1. Reduce the stress value of tempered glass
The distribution of stress in the tempered glass is that the two surfaces of the tempered glass are compressive stress, the core layer is in tensile stress, and the stress distribution on the glass thickness is similar to a parabola. The center of the glass thickness is the apex of the parabola, that is, the maximum tensile stress; the two sides are near the surface of the glass is the compressive stress; the zero stress surface is about 1/3 of the thickness. By analyzing the physical process of tempering and quenching, it can be seen that the surface tension of the tempered glass and the maximum tensile stress inside have a rough proportional relationship, that is, the tensile stress is 1/2 to 1/3 of the compressive stress. Domestic manufacturers generally have a tempered glass surface. The tension is set at around 100 MPa, and the actual situation may be higher. The tensile stress of the tempered glass itself is about 32 MPa to 46 MPa, and the tensile strength of the glass is 59 MPa to 62 MPa. As long as the tension generated by the expansion of the nickel sulfide is 30 MPa, it is sufficient to cause self-explosion. If the surface stress is lowered, the tensile stress of the tempered glass itself is reduced accordingly, which helps to reduce the occurrence of self-explosion.
American standard ASTMC1048 stipulates that the surface stress range of tempered glass is greater than 69 MPa; semi-tempered (heat-reinforced) glass is 24 MPa to 52 MPa. The curtain wall glass standard BG17841 is defined as semi-tempered stress range 24 < δ ≤ 69 MPa. China March 1 this year The implementation of the new national standard GB15763.2-2005 "Safety glass for construction Part 2: tempered glass" requires that the surface stress should not be less than 90MPa. This is 5MPa lower than the 95MPa specified in the previous old standard, which is conducive to reducing self-explosion.
2, make the stress of the glass uniform
The uneven stress of the tempered glass will significantly increase the self-explosion rate, which has reached a level that cannot be ignored. The self-explosion caused by uneven stress sometimes manifests itself very concentrated. In particular, the self-explosion rate of a specific batch of curved tempered glass can reach a shocking severity, and self-explosion may occur continuously. The main reason is the local stress unevenness and the deviation of the tension layer in the thickness direction, and the quality of the original glass film also has a certain influence. Uneven stress will greatly reduce the strength of the glass, which is equivalent to increasing the internal tensile stress to a certain extent, so that the self-explosion rate is improved. If the stress of the tempered glass can be evenly distributed, the self-explosion rate can be effectively reduced.
3. Hot dip treatment (HST)
Hot dip interpretation. Hot dip treatment is also called homogenization treatment, commonly known as "detonation". The hot dip treatment is to heat the tempered glass to 290 °C ± 10 °C, and keep it for a certain period of time, so that the nickel sulfide can quickly complete the crystal phase transformation in the tempered glass, so that the tempered glass which can be self-explosive after the original use is artificially broken in the factory. In the hot dip furnace, thereby reducing the self-explosion of the tempered glass in use after installation. This method generally uses hot air as a heating medium, which is called "HeatSoakTest" in foreign countries, referred to as HST, and is literally translated as hot dip.
Hot dip is difficult. In principle, hot dip treatment is neither complicated nor difficult. But actually achieving this process is very difficult. Studies have shown that there are many specific chemical structural formulas of nickel sulfide in glass, such as Ni7S6, NiS, NiS1.01, etc., not only the proportion of various components are different, but also may be doped with other elements. The phase change speed is highly dependent on the temperature. Studies have shown that the phase transition rate at 280 ° C is 100 times that at 250 ° C, so it must be ensured that each piece of glass in the furnace experiences the same temperature regime. Otherwise, on the one hand, the glass with low temperature is not enough for the holding time, and the nickel sulfide cannot completely change phase, which reduces the effect of hot dip. On the other hand, when the glass temperature is too high, it may even cause reverse phase transformation of the nickel sulfide, causing a greater hidden danger. Both of these conditions can cause hot dip treatment to be laborious and even counterproductive. The uniformity of temperature during hot-dip furnace operation is so important. When the heat-dip insulation of most domestic hot-dip furnaces was three years ago, the temperature difference in the furnace even reached 60 °C. It is not uncommon for foreign imported furnaces to have a temperature difference of about 30 °C. Therefore, although some tempered glass is treated by hot dip, the self-explosion rate is still high.
The new standard will be more effective. In fact, hot dip processes and equipment have also been continuously improved. The German standard DIN18516 specifies a holding time of 8 hours in the 90-year version, while the prEN14179-1:2001 (E) standard reduces the holding time to 2 hours. The effect of the hot dip process under the new standard is very significant, and there are clear statistical technical indicators: after hot dip, it can be reduced to one self-explosion per 400 tons of glass. On the other hand, the hot dip furnace is constantly improving the design and structure, and the heating uniformity is also significantly improved, which can basically meet the requirements of the hot dip process. For example, CSG's hot-dip glass has a self-explosion rate that meets the technical standards of the new European standards, and is extremely satisfied with the 120,000-square-meter Guangzhou New Airport super-large project.
Although the hot dip treatment does not guarantee that self-explosion will never occur, it does reduce the occurrence of self-explosion and solves the self-explosion problem that plagues the engineering parties. Therefore, hot dip is the most effective way to completely solve the problem of self-explosion in the world.
Studying the self-explosion of tempered glass is to find a better solution. Comparing the effectiveness and reliability of different solutions is to further reduce the self-explosion rate and reduce the loss caused by self-explosion. Based on the above analysis and comparison, combined with the actual situation of engineering glass, several suggestions are for reference only.
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