In the production of flat glass, the molten silica-based mix is cooled slowly under carefully controlled conditions. This annealing procedure removes undesirable stresses from the glass. Cooling occurs in an annealing "lehr"; hence, the glass is termed "annealed" or "ordinary" glass. Annealed glass which has been heated to a temperature near its softening point and forced to cool rapidly under carefully controlled conditions is described as "heat-treated glass." The heat treating process produces highly desirable conditions of induced stress (described below) which result in additional strength, resistance to thermal stress, and impact resistance. Heat-treated glasses are classified as either fully tempered glass or heat strengthened. According to Federal Specification DD-G-1403B, fully tempered glass must have a surface compression of 10,000 psi or more or an edge compression of 9,700 psi or more. Heat-strength glass must have a surface compression between 3,500 and 10,000 psi, or an edge compression between 5,500 and 9,700 psi. The fracture characteristics of heat- strengthened glass vary widely from very much like annealed glass near the 3,500 psi level to similar to fully tempered glass at the 10,000 psi level.
HEAT TREATMENT PRINCIPLE
Glass can fracture when its surfaces or edges are placed into tension. Under these conditions inherent surface or edge fissures may propagate into visible cracks.
The basic principle employed in the heat treating process is to create an initial condition of surface and edge compression. This condition is achieved by first heating the glass, then cooling the surfaces rapidly. This leaves the center glass thickness relatively hot compared to the surfaces. As the center thickness then cools, it forces the surfaces and edges into compression. Wind pressure, missile impact, thermal stresses or other applied loads must first overcome this compression before there is any possibility of fracture.
In the "heat-treatment" process the key procedure is application of a rapid air quench immediately upon withdrawal of hot (approx. 1200 ° F) glass from the "tempering furnace." The immediate and sustained application of an air quench produces the temper. As air direction against hot glass from arrays of fixed, reciprocation or rotating blast nozzles, it is important to extract heat uniformly from both surfaces (uneven heat extraction may produce bow or warp) and to sustain the quench long enough to prevent reheating of the glass surfaces from the still-hot glass core. A quenched condition becomes stable when the glass is reduced to a temperature of approximately 400-600 ° F.
There are two principal manufacturing methods for producing heat-treated glass. One process heat treats the glass in a horizontal position while the second method moves the glass through the furnace in a vertical position with each light of glass held by metal tongs.
Under wind pressure, tempered glass is approximately four times as strong as annealed glass. It resists breakage by small missiles traveling approximately twice as fast as missiles which break annealed glass. Tempered glass is also able to resist temperature differences (200 ° F - 300 ° F) which would cause annealed glass to crack.
Annealed Glass Tempered Glass Typical Breaking Stress (large light 60 sec. load) 6,000 psi 24,000 psi Typical Impact Velocity Causing Fracture (1/4" light 5 gm missile, impact normal to surface 30 ft/sec 60 ft/sec
Fully tempered glass is used in many applications because of its safety characteristics. Safety comes from strength and from a unique fracture pattern. Strength, which effectively resists wind pressure and impact, provides safety in many applications. When fully tempered glass breaks the glass fractures into small, relatively harmless fragments. This phenomenon called "dicing," markedly reduces the likelihood of injury to people as there are no jagged edges or sharp shards.
Fully tempered glass is a safety glazing material when manufactured to meet the requirements of the ANSI Z97.1 Standard and Federal Standard CPSC 16 CFR 1201. Federal Standard CPSC 16 CFR 1201, as well as state and local codes, require safety glazing material where the glazing might reasonably be exposed to human impact. This includes doors, tub and shower enclosures, side lights, and certain windows. Applicable building codes should be checked for specific information and requirements.
Toughened or tempered glass is a type of safety glass processed by controlled thermal or chemical treatments to increase its strength compared with normal glass. Tempering creates balanced internal stresses which cause the glass, when broken, to crumble into small granular chunks instead of splintering into jagged shards. The granular chunks are less likely to cause injury.
As a result of its safety and strength, tempered glass is used in a variety of demanding applications, including passenger vehicle 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. In the United States, federal safety laws require that window glass be tempered if each of the following criteria are met: sill height within 18 in (0.457 m) of the floor, top edge greater than 36 in (0.914 m) from the floor, area greater than 9 ft² (0.836 m²), and horizontal distance to nearest walking surface of less than 36 in (0.914 m).
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 MPa. For it to be considered safety glass, the surface compressive stress should exceed 100 MPa. 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, sharp impacts and sometimes even scratches after tempering will cause the glass to fracture. The glass solidified by dropping into water, known as "Prince Rupert's Drops", which will shatter when their "tails" are broken, are extreme examples of the effects of internal tension.
The strain pattern resulting from tempering can be observed with polarized light or by using a pair of polarizing sun glasses.
Toughened glass is used when strength, thermal resistance and safety are important considerations. The most commonly encountered tempered glass is that used for side and rear windows in automobiles. It is used for its characteristic of shattering into small cubes rather than large shards and is sometimes referred to as safety glass in this context. (The windscreen or windshield is instead made of laminated glass, which will not shatter when broken.)
Toughened glass is also used in buildings for unframed assemblies (such as frameless doors), structurally loaded applications, and any other application that would become dangerous in the event of human impact.
Rim-tempered indicates a limited area such as the rim of the glass or plate is tempered and is popular in food service.
Cooking and baking
Some forms of tempered glass are used for cooking and baking. Manufacturers include Pyrex, Corelle, and Arc International
Toughened glass is 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 above its annealing point of about 720 °C. The glass is then rapidly cooled with forced air drafts while the inner portion remains free to flow for a short time.
An alternative chemical toughening process involves forcing a surface layer of glass at least 0.1mm thick into compression by ion exchange of the sodium ions in the glass surface with the 30% larger potassium ions, by immersion of the glass into a bath of molten potassium nitrate. Chemical toughening results in increased toughness compared with thermal toughening, and can be applied to glass objects of complex shape
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 is 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 MPa (9427 psi) in pressure-resistance while heat strengthened glass is between 40 and 55 megapascals (5801 and 7977 psi respectively).
It is important to note that the tempering process does not change the stiffness of the glass. Annealed glass deflects the same amount as tempered glass under the same load, all else being equal. But tempered glass will take a larger load, and therefore deflect further at break.
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 the glass 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 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.
The surface of tempered glass does exhibit surface waves caused by contact with the rollers. This waviness is a significant problem in manufacturing of thin film solar cells
The first patent on tempered glass was held by chemist Rudolph A. Seiden, born in 1900 in Austria.
Though the underlying mechanism was not known at the time, the effects of "tempering" glass have been known for centuries. In about 1660, Prince Rupert of the Rhine brought the discovery of what are now known as "Prince Rupert's Drops" to the attention of King Charles II. These are teardrop shaped bits of glass which are produced by allowing a molten drop of glass to fall into a bucket of water, thereby rapidly cooling it. They have the curious ability to withstand a blow from a hammer on the bulbous end without breaking, but the drops will disintegrate explosively if the tail end is even slightly damaged. The teardrops were often used by the King as a practical joke
Why Tempered Glass?
Tempered glass is a type of safety glass regularly used in applications in which standard glass could pose a potential danger. Tempered glass is four to five times stronger than standard glass and does not break into large shards when broken. Tempered glass is manufactured through a process of extreme heating and rapid cooling, making it harder than normal glass.
The brittle nature of tempered glass causes it to shatter into small oval-shaped pebbles when broken. This eliminates the danger of sharp edges. Due to this property, along with its strength, tempered glass is often referred to as safety glass. The thermal process that cures tempered glass also makes it heat resistant. Tempered glass can be used in any situation where there is a danger of the glass breaking due to impact or extreme heat. Here at woodstove-fireplaceglass.com, we have the capability of tempering glass in many thicknesses and sizes. Availability of one of the largest tempering furnaces in Il, gives us the flexibility and speed that our customers have come to expect at wholesale pricing.
•Stronger Tempered glass is 5-8 times as impact resistant as ordinary glass. The bending strength of tempered glass is 4-5 times as powerful as ordinary glass. Tempered glass can stand higher static pressure loads than regular glass.
•Thermally Stable (Less Heat Sensitive) Thermal stability is the property that allows glass to resist temperature differences. It is measured by the temperature difference that would cause the glass to crack. The thermal stability of tempered glass is 1.5-2 times better than annealed (regular) glass. This means while annealed glass would crack at 100 degrees Celsius, tempered glass can withstand temperatures of up to 320 degrees Celsius before cracking. It’s this quality that makes tempered glass, perfect for the outdoor heat.
•Safer Tempered glass has many applications because of its safety characteristics. The safety of tempered glass comes from strength and its unique fracture pattern. When tempered glass breaks, it breaks into small, relatively harmless blunt glass fragments. This is called “dicing,” and reduces the likelihood of injury since there are no dangerous jagged edges or sharp shards as is the case with ordinary glass.
Annealed glass is hardened glass, but it lacks the safety features of tempered glass. Should annealed glass suffer breakage, it will break into completely separated small pieces. Annealed glass is ideal as a protective covering on existing furniture, and for indoor use
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