is found to form a transition zone of thickness 10–15 μm between the vanadium alloy and the steel, which is characterized by diffusional interaction and has a variable chemical composition. This zone consists of a series of solid solutions, without the deposition of brittle phases, and consequently the junction between the layers is strong. No pores, peeling, or defect are observed at the steel–alloy junction. However, in compressive tests of semiannular three-layer samples with a cut after hot pressing, a crack is formed in the steel layer at the tip of the cut. Annealing at 800°C improves the transition zone by increasing the thickness corresponding to diffusional interaction. Consequently, in mechanical tests, the sandwich behaves as a monolithic material, without cracking or peeling between the steel and the vanadium alloy.