Swelling behavior and stress analysis of hematite pellets: Predicting failure through distortion energy criteria

This study investigates the reduction swelling behavior and stress distribution in hematite pellets under varying hydrogen atmospheres (60–100 % H₂), temperatures (1073–1273 K), and pressures, employing distortion energy theory (DE) and a swelling model. The results indicate that higher H₂ increase the removal rates but significantly intensify the swelling due to rapid phase changes, porosity evolution, and the formation of a metallic iron whisker shell. The most severe swelling is observed at 1273 K and 100 % H₂, driven by intensified reduction kinetics and structural instabilities. Conversely, at 1073 K and 60 % H₂, slower reduction rates result in reduced swelling. Swelling consistently tends to increase with higher temperatures, H₂ concentrations, and pressures, with gas composition critically influencing the swelling index and stress distribution. Lower hydrogen concentrations reduce radial and tangential stress magnitudes, enhancing mechanical stability. Crack propagation is observed at 1273 K across all hydrogen levels, while at 1173 K, cracks occur only in 100 % H₂, and at 1073 K, no cracks form at any concentration, demonstrating the interplay between temperature and hydrogen concentration in influencing material integrity. Such findings underlined that the optimal operating conditions should be selected to minimize swelling and mechanical failure under the reduction process.