Tempered Glass Emergency Measures

Quality
Tempered glass is obtained by cutting ordinary annealed glass into the required size, then heating it to a near softening point, and then cooling it quickly and evenly. After tempering, uniform compressive stress is formed on the surface of the glass, while tensile stress is formed inside, which greatly improves the performance of the glass. The tensile strength is more than 3 times that of the latter, and the impact resistance is more than 5 times that of the latter.
It is precisely this feature that stress characteristics become an important sign for distinguishing true and false tempered glass, that is, tempered glass can see colored stripes on the edge of the glass through the polarizing film, and black and white spots can be seen on the surface of the glass. Polarizing films can be found in camera lenses or glasses. Pay attention to the adjustment of the light source when observing, so that it is easier to observe.
Self-explosion defect
The automatic explosion of tempered glass without direct mechanical external force is called the self-explosion of tempered glass. According to industry experience, the self-explosion rate of ordinary tempered glass is about 1~3‰. Self-explosion is one of the inherent characteristics of tempered glass.
There are many reasons for the expansion of spontaneous explosion, which can be simply summarized as follows:
① The influence of glass quality defects
A. There are stones, impurities and bubbles in the glass: impurities in the glass are the weak points of tempered glass and the stress concentration points. In particular, if the stones are in the tensile stress zone of tempered glass, it is an important factor leading to explosion.
Stones exist in the glass and have different expansion coefficients from the glass body. After the glass is tempered, the stress concentration in the crack area around the stones increases exponentially. When the expansion coefficient of the stones is smaller than that of the glass, the tangential stress around the stones is in a tensile state. Crack expansion with stones is very easy to occur.
B. Nickel sulfide crystals in the glass
Nickel sulfide inclusions generally exist in small crystal spheres with a diameter of 0.1-2 mm. The appearance is metallic. These inclusions are Ni3S2, Ni7S6 and Ni-XS, where X=0-0.07. Only the Ni1-XS phase is the main cause of the spontaneous explosion of tempered glass.
The theoretical NIS is known to be 379. C, there is a phase transition process, from the high-temperature α-NiS hexagonal system to the low-temperature β-NiS trigonal system, accompanied by a 2.38% volume expansion. This structure is preserved at room temperature. If the glass is heated in the future, the α-β state may quickly change. If these impurities are inside the tempered glass under tensile stress, the volume expansion will cause spontaneous cracking. If a-NIS exists at room temperature, it will slowly change to the β state after several years or months. The slow increase in volume during this phase transition may not cause internal rupture.
C. The glass surface has scratches, cracks, deep cracks and other defects due to improper processing or operation, which can easily cause stress concentration or cause tempered glass to explode.
② Uneven and offset stress distribution in tempered glass
The temperature gradient generated along the thickness direction of the glass when heated or cooled is uneven and asymmetric. The tempered product tends to explode by itself, and some produce "wind explosion" when quenching. If the tensile stress area deviates to one side of the product or to the surface, the tempered glass will explode by itself.
③ The influence of the degree of toughening. Experiments have shown that when the degree of toughening is increased to 1 level/cm, the number of self-explosion reaches 20%~25%. This shows that the greater the stress, the higher the degree of toughening, and the greater the number of self-explosion.