Over the years use of glass has increased tremendously. Modern structural design considerations, aesthetic tastes, energy saving and comfort consciousness demands glasses having increased strength, safety and thermal performances. This situation demands increased use of heat treated glass.
The Principle of Heat Treatment
The basic objective of heat treatment process is to create an initial condition of surface and edge compression. This condition is achieved by first heating the glass, then cooling the surface rapidly. This cases the center of the glass to be hotter than the surface. As the center cools it forces the surfaces and edges into compression. Therefore such glasses can be Brocken only after overcoming this compressed extra surface.
The heat treated glass is fractured into small relatively harmless pieces by the tensile forces released from the glass center core and this phenomenon of breaking glass into small particle is called dicing.
The Process of Heat Treatment
Annealed float glass is cut into the required final size, the edges either seamed or polished and washed. There are two principle manufacturing method for heat treatment. One process heat treat it in a horizontal position while the second method moves the glass through the furnace in a vertical position with each light held by metal tongs. It is then heated to approximately 650 Degree Celsius. Upon withdrawal of hot glass from the tempering furnace, it is rapidly cooled (quenched) by blowing carefully controlled air flow from fixed or reciprocating or rotating blast nozzles uniformly onto both surfaces of glass simultaneously. Higher airflow rates produce fully tempered glass having a surface compression of 700kg/m2 or more and an edge compression of 680 or more and much lower air flow rates produce heat strengthened glass having a surface compression between 250 to 700kg/m2 and an edge compression between 390 to 680 kg/m2..
Response of Glass Properties to Heat treatment
The colour, solar properties, chemical compositions, hardness, specific gravity, coefficient of thermal expansion, softening point, thermal conductivity and stiffness remain unchanged after heat treatment. The only physical properties that changed are improved flexural and tensile strength, and improved resistance to thermal stress and thermal shock. Heat treatment changes the break pattern of glass. Fully tempered glass disintegrates into relatively small harmless pieces.
Type of Heat Treated Glass
Heat treated glass are classified into two types
1. Fully Tempered Glass
2. Heat Strengthened Glass
Fully Tempered Glass
Both fully tempered and Heat Strengthened glass is having the same heat treatment process and only difference is in the method of quenching. Higher airflow rates produce fully tempered glass.
Characteristics of fully tempered glass
Fully tempered glass is approximately 4 times stronger than the normal annealed glass and 6 times thermal resistant to that of annealed glass. It can be used up to a working temperature of 300Degree Celsius. Tempered glass is also able to resist temperature differences of 100 to 150 Degree Celsius which could cause annealed glass to crack. Typically breaks into small particles which are normally non-injurious and can be used as safety glass. It can be laminated with a suitable number of PVB interlayer. It can’t be cut or drilled after tempering. Since tempering improves the structural property. Tempered glass is considered as a structural element and it can be used for frameless glass facades, doors and point support systems and also can be used with any type of windows and curtain walls.
Uses of Tempered Glass
Tempered is using widely in Construction Industry for windows, curtain walls, frameless structural glazing work, sky lights etc were applications requiring increased strength and safety.
It also using in automotive industry as side and rear windows in automobiles.
It is also find application in refrigerators, furniture, ovens, shelving etc.
Tempered glass should not be used were building codes require wired glass for fire resistance.
Heat Strengthened Glass
As described earlier low air flow rates at the time of quenching produce Heat Strengthened Glass.
Characteristics of Heat Strengthened Glass
Heat Strengthened one is approximately 2 times stronger than the normal annealed one and 2 times thermal resistant to that of annealed one. It can be used up to a working temperature of 150 Degree Celsius. It breaks into large pieces almost like annealed glasses and cannot be used as safety glazing. It can be laminated with PVB interlayer. It can’t be cut or drilled after tempering. Heat Strengthened glass is not a structural glass and must be glazed on all four edges in conventional or structural silicone Aluminium system. It is not suitable for frameless facades. It is ideal for use in high-rise buildings and towers subjected to elevated wind load and temperature. Surface defects like optical distortion and bow etc are less than fully tempered. Nickel Sulphide inclusion is not considered as a significant source of fracture as in the cause of fully tempered.
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