Toughened or tempered glass is a type of safety glass processed by controlled thermal treatments to increase its strength compared with normal glass. Tempering puts the outer surfaces into compression and the inner surfaces into tension. Such stresses cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards as plate glass (a.k.a. annealed glass) creates. The granular chunks are less likely to cause injury. As a result of its safety and strength, toughened glass is used in a variety of demanding applications, including windows, shower doors, architectural glass doors and tables, refrigerator trays, as a component of bulletproof glass, for diving masks, and various types of plates and cookware.
Toughened glass is physically and thermally stronger than regular glass. The greater contraction of the inner layer during manufacturing induces compressive stresses in the surface of the glass balanced by tensile stresses in the body of the glass. For glass to be considered toughened, this compressive stress on the surface of the glass should be a minimum of 69 megapascals (10,000Â psi). For it to be considered safety glass, the surface compressive stress should exceed 100 megapascals (15,000Â psi). The greater the surface stress, the smaller the glass particles will be when broken. It is this compressive stress that gives the toughened glass increased strength. This is because any surface flaws tend to be pressed closed by the retained compressive forces, while the core layer remains relatively free of the defects which could cause a crack to begin. Any cutting or grinding must be done prior to tempering. Cutting, grinding, and sharp impacts after tempering will cause the glass to fracture.
The strain pattern resulting from tempering can be observed with polarized light or by using a pair of polarizing sun glasses.
Toughened glass is also used in buildings for unframed assemblies (such as frameless glass doors), structurally loaded applications, and any other application that would become dangerous in the event of human impact. Tempered and heat strengthened glass can be three to seven times stronger than annealed glass. Tempered glass is used in many applications like skylights, doorways and stairways, large windows, windows which extend close to floor level, sliding doors, elevators, fire department access panels, and near swimming pools.
Toughened glass can be made from annealed glass via a thermal tempering process. The glass is placed onto a roller table, taking it through a furnace that heats it well above its transition temperature of 564 °C (1,047 °F) to around 620 °C (1,148 °F). The glass is then rapidly cooled with forced air drafts while the inner portion remains free to flow for a short time.
The term "toughened glass" is generally used to describe fully tempered glass but is sometimes used to describe heat-strengthened glass as both types undergo a thermal "toughening" process.
There are two main types of heat-treated glass: heat-strengthened and fully tempered. Heat-strengthened glass is twice as strong as annealed glass while fully tempered glass has typically four to six times the strength of annealed glass and withstands heating in microwave ovens. The difference is the residual stress in the edge and glass surface. Fully tempered glass in the US is generally rated above 65 megapascals (9,400 psi) in pressure-resistance, while heat-strengthened glass is between 40 and 55 megapascals (5,800 and 8,000 psi). The tempering process does not change the stiffness of the glass. Annealed glass undergoes a similar deflection compared to tempered glass under the same load, but tempered glass can take a higher load and, therefore, deflects further before breaking.
STRONG Tempered glass is 4-5 times stronger than normal annealed glass.
HIGH THERMAL SHOCK STRENGTH Ability to withstand high temperature variations upto 250°c.
WIND LOADS Used in building facades as it have the ability to withstand heavy wind loads.
SAFE Breaks into small, relatively harmless fragments. This reduces the likelihood of injury as there are no jagged edges or sharp corners.
Toughened glass must be cut to size or pressed to shape before toughening, and cannot be re-worked once toughened. Polishing the edges or drilling holes in the glass is carried out before the toughening process starts. Because of the balanced stresses in the glass, damage to any portion will eventually result in the glass shattering into thumbnail-sized pieces. The glass is most susceptible to breakage due to damage to the edge of the glass, where the tensile stress is the greatest, but shattering can also occur in the event of a hard impact in the middle of the glass pane or if the impact is concentrated (for example, striking the glass with a hardened point). Using toughened glass can pose a security risk in some situations because of the tendency of the glass to shatter completely upon hard impact rather than leaving shards in the window frame.
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| Properties | Tempered Glass | Heat strengthened glass | Annealed Glass |
|---|---|---|---|
| Thermal stress | Upto 250°C | Upto 130°C | Upto 50°C |
| Strength | 4 times of annealed | 2 times of annealed | Not strong |
| Bending strength | 120 – 200 N/mm² | 60 – 100 N/mm² | 40 N/mm² |
| Design stress | 50 Mpa | 27Â Mpa | 17Â Mpa |
| Surface compression | >10000 psi (69 Mpa) | 3500-7500psi (24-52 Mpa) | Upto 2800 psi (19 MPa) |
| Safety | Approved as safety glazing for most applications | Not approved as safety glazing | Not approved as safety glazing |
| Site alterations | Not possible | Not possible | Possible |
| Flatness | Slight bow and corrugation | Bow and corrugation lower than tempered glass | Very flat |
| Optics | Reasonably good | Good. Comparatively better than tempered glass | Very good |
| Lamination | Min 1.52 PVB | Min 1.52 PVB | Min 0.38mm PVB |
| NiS breakage | Prone | Extremely rare possibility | Non existent |
| Heat Soaking | Recommended | Not necessary | Not required |
- Glass Doors
- Glass for Shower Cubicals
- Glass for Facades
- Glass for Windows
- Glass for Stairs
- Glass Partitions
- Glass for Railings
- Glass Walkways
- Glass for Squash Courts
- Glass for Lighting Fixtures
- Glass for Kitchen Appliances
- Glass for Furnitures
- Glass for Automobiles