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Chapter 13: Problem 17

In your own words, briefly describe what happens as a glass piece is thermally tempered.

Short Answer

Expert verified
Answer: During the thermal tempering process, a glass piece is heated to a specific temperature (around 620掳C to 650掳C) and then rapidly cooled, creating compressive stresses on the outer surfaces and tensile stresses in the interior. This results in a stronger and more durable glass piece that is more resistant to breakage caused by impacts, temperature changes, and other physical stresses. Additionally, when it does break, the thermally tempered glass breaks into small, relatively harmless pieces, reducing the risk of injury.

Step by step solution

01

Understanding Thermal Tempering

Thermal tempering is a process used to increase the strength and durability of glass by heating it to a specific temperature and then rapidly cooling it. This process causes the outer surfaces of the glass to contract, creating compressive stresses, while the interior remains in tension. The result is a stronger, more durable glass piece that is less likely to break and can better withstand temperature changes.
02

Heating the Glass

During the tempering process, the glass piece is placed inside a tempering oven and heated to a temperature of around 620掳C to 650掳C (1148掳F to 1202掳F). The goal is to heat the glass evenly and bring it to a uniform temperature so that it becomes soft and malleable. This needs to be done carefully to avoid any deformation or irregularities on the glass surface.
03

Rapid Cooling

After reaching the desired temperature, the glass piece is rapidly cooled, typically using high-pressure air jets, over a period of a few seconds to a few minutes. This fast cooling causes the outer surfaces of the glass to contract and solidify quickly, while the interior remains in a molten state for a longer period.
04

Formation of Compressive Stresses

As the outer surfaces of the glass contract and solidify while the interior remains molten, compressive stresses are created within the glass piece. These stresses make the surface tougher and more resistant to breakage, as they can help to maintain the glass' structural integrity even if the surface is subjected to external forces.
05

Strengthening and Benefits

The combination of compressive stresses on the outer surfaces and tensile stresses in the interior of the glass result in a thermally tempered glass piece with increased strength and durability. This means that it is more resistant to breakage caused by impacts, temperature changes, and other physical stresses. Additionally, thermally tempered glass breaks into small, relatively harmless pieces when it does break, reducing the risk of injury.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Compressive Stresses
Compressive stresses are crucial to the effectiveness of thermally tempered glass. When a piece of glass undergoes thermal tempering, the process instills a layer of compressive stresses on the outer surfaces. These stresses are created when the glass cools and solidifies more quickly outside than inside. This rapid cooling makes the glass surfaces shrink and become tightly bonded.
As a result, the exterior of the glass is essentially "squeezed" inward by these stresses, enhancing its toughness.
  • Compressive stresses help prevent surface cracks.
  • They enhance the glass's ability to resist external pressures.
  • They increase safety, as tempered glass tends to shatter into small, less dangerous pieces.
These stresses contribute significantly to why thermally tempered glass is much more resistant to damage compared to untreated glass.
Rapid Cooling
Rapid cooling is a vital step in the thermal tempering process. After glass is heated to a high, uniform temperature, it is rapidly cooled using techniques such as high-pressure air jets.
This cooling happens in seconds to minutes and is necessary to generate the compressive stresses on the glass surfaces. Without rapid cooling, the glass would not achieve the necessary surface strength.
This method ensures:
  • The outer layers solidify quickly, creating compressive forces.
  • The inner part remains slightly softer, retaining some elasticity.
Rapid cooling ensures that new compressive stresses are locked into the outer surface, making the glass more resistant to scratching, cracking, and breaking.
Glass Strength
The strength of thermally tempered glass is significantly enhanced through the tempering process. By generating compressive stresses on the outer layer and maintaining tensile stresses inside, tempered glass achieves remarkable toughness.
The balance between these stresses is key to its structural integrity.
  • Tempered glass can resist higher impacts compared to ordinary glass.
  • It is able to withstand significant thermal fluctuations, reducing breakage risks.
This increased strength not only extends the lifespan of the glass but also makes it a suitable choice for many applications where safety is a prime concern.
Tempering Process
The tempering process involves a series of controlled steps to enhance the properties of glass. Initially, the glass is heated uniformly to a high temperature, making it malleable without causing deformation. This stage sets the groundwork for the next crucial step.
Once the glass achieves the desired heat, it is swiftly cooled to lock in the compressive forces on the surface.
This two-part process of heating and rapid cooling is what imbues the glass with its strengthened properties.
  • Enhances durability and resistance to breakage.
  • Ensures the glass breaks into small, non-sharp pieces.
In essence, the tempering process is essential for creating glass that is safe and robust, serving as an important method for producing high-performance glass materials.

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Most popular questions from this chapter

Some ceramic materials are fabricated by hot isostatic pressing. Cite some of the limitations and difficulties associated with this technique.

Glass pieces may also be strengthened by chemical tempering. With this procedure, the glass surface is put in a state of compression by exchanging some of the cations near the surface with other cations having a larger diameter. Suggest one type of cation that, by replacing \(\mathrm{Na}^{+},\) will induce chemical tempering in a soda-lime glass.

(a) What are the three main components of a whiteware ceramic such as porcelain? (b) What role does each component play in the forming and firing procedures?

For many viscous materials, the viscosity \(\eta\) may be defined in terms of the expression $$\eta=\frac{\sigma}{d \epsilon / d t}$$ where \(\sigma\) and \(d \epsilon / d t\) are, respectively, the tensile stress and the strain rate. A cylindrical specimen of a borosilicate glass of diameter \(4 \mathrm{mm}(0.16 \text { in. })\) and length \(125 \mathrm{mm}\) (4.9 in.) is subjected to a tensile force of \(2 \mathrm{N}\left(0.45 \mathrm{lb}_{\mathrm{f}}\right)\) along its axis. If its deformation is to be less than \(2.5 \mathrm{mm}(0.10 \text { in. })\) over a week's time, using Figure \(13.7,\) determine the maximum temperature to which the specimen may be heated.

Cite one reason why drying shrinkage is greater for slip cast or hydroplastic products that have smaller clay particles.
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