Quenching is a critical step in the formation of stress in tempered glass. The basic requirement for glass quenching is to cool the glass down at a predetermined rate uniformly for even stress in glass.
The cooling rate of air quenching depends on many factors in the process such as air pressure, temperature, and air flow, as well as the gap from the air nozzle to the glass surface. In general, air pressure and the gap from the nozzle to the surface are controllable factors in the production.
Stress in glass gets released when it’s heated. At the start of quenching, the glass sheet begins to cool down from the surface at a preset speed, producing a center-to-surface temperature gradient. While the surface is contracting, the volume in the center part remains basically unchanged at a comparatively high temperature. As a result, the surface layer of the glass creates temporary tensile stresses while inner layer produces a temporary compressive stress and the center portion is left with stress relaxation. The glass at this stage is quite easy to break up in case there is any defect on the surface. Not until the residual stress is completely released, the restructuring of stress it causes often leads to the tempered glass cracking at its early stage. The reason for this could be the heating temperature not being high enough, the heating time being too short, or uneven heating.
As the inner layer cooling process continues, the surface temperature drops below glass transition temperature. The surface layer hardening stops from contracting while the contracting process continues on the inner layer until it also reaches the glass transition temperature, resulting in compressive stress by the surface layer and tensile stress in the inner layer. At this point, even though the temperature gradient is eliminated, the stress remains inside the glass sheet and becomes permanent.