Overview of Tempered Glass:
Tempered glass is made using special heated furnaces, which heat the annealed glass to a uniform temperature of approximately 700ºC. When the glass exits the furnace, it is rapidly cooled. This rapid cooling puts the glass surface into a state of compression while the centre core remains in tension.
Tempering Process: Tempered glass is made by uniformly heating annealed glass to a temperature of up to 700°C and immediately cooling it. The glass first goes through the furnace, carried in by rollers, where it is heated using three modes - conduction, convection and radiation. The rollers conduct the heat, while the coils help in the radiation of heat. The convection effect is brought about by hot air blowing through the nozzles, thus circulating the heat inside the furnace and making sure that the glass is heated evenly on all sides. This super heated glass then undergoes a rapid cooling process by a uniform and simultaneous blast of air on both surfaces.
After tempering, the stress pattern developed by the glass is high, and the mechanical strength of tempered glass is four to five times higher than that of annealed glass.
Mechanical Strength of Tempered Glass = 4 x Mechanical strength of Annealed Glass
When tempered glass is broken, all the stored energy is released at once and the glass breaks into small pieces.
Characteristics of Tempered glass:
Tempered glass is four to five times stronger than annealed glass of the same size and thickness against impact. Tempered glass has higher thermal strength, and it can withstand high temperature changes up to 250°C. Tempered glass is considered as safety glass. It is difficult to break and even in the event of a breakage, disintegrates into small relatively harmless globules. Tempering does not alter the basic characteristics of glass such as light transmission and solar radiant heat properties of normal glass. The glass surface after heat treatment has the same resistance to surface damage as annealed glass. Tempered glass cannot be cut or altered.
One problem that can also exist with tempered glass is spontaneous breakage, where the glass can break for no apparent reason. Spontaneous breakage occurs when nickel sulfide stones increase in size due to repeated heating and cooling.
Properties of Tempered Glass:
Need for Tempered Glass: Tempering increases tensile strength, thermal shock resistance, and safety of normal glass. It also increases its heat withstanding capacity. Because of these properties, tempered glass is preferred in applications where strength, safety and thermal resistance are important considerations.
Strength & Safety Considerations Four to five times stronger than annealed glass Two to three times stronger than heat strengthened glass Fragments into small, relatively harmless pieces, reducing the likelihood of injury.
Typical Breakage Pattern
Heat resistance Withstands both uneven heating and high temperatures due to its superior edge strength compared to normal annealed glass. Because of its high edge strength tempered glass is used in spider and point fixed glazing.
Thermal Breakage: Uneven heating due to direct sunlight makes glass expand and contract at different rates. This difference in the rates of expansion (central part of glass) and contraction (edges of glass) create stresses in glass which may lead to breakage. When temperature difference is below 32°C, the risk of thermal breakage is low, but when the difference becomes more than this, the risk increases significantly.
Thermal Breakage Pattern
Thermal breakage starts at the edge and is perpendicular to the glass edge. Breakage may be single or multiple depending on the built up stress. Several cracks indicate high thermal stresses across the glass pane. A fully tempered glass has significantly higher edge strength to withstand chances of thermal breakage
Application of Tempered Glass:
Tempered glass is specifically designed for use in areas where there is a high risk of contact and breakage. It is not harder or softer than annealed glass but is tougher. Tempering does not make glass more porous nor is tempered glass more prone to scratches than normal glass. Tempered glass offers a wide variety of uses in industries including building, manufacturing, automotive, to name a few.
There is a wide scope for the use of tempered glass in the building industry. It can be used to raise the elemental beauty of exteriors and interiors by using it on facades, showroom shelves, dividers, shower cabins, doors and the like. High resistance to thermal stress and small missile impact makes tempered glass an ideal choice for sloped glazing. The manufacturing industry uses tempered glass in ovens, refrigerators, fireplace screens, and shelves. In the motor vehicle industry, tempered glass is commonly used inside and rear windows.
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