HEAT TREATED GLASS

INTRODUCTION

“Heat-treated glass” is a general term used in the glass fabrication industry to describe glass that has been processed through a tempering oven to change its strength and breakage characteristics There are two distinct heat-treated products, heat-strengthened glass and fully tempered glass, as defined in ASTM C1048 Standard Specification for Heat-treated Flat Glass–Kind HS, Kind FT Coated and Uncoated Glass. Compared to annealed glass (nonheat-treated glass), both have increased strength to resist higher levels of impact, mechanical load and thermal stress. Heat-strengthening adds strength to the glass while limiting the change in its breakage characteristics. Tempered glass is stronger than heat-strengthened glass and significantly reduces the broken piece sizes to meet the safety glazing standards.

DESCRIPTION

Glass is heat-treated by heating annealed glass to a temperature of approximately 621°C, then rapidly cooling it. The glass is cooled by a carefully controlled airflow (also known as quenching), which uniformly cools all glass surfaces simultaneously. High airflow rates produce tempered glass and much lower airflow rates produce heat-strengthened glass.

Fully  Tempered  Glass  Fully  tempered  glass,  normally  referred  to  as  just  “tempered glass”, is approximately four times stronger than annealed glass of the same thickness and configuration.  When it is broken,tempered glass fractures into small fragments  that reduce the probability of serious injury as compared to annealed glass. Tempered glass  meets  safety  glazing  standards.  Because  tempered glass fractures  into  many small pieces, it tends to vacate the opening, when broken, more than heat-strengthened and annealed glass does.


Heat-Strengthened  Glass  Heat-strengthened  glass  is  approximately  twice  as  strong as annealed glass of similar thickness and configuration. Heat-strengthened glass generally fractures in a manner similar to annealed glass and tends to remain in the opening when broken. It is intended for general glazing where additional strength and/or  resistance  to  mechanical  and/or  thermal  stress  are  desired.  Heat-strengthened glass is NOT a safety-glazing product and therefore should not be used where safety glazing is required.

CAPABILITIES

Glass Options

Most architectural glass products can be heat-treated. Some patterned glass and decorative glass with a deep surface pattern may not be heat- treatable. Silk-screened and ceramic spandrel glass are always either heat-strengthened or tempered as part of their fabrication process. Heat- absorbing glasses, such as tints, reflective glasses and Low-E glass, may require heat treatment to reduce the probability of thermal-stress breakage, especially when used as part of an insulating glass unit.

Thickness

Glass thicknesses from 3.8mm through 20mm can be tempered. Glass thicknesses from 3.8mm through 8mm are commonly heat-strengthened.

Size

The minimum and maximum heat-treated glass sizes start from 250mm × 250mm up to 2440mm x 4800mm

Applications

Heat-strengthened

Due to its good glass retention properties, heat-strengthened glass is can be used for applications where additional strength is needed to meet mechanical loads or thermal loads caused by certain tinted or coated glasses. Heat- strengthened glass is widely used in laminated glass for additional strength, such as in overhead and sloped glazing. Heat-strengthened glass cannot be used in any safety glazing applications.

Tempered

Tempered glass is used when the strength requirements exceed the capabilities of heat-strengthened glass, and for all safety glazing applications. Tempered glass is commonly used in curtain walls, sliding doors, partitions, windows, storefronts, display cases, bath and shower enclosures and all- glass doors and entrances. Tempered glass can be installed in overhead glazing and skylights in the top pane only of the IG unit. The bottom pane must be laminated glass.

CHARACTERISTICS

Properties Unaffected by Heat-treating: The color, chemical composition and light transmission characteristics of glass remain unchanged after the heat-treating process. The physical properties of glass, such as the compressive strength, hardness, specific gravity, the softening point, thermal conductivity, solar transmittance, stiffness and expansion coefficient, also remain unchanged.

Deflection: It is important to note that heat-treating does not change the deflection characteristics of glass. In many cases, even though thinner heat-treated glass may be strong enough for a specific application, thicker glass may need to be specified in order to reduce the amount of glass deflection.

Breakage Characteristics: The higher the amount of residual stress in a piece of glass, the smaller the particle size will be when the glass fractures. When annealed glass fractures, the cracks are far apart and the pieces are normally quite large with sharp edges. As a result of the heat-treating process, tempered glass fractures into small particles when broken, thus meeting safety glazing requirements. The breakage characteristics of heat-strengthened glass can vary within the allowable stress range of the product (3,500 to 7,500 psi surface compression). Heat-strengthened glass typically fractures into large pieces that are more similar to annealed glass than to tempered glass.

Fabrication : Fabrication work such as cutting, polishing, grinding, drilling, notching, sandblasting, etching or any other process that modifies the glass, must be completed prior to heat-treating the glass. ASTM C1048 provides specific limitations and requirements for the size and location of holes and notches. Any fabrication process completed after the glass is heat-treated, such as sandblasting or V-grooving, will reduce the strength of the glass.

Thermal Shock Resistance: Heat-treated glass will withstand greater thermal shock than the same thickness and configuration of annealed glass. Thermal shock results when a rapid temperature change between the surface and core of the glass occurs. When this temperature differential is of sufficient magnitude, the glass will fracture. To fracture (6 mm) annealed glass, the average temperature differential would be approximately (38°C). To fracture (6 mm) heat-strengthened and tempered glass, the average temperature differential would be about (121°C) and (204°C), respectively.