Heat treated glass refers to glass that has undergone rapid heating and cooling to deliberately strengthen its physical and thermal properties. During heat treatment, flat or shaped glass sheets are subjected to temperatures close to their deformation point to build up residual surface compressive stresses while also minimizing edge defects. The results are glass panels that offer improved product performance and safety benefits over standard annealed glass.
Types of Heat Treatment Processes
There are three main types of heat treatment processes used in the glass industry today:
Tempering
Heat Treated Glass tempering involves heating glass to just above its softening point, around 600°C, and then rapidly cooling using either a blast of air or contacting surfaces. This causes the outer surfaces to contract much more sharply than the inner portion. The uneven rate of cooling results in the generation of residual compressive surface stresses and balancing tensile core stresses. The compression stress layer makes the tempered glass at least 4 times stronger and safer than its annealed counterpart. It completely shatters on breakage into small granular pieces instead of sharp shards.
Heat Strengthening
Heat strengthening is a milder form of tempering where the glass is heated to slightly below the softening point, around 550°C, and cooled at a slower controlled rate. This induces a partial tempering effect with lower compressive stresses of around 70-90 MPa. While heat strengthened glass does not completely disintegrate upon failure, it is safer than annealed glass but not as tough as fully tempered glass.
Chemical Strengthening
In chemical strengthening, small alkali metal ions like potassium are ion-exchanged on the surface of glass by submerging it in a molten salt bath. Larger ions diffuse into the glass when it is heated, creating a dense compressive layer. After cooling, these larger ions are trapped just below the surface, inducing high compressive stresses up to 250-350 MPa. Chemically strengthened glass is extremely damage-resistant but more brittle than thermally tempered glass.
Applications of Heat Treated Glass
The unique advantages of heat treated glass have enabled its widespread use in commercial, residential and transportation applications:
Architectural Uses
Fully tempered and heat strengthened glass find extensive usage in glass facades, curtain walls, doors, skylights and sloped glazing due to their safety and durability properties. The floor-to-ceiling glass panels can be larger without posing fragmentation hazards. chemically strengthened glass is growing in popularity for its exceptional strength in thin profiles.
Automotive Applications
All car windows except the windshield are made from tempered glass for passenger safety. It breaks into small chunks instead of sharp pieces if shattered. Other windshield variants like heat strengthen laminated glass provide impact resistance and projectile retention.
Home Appliances
Glass cooktops, oven doors and glass shelves in appliances employ tempered glass to withstand impacts and thermal shocks. Chemically strengthened glass is used for thin, durable covers on portable electronics and screens.
Benefits Over Annealed Glass
The key advantages of heat treated glass versus regular annealed glass include:
Improved Strength & Safety
Fully tempered and chemically strengthened glass are 4-5 times stronger than annealed glass. On breakage, they fracture into small blunt pieces rather than dangerous shards.
Thermal Shock Resistance
The compression stresses allow heat treated glass to better withstand sudden temperature changes without cracking. This enables uses like ceramic glass cooktops.
Large Sizes & Uniform thickness
The strengthening process permits manufacturing glass panels in much larger dimensions and thicknesses without warping or edge defects compared to annealed versions.
Scratch Resistance
The compressive layer in strengthened glass variants makes them more resistant to surface abrasions and scratches from everyday use.
heat treating has enabled practical applications for glass by vastly enhancing its mechanical and thermal performance properties. As advanced fabrication methods are developed, chemically strengthened glass is emerging as the optimum solution for next-generation architectural and consumer applications demanding exceptional strength in delicate thin form factors.
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Money Singh is a seasoned content writer with over four years of experience in the market research sector. Her expertise spans various industries, including food and beverages, biotechnology, chemical and materials, defense and aerospace, consumer goods, etc. (https://www.linkedin.com/in/money-singh-590844163)