What is thermal tempering of glass? | ACI Industriearmaturen GmbH

What is thermal tempering of glass?

Technical glasses such as borosilicate glass, soda-lime glass, quartz glass or sapphire glass play an important role in the field of optical process observation. In the following, we will deal with the thermal tempering of glass.

During thermal tempering, glass is heated to a temperature that is about 100°C above the transformation temperature. At the transformation temperature, the glass changes from the plastic to the rigid state during cooling. For this purpose, the material is heated to approx. 620°C to 670°C in a tempering furnace and then cooled down by blowing in air.

Glass_tempering_oven
Glass_Cooling_with_air

During thermal tempering, glass is heated to a temperature that is about 100°C above the transformation temperature. At the transformation temperature, the glass changes from the plastic to the rigid state during cooling. For this purpose, the material is heated to approx. 620°C to 670°C in a tempering furnace and then cooled down by blowing in air.

However, these stresses still show very small differences at this stage, as they are quickly dissipated by the high viscosity. The initial temperature is therefore very important for the reduction of tensile stresses.

Tensile_and_compressive_stress_in_the_glass_cross_section

If the initial temperature is too low and the viscosity of the core too high, the resulting internal stresses may lead to glass breakage. The controlled, rapid cooling causes the glass surface to shrink more than the core. As a result, a defined internal stress builds up in the material, giving the glass greater strength.

What are the advantages of thermally tempered glass?

A very important aspect is the safety against breakage. This is greatly increased by the thermal tempering. A thermally toughened glass has a different breakage behaviour than a conventional float glass. The thermally toughened sight glass does not break into large, sharp-edged shards. In the event of glass breakage, it shatters into small, more rounded fragments. This prevents serious cuts. This also virtually eliminates cuts when cleaning the affected area.

Chipped_tempered_borosilicate_glass plate

Thermal toughening greatly increases the scratch resistance. However, subsequent damage to the surface due to scratches reduces the thermal toughening and thus also the pressure resistance of the glass. The pressure resistance is considerably increased with thermally toughened glass. This is a further advantage, especially for process monitoring using sightglasses.Another important advantage achieved by thermal toughening is the increase in thermal shock resistance. The thermal shock resistance of untempered borosilicate glass is 133°C. A thermally toughened borosilicate glass has a higher thermal shock resistance. A thermally toughened borosilicate glass, on the other hand, has a thermal shock resistance of +230°C. This increase in thermal shock resistance offers a great advantage in processes in which the operating temperatures change rapidly.

Which types of glass can be thermally tempered?

Borosilicate glass possible
Soda-lime glass possible
Quartz glass not possible
Sapphire glass not possible


Due to the wide variety of glass types available, only those glass materials that are primarily used in the applications of ACI's customers are discussed above.

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