Chemical strengthening and heat strengthening/tempering are both processes for increasing the strength and durability of soda lime and other glass substrates.
Chemical Strengthening
![Heat Strengthened Glass Break Pattern](/media/chemically-strengthened-glass-break-pattern.jpg" "Heat Strengthened Glass Break Pattern")
Chemical strengthening is best suited for thin display applications (3mm and under, though can be up to 6mm) as well as applications where optical distortion must be kept to a minimum. The chemical strengthening process toughens soda lime and other glass substrates through a sodium and potassium ion-exchange process in a salt bath. The process imparts a higher strength, durability, and MOR (Modulus of Rupture, which is resistance to breaking in bending) as well as maintaining higher surface quality (80/50 and up to 60/40 for smaller applications). Glass can be strengthened from 8 to 16 hours imparting an MOR of 165 Mpa (24 Kpsi) and case depth of 16-19 um for the 8 hour cycle and an MOR of 220 Mpa (32 Kpsi) and case depth of 22-27 um for the 16 hour cycle.
Heat Strengthening
Heat strengthening and full tempering require a 3mm or thicker glass substrate and generally can only maintain 120/80 surface quality while imparting a higher thermal strength and a safety dicing break pattern (when broken dices into many small pieces for safety) for fully tempered parts. The heat strengthening process is utilized when a full temper is not possible due to thickness, size, or low thermal expansion rate of the substrate. Heat strengthened glass is generally twice as strong as standard annealed glass, while full tempered glass is typically four to six time stronger than annealed glass. Often of high importance is that full tempered glass imparts the safety dicing pattern when broken. The quality of heat treated parts (H/S or F/T) such as cosmetics and warping, as also the specifications/ requirements of machined features such as holes being a certain distance away from the edges of the glass is specified in ASTM C-1048-04.