Mechanical Degradation and Tempering-Induced Recovery of Nanobainitic Steel After Moderate Sub-Zero Exposure

The exploitation of advanced high-strength steels in cold climates requires a deep understanding of their structural stability and mechanical reliability. This study investigates the mechanical response of a 0.45C-1.57Si-2.61Mn (wt.%) steel with nanobainite microstructure to moderate sub-zero exposure (SZE) followed by stress-relief tempering. It was found that SZE at –25 °C and –50 °C induces a progressive degradation of tensile properties and impact toughness that persists after rewarming to room temperature. This deterioration is primarily driven by the accumulation of residual stresses due to thermal expansion mismatch between phases, with only minor contributions from athermal martensitic transformation of retained austenite. Notably, stress-relief tempering at 220 °C effectively restores the tensile performance and doubles the impact toughness compared to the as-austempered condition. Despite Mn and Si segregation which caused a scatter in retained austenite content (13.6–21.5 vol.%), the austenite remains stable throughout the SZE and tempering cycles. These results identify a critical threshold (between 0 °C and –25 °C) for SZE-induced degradation in nanobainitic steels and demonstrate that stress-relief tempering is essential for enhancing their performance in cold-climate applications.