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 self-explosion due to expansion, which can be briefly summarized as follows：
①The impact 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 are also the places where stress is concentrated. Especially if the stone is located in the tensile stress area of tempered glass, it is an important factor leading to explosion.
Stones are found in glass and have a different coefficient of expansion than the vitreous body. The stress concentration in the crack area around the stone increases exponentially after glass tempering. When the expansion coefficient of the stone is smaller than that of glass, the tangential stress around the stone is in tension. The crack propagation that accompanies stones can easily occur.
B. Glass contains nickel sulfide crystals
Nickel sulfide inclusions generally exist in the form of small crystallized spheres with a diameter of 0.1-2mm. The appearance is metallic, and these inclusions are NI3S2, NI7S6 and NI-XS, where X=0-0.07. Only the NI1-XS phase is the main reason for the spontaneous explosion of tempered glass.
The theoretical NIS is known to be 379. There is a phase transition process at C, from the a-NIS hexagonal crystal system in the high-temperature state to the B-NI trigonal crystal system in the low-temperature state, accompanied by a volume expansion of 2.38%. This structure is preserved at room temperature. If the glass is heated in the future, the a-B state transition may occur rapidly. If these debris are inside the tempered glass that is subject to tensile stress, the volume expansion will cause spontaneous explosion. If a-NIS exists at room temperature, it will slowly transform to the B state over several years or months. The slow increase in volume during this phase transition may not necessarily 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 the tempered glass to self-explode.
② Uneven stress distribution and offset in tempered glass
When glass is heated or cooled, the temperature gradient generated along the thickness of the glass is uneven and asymmetrical. This makes tempered products have a tendency to self-explode, and some produce "wind explosion" when chilled. If the tensile stress zone is offset to a certain side of the product or to the surface, the tempered glass will self-explode.
③Influence of the degree of tempering.
Experiments have shown that when the degree of tempering is increased to level 1/cm, the number of self-destructions reaches 20-25%. It can be seen that the greater the stress, the higher the degree of tempering and the greater the amount of self-explosion.
Tempered glass self-explosion solution
1. Reduce the stress value of tempered glass
The distribution of stress in tempered glass is that the two surfaces of the tempered glass are under compressive stress, the core layer is under tensile stress, and the stress distribution across the thickness of the glass is similar to a parabola. The center of the glass thickness is the apex of the parabola, which is where the tensile stress is maximum; the two sides close to the two surfaces of the glass are compressive stress; the zero-stress surface is located approximately 1/3 of the thickness. By analyzing the physical process of tempering and rapid cooling, it can be seen that the surface tension of tempered glass and the maximum internal tensile stress have a rough numerical proportional relationship, that is, the tensile stress is 1/2 to 1/3 of the compressive stress. Domestic manufacturers generally use the surface tension of tempered glass as the The tension is set at around 100MPa, but the actual situation may be higher. The tensile stress of tempered glass itself is about 32MPa ~ 46MPa, and the tensile strength of glass is 59MPa ~ 62MPa. As long as the tension generated by the expansion of nickel sulfide is 30MPa, it is enough to cause self-explosion. If the surface stress is reduced, the tensile stress inherent in the tempered glass[1] will be reduced accordingly, thus helping to reduce the occurrence of self-explosion.
The American standard ASTMC1048 stipulates that the surface stress range of tempered glass is greater than 69MPa; semi-tempered (heat-reinforced) glass is 24MPa ~ 52MPa. The curtain wall glass standard BG17841 stipulates that the stress range of semi-tempered glass is 24<δ≤69MPa. my country's March 1 this year The implemented new national standard GB15763.2-2005 "Safety Glass for Construction Part 2: Tempered Glass" requires that its surface stress should not be less than 90MPa. This is 5MPa lower than the 95MPa specified in the old standard, which is beneficial to reducing self-explosion.
2. Make the stress of the glass uniform
The uneven stress of tempered glass will significantly increase the self-explosion rate, which has reached a level that cannot be ignored. Self-explosion caused by uneven stress is sometimes very concentrated. In particular, the self-explosion rate of a specific batch of curved tempered glass can reach a shocking degree of severity, and self-explosion may occur continuously. The main reasons are local uneven stress and deviation of the tension layer in the thickness direction. The quality of the original glass sheet itself also has a certain impact. Uneven stress will significantly reduce the strength of the glass, which is equivalent to increasing the internal tensile stress to a certain extent, thereby increasing the self-explosion rate. If the stress of tempered glass can be evenly distributed, the self-explosion rate can be effectively reduced.
3. Hot soak treatment (HST)
Heat soak explained. Hot soak treatment is also called homogenization treatment, commonly known as "detonation". The heat dipping treatment is to heat the tempered glass to 290℃±10℃ and keep it warm for a certain period of time, which prompts the nickel sulfide to quickly complete the crystal phase transformation in the tempered glass, causing the tempered glass that is likely to explode after use to be artificially broken in advance in the factory. Heat soaking furnace, thereby reducing the self-explosion of tempered glass in use after installation. This method generally uses hot air as the heating medium. It is called "HeatSoakTest" abroad, or HST for short, which is literally translated as heat soak treatment.
Heat Soaking Difficulties. In principle, heat soak treatment is neither complicated nor difficult. But in fact it is very difficult to achieve this process indicator. Research shows that there are many specific chemical structural formulas of nickel sulfide in glass, such as Ni7S6, NiS, NiS1.01, etc. Not only do the proportions of various components vary, but they may also be doped with other elements. The speed of its phase change is highly dependent on the temperature. Research shows that the phase change rate at 280°C is 100 times that at 250°C, so it is necessary to ensure that each piece of glass in the furnace experiences the same temperature regime. Otherwise, on the one hand, the glass with low temperature cannot be completely phase-changed due to insufficient heat preservation time, which weakens the effect of heat soaking. On the other hand, when the glass temperature is too high, it may even cause reverse phase transformation of nickel sulfide, causing greater hidden dangers. Both situations can make heat soaking ineffective or even counterproductive. The uniformity of temperature when the hot soak furnace is working is so important. Three years ago, the temperature difference in the furnace during hot soak insulation in most domestic hot soak furnaces even reached 60°C. It is not uncommon for imported furnaces to have temperature differences of about 30°C. Therefore, even though some tempered glass has been heat-dipped, the self-explosion rate remains high.
The new standards will be more effective. In fact, the hot dip process and equipment have been continuously improved. The German standard DIN18516 specified a holding time of 8 hours in the 1990 edition, while the prEN14179-1:2001(E) standard reduced the holding time to 2 hours. The effect of the hot-dipping process under the new standard is very significant, and there are clear statistical technical indicators: after hot-dipping, it can be reduced to one case of self-explosion per 400 tons of glass. On the other hand, hot dip furnaces are constantly improving their design and structure, and the heating uniformity has also been significantly improved, which can basically meet the requirements of the hot dip process. For example, the self-explosion rate of the heat-dip treated glass of CSG Group has reached the technical indicators of the new European standards, and it performed extremely satisfactorily in the 120,000-square-meter Guangzhou New Airport project.
Although the heat soak treatment cannot guarantee that self-explosion will never occur, it does reduce the occurrence of self-explosion and truly solves the self-explosion problem that plagues all parties in the project. Therefore, heat soaking is the most effective method unanimously recognized in the world to completely solve the problem of self-explosion.
