To characterize the cross-scale coupling between macroscopic deformation and microscopic rough-surface contact in metal O-rings, this study proposes a macro–micro contact analysis and leakage flow regime identification method for metal O-ring sealing interfaces. Finite element analysis was employed at the macroscopic scale to obtain the sealing contact width and pressure distribution, while microscopic rough-surface morphology was characterized using fractal theory. Based on asperity contact analysis, the equivalent leakage channel height was determined, and the Knudsen number (Kn) was introduced to identify the fluid flow regime within micro-gaps. The results show that the O-ring cross-section flattens into an elliptical shape under compression, while the contact pressure exhibits a saddle-shaped distribution, and micro-gap leakage channels remain present. As the compression ratio increased from 5% to 25%, the sealing contact width and contact pressure increased, whereas the leakage gap height decreased significantly, resulting in an increase in the Kn values from 0.00176 to 0.045, 0.07, 0.155, and 2.78. Consequently, the flow regime evolved from continuum flow through the transition regime to molecular flow. The findings reveal the cross-scale coupling mechanism between macro–micro contact behavior and leakage flow regime evolution, providing practical guidance for determining the preload level and selecting appropriate leakage models in the engineering design of metal O-ring seals.

Lead-based alloy has received widespread attention as a coolant in nuclear reactors. However, there is limited research on pure lead after solidification. In this study, a systematic investigation was conducted on the long-term evolution of the microstructure and physical properties of pure lead samples solidified under different cooling rates, with a comparative analysis against of lead–bismuth eutectic (LBE). Microscopic detection (using optical and electron microscopes), density measurement, and compressive mechanical testing were carried out. The study results show that during the long-term evolution process after solidification (at room temperature of 27 °C), pure lead samples spontaneously undergo recovery and recrystallization, with larger grain size and more uniform microstructure. The density of samples remains within a stable range. The yield strength of samples after solidification will gradually decrease over time. For example, after 180 days of evolution, the yield strength of the rapidly cooled sample (10 K/min) decreased from 4.879 MPa to 3.766 MPa.