Annealing is a process of slowly cooling glass to relieve internal stresses after it was formed. Glass in the common sense refers to a Hard, Brittle, transparent Solid, such as that used for Windows many The process may be carried out in a temperature-controlled kiln known as a Lehr. A lehr is a temperature-controlled Kiln for annealing objects made of Glass. [1] Glass which has not been annealed is liable to crack or shatter when subjected to a relatively small temperature change or mechanical shock. Annealing glass is critical to the durability of glass. If glass is not annealed, it will retain many of the thermal stresses caused by quenching and significantly decrease the overall strength of the glass.
The glass is heated until the temperature reaches a stress-relief point, that is, the annealing temperature (also called annealing point) at a viscosity, η, of 1013 Poise = 1012 Pa·s, at which the glass is still too hard to deform, but is soft enough for the stresses to relax. Temperature is a physical property of a system that underlies the common notions of hot and cold something that is hotter generally has the greater temperature Stress is a measure of the average amount of Force exerted per unit Area. Viscosity is a measure of the resistance of a Fluid which is being deformed by either Shear stress or Extensional stress. The poise (symbol P pwɑːz is the unit of dynamic Viscosity in the Centimetre gram second system of units. In Materials science, deformation is a change in the shape or size of an object due to an applied force. The piece is then allowed to heat-soak until its temperature is even throughout. The time necessary for this step varies depending on the type of glass and thickness of the thickest section. The glass is then slowly cooled at a predetermined rate until its temperature is below the strain point (η = 1014. 5 Poise). Following this, the temperature can safely be dropped to room temperature at a rate limited by the heat capacity, thickness, thermal conductivity, and thermal expansion coefficient of the glass. Room temperature (also referred to as ambient temperature) is a common term to denote a certain Temperature within enclosed space at which humans are accustomed Specific heat capacity, also known simply as specific heat, is the measure of the heat energy required to increase the Temperature of a unit quantity In Physics, thermal conductivity, k is the property of a material that indicates its ability to conduct Heat. When the Temperature of a substance changes the energy that is stored in the Intermolecular bonds between atoms changes After the annealing process the material can be cut to size, drilled or polished.
At the annealing point (η = 1013 Poise) stresses relax within several minutes, while at the strain point (η = 1014. 5 Poise) stresses relax within several hours. [2] Stresses that are still present below the strain point are permanent.
Tempered glass must be converted to annealed glass before cutting or grinding. Toughened or tempered glass is Glass that has been processed by controlled thermal or chemical treatments to increase its strength compared with normal glass
See also
References
- ^ E. Annealing, in Metallurgy and Materials science, is a Heat treatment wherein a material is altered causing changes in its properties such as strength Float glass is a sheet of Glass made by floating molten glass on a bed of molten Tin. Toughened or tempered glass is Glass that has been processed by controlled thermal or chemical treatments to increase its strength compared with normal glass Optical fabrication and testing spans an enormous range of manufacturing procedures and F. Collins (1921) Electrically heated glass annealing lehr. Journal of the American Ceramic Society 4 (5), 335–349.
- ^ Werner Vogel: "Glass Chemistry"; Springer-Verlag Berlin and Heidelberg GmbH & Co. K; 2nd revised edition (November 1994), ISBN 3540575723