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Tempered glass


Tempered (toughened) glass is up to six times stronger than annealed glass. When broken, it shatters into many tiny relatively harmless fragments which can prevent major injuries. The thermal strength of toughened glass allows it to withstand up to 250 degrees centigrade variance from ambeient temperature. This type of glass is intended for glass facades, sliding doors, building entrances, bath and shower enclosures, kitchen splashbacks, and colourbacked painted walls claddings, and other uses requiring superior strength and safety.

Production

There are two different methods used to produce tempered glass

  • Heat treating: Where the annealed glass is subjected to a special heat-treatment in which it is heated to about 600°C and afterwards cooled. If it is cooled rapidly, the glass is up to six times stronger then annealed ordinary glass and it breaks into many small fragments (fully-tempered). If it is cooled slowly, the glass is twice as strong as annealed glass and the fragments of the broken glass are linear and more likely to remain in a frame.
  • Chemical Strengthening: The glass is covered by a chemical solution which produces a higher mechanical resistance. Chemically-strengthened glass has similar properties to thermal-treated glass. The product is not generally used for window glass, but more commonly seen in industries where thin, strong glass is needed.

DEFINITIONS

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." Or "toughened" glass. The heat treating process produces highly desirable conditions of induced compression (described below) which result in additional strength, resistance to thermal stress, and impact resistance.

Heat-treated glasses are classified as either fully tempered 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-strengthened 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.

MANUFACTURING PROCESSES

In the "heat-treatment" process the key procedure is application of a rapid air quench immediately upon withdrawal of hot (approx. 620° C) 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°C.

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.

STRENGTH

Under wind pressure, tempered glass is approximately four to six times as strong as annealed glass. It resists breakage by small missiles travelling approximately twice as fast as missiles which break annealed glass. Tempered glass is also able to resist temperature differences of up to 250°C 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

SAFETY

Fully tempered glass is used in many applications because of its safety characteristics. Safety comes from strength and from a unique fracture pattern and thermal resistance. 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 long sharp shards.

Fully tempered glass is a safety glazing material when manufactured to meet the requirements of the AS/NZS 2188 , where the codes require safety glazing material where the glazing might reasonably be exposed to human impact. This includes doors, bath and shower enclosures, side lights, and certain windows. Applicable building codes should be checked for specific information and requirements.

USES FOR TEMPERED GLASS

Fully tempered glass is used traditionally in place of other glass products in applications requiring increased strength, thermal resistance, and reduced likelihood of injury in the event of breakage. The building industry, motor vehicle industry and certain manufacturing industries find tempered glass is effective and economical in a wide range of applications.

Fully tempered glass can satisfy federal, state and local building code requirements for safety glazing in such applications as doors, side lights, shower and bath enclosures, and interior partitions. It is also used in storm doors, patio-door assemblies, and escalator and stairway balustrades. As a glazing product it is used in windows and in spandrel areas (for wind pressure, small missile impact and thermal stress resistance). Special building applications include sloped glazing, racquetball courts, skylights (see below), and solar panels. Any conditions or requirements imposed in the applicable safety glazing laws and building codes limiting such special uses should be determined prior to glazing.

The domestic motor vehicle industry employs tempered glass as side and rear windows in automobiles, trucks, and multi-purpose vehicles. Manufacturing industries use tempered glass in refrigerators, furniture, ovens, and shelving.

Tempered glass should not be used where building codes require wired glass for fire-spread resistance. Tempered glass should not be used, alone, where the objective is to provide security against forced entry or bullet passage. Combinations of annealed, tempered glass and high performance acrylics can be effective barriers against forced entry and bullet impact, if properly designed and constructed..

TEMPERED GLASS IN SLOPED GLAZING AND SKYLIGHTS

Because of its high resistance to thermal stresses and small missile impact, tempered glass is used in skylights and sloped glazing. On rare occasions when tempered glass in these applications fails, it may fail completely from the exposed glass area. Individual fragments from tempered glass are relatively small and harmless. A number of these fragments may be loosely joined and fall in this manner. Such pieces do not have the sharp edges normally associated with broken glass but may have significant weight. Some building codes may require the use of screens under skylights or laminated toughened glass to be used. The use of screens may also be dictated by considering the risk of breakage and the resulting consequences.

HANDLING AND INSTALLATION

Tempered glass should receive the same care as annealed glass. Unfortunately, familiarity with the greatly improved strength of tempered glass may mislead people to exert less care in handling it. Careless handling and improper installation sometimes will produce edge damage. Delayed breakage can ensue when edge-damaged tempered glass is subjected to a moderate thermal of mechanical stress. Full penetration of the compression layer will likely produce instantaneous total fragmentation of tempered glass. Hence, tempered glass cannot be cut or modified following heat treatment.

IMPERFECTIONS

Inclusions in glass originate from impurities in the batch or cullet, or are combined impurities from furnace refractories. Common forms of inclusions include aluminous stones, iron stones, and silicon. Nickel sulphide stones are uncommon, microscopic defects in glass, and may cause breakage. Delayed breakage may occur when a nickel sulfide stone is present near the center of the glass thickness.

The tempering process rarely introduces imperfections into glass. The basic glass may contain bubbles, vents, chips, and inclusions which, if accepted or not revealed by inspection before tempering can cause breakage in the initial heating or final quench operations. If inclusions are not eliminated by self destruction during the tempering process, in rare cases they may lead to failure at a later time.

VISUAL APPEARANCE

Tempered glass possesses the basic optical qualities of annealed glass. The induced stress condition sometimes produces a slight bow in tempered glass lights. Tempered glass that has been manufactured in a vertical tempering oven contains small surface depressions resembling dimples along one edge. These marks are caused by the pointed metal tongs which support the glass during its passage through the oven. Glass which is passed horizontally through an oven may contain a very slight surface wave caused by contact with the rollers. The waviness can sometimes be detected when viewing reflected images from a large distance. Finally, the air quench nozzles discharge air in a fixed, reciprocating or rotating motion. The area of air quench can be seen through polarized glass as arrays of irridescent spots or lines.

Under some lighting conditions these patterns can be seen in ordinary light.
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