Tempered glass belongs to safety glass. Tempered glass is actually a kind of prestressed glass. In order to improve the strength of glass, compressive stress is usually formed on the surface of glass by chemical or physical methods. When the glass bears external force, the surface stress is first offset, which improves the bearing capacity and enhances the wind pressure resistance, cold and heat resistance and impact resistance of the glass itself.
Tempered glass is made by cutting ordinary annealed glass into the required size, then heating it to about 700 degrees near the softening point, and then rapidly and uniformly cooling it (usually, 5-6MM glass is heated at 700 degrees for about 240 seconds, and the temperature is reduced by about 150 seconds).
8- 10MM glass is heated at 700℃ for about 500 seconds and cooled for about 300 seconds. In short, according to the thickness of the glass, the heating and cooling time is different.
After tempering, uniform compressive stress is formed on the glass surface and tensile stress is formed inside, which improves the bending and impact strength of the glass, and its strength is about four times that of ordinary annealed glass.
Physical tempered glass is also called quenched tempered glass. When ordinary flat glass is heated to near the glass softening temperature (600℃) in the heating furnace, the internal stress is eliminated by its own deformation.
Then the glass is moved out of the heating furnace, and then the high-pressure cold air is blown to both sides of the glass by multi-head nozzles, so that it can be quickly and evenly cooled to room temperature, and the tempered glass can be made.
This kind of glass is in a stress state of internal tension and external pressure. Once damaged locally, the stress will be released and the glass will be broken into countless small pieces. These small pieces have no sharp edges and corners, so they are not easy to hurt people.
Extended data:
I. Advantages and disadvantages of tempered glass
1, advantages:
1) The strength is several times higher than that of ordinary glass, and it is resistant to bending.
2) Safe use, increased bearing capacity and improved brittleness. Even if the tempered glass is damaged, it will appear as small fragments without acute angle, which greatly reduces the harm to human body.
Compared with ordinary glass, the quenching and heat resistance of tempered glass is improved by 3~5 times, and it can generally withstand the temperature difference of more than 250 degrees, which has obvious effect on preventing hot cracking. This is a kind of safety glass. Provide qualified materials to ensure the safety of high-rise buildings.
2. Disadvantages:
1) Tempered glass can no longer be cut and processed. Only the glass can be processed into the required shape before tempering, and then tempered.
2) Although the strength of tempered glass is stronger than that of ordinary glass, tempered glass has the possibility of self-explosion, while ordinary glass has no possibility of self-explosion.
3) The surface of tempered glass will be uneven (wind spots) and its thickness will be slightly thinner. The reason for the thinning is that the glass is softened by hot melt and quickly cooled by strong wind, which makes the crystal gap inside the glass smaller and the pressure greater, so the tempered glass is thinner than before.
Generally, the tempered thickness of 4~6mm glass is 0.2~0.8mm, and the tempered thickness of 8~20mm glass is 0.9 ~1.8mm.. The specific degree depends on the equipment, which is why tempered glass can't be used as a mirror.
4) The flat glass for building will generally deform after being tempered in a tempering furnace (physical tempering), and the degree of deformation is determined by the process of equipment and technicians. To some extent, it affects the decorative effect (except for special needs).
Second, the development history
The development of tempered glass can be traced back to the middle of17th century. A prince named Robert from the Rhine once did an interesting experiment. He put a drop of molten glass into cold water and made a very hard glass.
This kind of high-strength granular glass is like a water drop with a long and curved tail, and is called "Prince Robert Granule". However, when the tail of the small grain bends and breaks, the whole small grain suddenly collapses violently and even becomes fine powder, which is very strange.
The above practice is very similar to the quenching of metal, which is the quenching of glass. This quenching does not change the composition of glass, so it is called physical tempering, so tempered glass is called tempered glass.
The first patent for glass tempering was obtained by the French in 1874. Tempering method is to heat the glass to near the softening temperature and immediately put it into a relatively low temperature liquid tank to improve the surface stress.
This method is the early liquid tempering method. Frederick Siemens of Germany obtained a patent in 1875, and Geovge E. Rogens of Massachusetts of the United States applied the tempering method to glass and lamp posts in 1876. In the same year, HughO'heill of New Jersey received a patent.
In 1930s, Saint-Gobain Company of France, Triplex Company of the United States and pilkington Company of the United Kingdom all began to produce large-area flat tempered glass for automobile windshields. Japan also industrialized the production of tempered glass in 1930s. Since then, the world has begun the era of mass production of tempered glass.
Following 1970, British Triplex Company successfully toughened glass with a thickness of 0.75~ 1.5mm with liquid medium, ending the history that physical tempering cannot toughen thin glass, which is a major breakthrough in toughened glass technology.
The history of tempered glass in China began at 1955, with trial production in Hua Yao glass factory and trial production in Qinhuangdao tempered glass factory at 1958.
1965, Qinhuangdao Hua Yao glass factory began to produce military tempered glass. In 1970s, Luoyang Glass Factory took the lead in introducing Belgian tempering equipment. At the same time, the chemically toughened glass of Shenyang Glass Factory was put into production.
Since 1970s, toughened glass technology has been widely promoted and popularized all over the world. Tempered glass has been used in automobile, architecture, aviation, electronics and other fields, especially in architecture and automobiles.
References:
Baidu encyclopedia-toughened glass