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Heat treated glass speaks to what is treated glass and why use treated glass both the process and manufacture of this glass, as well as tempered glass and heat strengthened glass.
This glass is produced by reheating annealed glass in an oven to almost 1150 degrees Fahrenheit (620 C) and then cooling by quenching both of its surfaces rapidly with blasts of air while its core cools much more slowly.
This process induces permanent compressive stresses in the edges and faces of the glass and tensile stresses in the core. The resulting glass is stronger in bending than annealed glass and more resistant to thermal stress and impact. These properties make heat treated glass useful for windows exposed to heavy wind pressures, impact or intense heat and cold.
By adjusting the quenching process, greater or lesser degrees of residual stress may be introduced into the glass, producing products referred as either “tempered” or heat strengthened glass.
Tempered glass has higher residual stresses than heat strengthened glass and is about four times as strong as bending as annealed glass. If it does break, the sudden release of the internal stresses reduces tempered glass instantaneously to small, square edged granules rather than long, sharp edged granules, rather than long sharp edged strands.
This characteristic combined with its high strength, qualifies it for use as safety glazing that is, in situations of possible occupant impact. Tempered glass is also used for all glass doors that have no frame at all, for whole walls of squash and handball courts, for hockey rink enclosures and for basketball backboards.
Tempered glass is more costly than annealed glass. It often has noticeable optical distortions created by the tempering process. In addition all cutting to size, drilling and edging must be done before the heat treatment of the glass because any such operations before the heat treatment of the glass because any such operations after tempering will release the stresses in the glass and cause it to disintegrate.
Tempered glass is also sometimes referred to as fully tempered glass to distinguish it more clearly from heat strengthened glass.
Heat Strengthened Glass
For many applications, lower cost heat strengthened glass may be used instead of tempered glass. The induced compressive stresses in the surface and edges of heat strengthened glass are about one tird as high as those in fully tempered glass. This is typically 5000 psi compared to 15000 psi for tempered glass (34 MPa to 104 1o4 MPa.
Heat strengthened glass is about twice as strong in bending as annealed glass and is more resistant to thermal stress. It usually has fewer distortions than tempered glass. Its breakage behaviour is more like that of annealed glass than tempered glass. For that reason, it cannot be used where safety glazing is required except in laminated form.