Working Principle of Physical Tempering
Heating Stage: Ordinary annealed glass is uniformly heated to 620–680°C-approaching the glass softening temperature-to relieve initial internal stresses.
Rapid Cooling (Quenching) Stage: High-pressure cold air is blasted onto both sides of the glass, causing the surface layer to cool, harden, and set rapidly, while the glass interior remains in a high-temperature, plastic state.
Stress Formation Stage: As the inner layer of the glass subsequently cools and contracts naturally, the already-solidified surface layer constrains this contraction. This results in the formation of uniform compressive stress on the surface and a balancing tensile stress layer in the interior.
Strength Enhancement Logic: When the glass is subjected to external force, the force must first counteract the pre-existing surface compressive stress, thereby significantly increasing load-bearing capacity; its flexural and impact strengths can reach 4–5 times that of ordinary glass.
Working Principle of Chemical Tempering
Ion Exchange Process: Glass is immersed in a molten alkali salt solution at approximately 400°C. At temperatures below the glass transition temperature, smaller sodium ions (or lithium ions) in the glass surface layer are exchanged for larger potassium ions from the salt solution.
Compressive Stress Generation Logic: The embedding of larger-volume potassium ions into the glass surface creates a crowding effect, forming a deep compressive stress layer that enhances the glass's strength.
Characteristic Differences: The compressive stress layer formed by chemical tempering is relatively thin, and the balancing internal tensile stress is minimal. Consequently, upon breakage, the glass does not shatter into the multitude of small fragments typical of physically tempered glass; this method is better suited for strengthening thin glass (2–4 mm).