Karaganda State Industrial University

Zirconium and its alloys are widely used in nuclear power engineering due to their favorable physical and mechanical properties and their low thermal-neutron absorption cross-section. Their high corrosion resistance in aqueous and steam environments at elevated temperatures is essential for the reliable operation of fuel assemblies and is associated with the formation of a stable, compact ZrO2 oxide layer. However, under reactor conditions, the presence of hydrogen, iodine and other fission products can reduce corrosion resistance, making detailed corrosion assessment necessary. Manufacturing technology, alongside alloy composition, also plays a decisive role in determining corrosion behavior. This study presents corrosion test results for a Zr-1%Nb alloy processed under thermomechanical conditions corresponding to rolling in a special type of three-roll skew rolling–Radial-Shear Rolling (RSR). The applied rolling technology ensured the formation of a pronounced ultrafine-grained (UFG) structure in the near-surface layers, with an average grain size below 0.6 µm. EBSD and TEM observations revealed a largely equiaxed microstructure with refined grains and increased grain boundary density. The corrosion testing was performed in high-temperature steam vessels at 400 °C and 10.3 MPa for 72, 336, 720 and 1440 h. The results demonstrate that RSR processing is an efficient alternative to conventional multi-pass normal bar rolling with vacuum heat treatments, allowing a significant reduction in processing steps and eliminating the need for expensive tooling and intermediate thermal or chemical treatments. Bars manufactured using this method meet the ASTM B351 requirements. The specific weight gain did not exceed 22 mg/dm2 after 72 h and 34.5 mg/dm2 after 336 h. After 1440 h, the samples exhibited a continuous, uniform dark-grey oxide layer with an average thickness below 5.3 µm.

In this study, finite-element modeling of the microstructure evolution of the combined drawing process with cryogenic cooling by JMAK and Cellular Automata methods was carried out. The use of different methods is due to the fundamental difference in the results obtained—in the JMAK method, it is possible to obtain only grain size data, but this is calculated for the entire workpiece volume. In the Cellular Automata method, the calculation is performed only at predefined points, but in this case the user receives information about the size and shape of the grains. An analysis of the initial microstructure of 25 µm showed that the most optimal option is a reduced drawing speed and the absence of intermediate heating—in both considered blanks, a grain size of 0.5 µm approximately was obtained on the surface. At the same time, variants with an increased deformation rate also have fairly good grain-grinding results up to 0.7 µm. At the same time, it was found that the use of intermediate heating to ambient temperature in the proposed thermomechanical processing scheme will lead to a higher grain size gradient along the wire section.