In addition to chemical and thermal stresses, sight glasses are also subjected to stress from pressure and bending moments in many applications. To help withstand this, the glass is normally toughened. There are a variety of methods used to toughen glass. Since the manner in which it is toughened may affect its suitability for a particular application, it is important that engineers be aware of the different methods so that the appropriate glass is specified.
Flat glass is produced utilizing the float glass process, where molten glass is floated on a bed of molten tin. The hot glass passes through an “annealing lehr,” where it is slowly cooled to prevent buildup of internal stresses. This allows the glass to be cut and otherwise processed afterward. Annealing is commonly used in the production of ordinary glass. (See Photo A.) Further strength improvement can be accomplished by toughening or tempering the glass.
To temper glass, one of two methods may be used:
Chemical Strengthening: A chemical solution may be applied to the glass in order to increase the mechanical resistance of the glass. The properties of chemically strengthened glass are similar to those of thermally treated glass. Chemically treated glass is commonly used in applications where thin, strong glass (stronger than window glass) is required.
Heat Treating: Annealed glass may be subjected to additional heat treating to further increase its strength. Glass is heated to approximately 680ºC, almost its softening point. It is then quenched or cooled with a steady high-pressure flow of compressed air. This process quickly cools the surfaces which go into compression while the core (inside) is allowed to cool more slowly and goes into tension. This process makes heat tempered glass three to five times as strong as annealed glass. When fractured, rapidly cooled (fully tempered) glass releases the tensile energy stored in the core and breaks into many fragments, similar to automotive rear window safety glass and glass basketball backboards. (See Photo B.) If, instead of cooling the glass rapidly, it is allowed to cool slowly, it becomes twice as strong as annealed glass. When fractured, these broken glass fragments are linear and will normally remain in the frame.
Thermally tempered glass is often used in conventional sight glasses.
Federal Specification DD-G-1403C for heat-treated glass divides it into two categories, fully tempered glass and heat-strengthened glass, and it specifies minimum compression values for each. For fully tempered glass, the surface compression must be 10,000 psi or more, or the edge compression must be 9,700 psi or more. For heat-strengthened glass, the surface compression must be between 3,500 psi and 10,000 psi, or the edge compression must be between 5,500 psi and 9,700 psi. With this wide range of compression values, the fracture characteristics of heat-strengthened glass may range from those typical of annealed glass (3,500 psi level) to those of fully tempered glass (9,700 psi level).