Thermal strengthening process
Thermal strengthening, also known as heat strengthening, is a process where glass is heat soaked and then annealed or rapidly cooled in a controlled manner. During this process, the glass is heated to its softening point and then rapidly cooled by blasting air or gas across one of its surfaces, which creates a compressive stress layer and increases its strength. The rapid quenching prevents the glass molecules from moving back into their original positions, trapping stresses into the surface in compression and inside in tension. This increases the strength of the glass, making it up to 4 times stronger than regular annealed glass and able to withstand higher wind loads or impacts from hail or stones.
Typical temperatures used for Heat Treated Glass strengthening range between 600°C to 650°C. The glass is soaked at these temperatures for a period of fewer than 10 minutes before rapid cooling. The glass emerges from the furnace evenly cooled with compression stresses in the surface and parallel tensile stresses inside. This process produces glass that is stronger and more resistant to thermal stresses than standard annealed glass. However, it is not as strong as fully tempered glass. If a heat strengthened glass breaks, it does not shatter into small safe pieces like tempered glass but instead breaks into larger dangerous jagged shards.
Properties of heat strengthened glass
Heat strengthened glass has higher strength compared to annealed glass, making it suitable for applications where impact resistance and safety are primary concerns. Some key properties of heat strengthened glass include:
- Strength: Heat strengthened glass is around 2-4 times stronger than regular annealed glass. Its breaking strength allows it to withstand higher wind loads and impacts.
- Surface compression: A high compressive stress layer is introduced on the surface through rapid quenching, increasing its resistance to surface damage from minor impacts or abrasions.
- Thermal endurance: It has better resistance to thermal stresses like expansion and contraction during heating and cooling cycles compared to annealed glass. This makes it suitable for applications prone to temperature fluctuations.
- Safety: Although it does not disintegrate into small rounded pieces like fully tempered glass, if heat strengthened glass breaks, it resists shattering completely into very sharp dangerous shards. Breakage typically results in larger blunt pieces.
- Clarity: There is no change to the optical clarity and transparency of the glass after heat strengthening. Its strength enhancement does not compromise on visibility.
- Weight: The weight of heat strengthened glass remains identical to the annealed glass from which it was made as only internal stresses are altered through thermal processing.
- Dimensional stability: Strengthening through controlled heating and cooling does not induce any changes to the physical dimensions and installed measurements of the glass.
Applications of heat strengthened glass
Some major applications where heat strengthened glass finds use include:
Exterior glass applications: Due to its improved resistance against wind loads, thermal stresses and minor surface damage from hail or impacts, heat strengthened glass is commonly used for applications like balcony railings, frames for shower or bathroom enclosures, glass floors, canopies etc.
Insulating glass units: The individual glass lites used in insulating glassunits for windows are often heat strengthened on the external surface for enhanced safety and durability under extreme weather conditions.
Safety glazing in doors: Glass doors in commercial, office or residential buildings are made impact resistant through heat strengthening for safety against accidental human impact.
Residential windows: Low to mid-rise residential constructions favor the use of heat strengthened over annealed glass in windows for strength during extreme weather events like high winds or storms while being safer than regular glazing.
Automotive applications: Side and rear windows in vehicles, especially commercial transports, are made of heat strengthened glass instead of annealed to resist cracks from everyday rock impacts encountered on the road.
Skylights and sloped glazing: Glazing used for skylights or in overhead applications where glass is exposed to higher wind pressure at slope are strengthened through heat treatment for added strength security.
Tempering process
Fully tempered or toughened glass is produced through an even more intense thermal strengthening process than simple heat strengthening. During tempering, the glass is heated to its softening point, which is around 660°C and is then rapidly cooled using a blast of air or gas.
Tempering makes glass up to 5-10 times stronger than regular annealed glass. The heating and rapid quenching through tempering distorts the molecular structure of the glass and puts its full surface and edges in a state of permanent compressive stress. This greatly enhances its strength and impact resistance properties.
If a piece of tempered glass breaks due to an applied force exceeding its limits, it degrades into small granular chunks instead of splintering into jagged sharp shards. These shattered pieces are less likely to cause injury. This makes it an indispensable material for applications where human safety against glass breakage is a prime priority like shower doors, automobile windshields, laminated glass in phone screens etc.
The tempering process also minimizes thermal stresses and prevents spontaneous breakageoccurrence from temperature fluctuations due to the balance of internal compressive and tensile stresses trapped inside the glass. However, tempered glass (unlike laminated glass) provides no protection against penetrating impacts like bullets. It also costs more than heat strengthened or regular annealed glass.
heat treating is a widely used thermal strengthening process that can upgrade the mechanical durability and safety properties of glass depending on the degree of thermal exposure and rapid cooling employed. This article discussed the thermal strengthening and tempering procedures and compared the resulting strengthened glass varieties in terms of their mechanical performance benefits, applications, advantages and limitations over regular unmodified annealed glazing materials. Heat treated glass finds extensive usage globally across industries and in everyday life through varied architectural, automotive and consumer applications whenever enhanced strength, safety, weather resistance or security of glazing is essential.
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About Author:
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)
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