1. Self-explosion of tempered glass and its classification

1. Self-explosion classification of tempered glass Since the birth of tempered glass, it has been accompanied by self-explosion. The self-explosion of tempered glass can be described as the phenomenon that the tempered glass is automatically broken without direct external action. Self-explosion of tempered glass can occur during tempering, storage, transportation, installation, and use. Self-explosion can be divided into two types according to different causes: - Self-explosion caused by visible defects in the glass, such as stones, sand, air bubbles, inclusions, notches, scratches, burst edges, etc.; the second is caused by nickel sulfide (NIS) in the glass. Impurities and heterogeneous phase particles cause the self-explosion of tempered glass.

2. The characteristics of uncontrollable self-explosion of tempered glass The reason for the self-explosion of tempered glass is unclear, and the responsibility is difficult to understand. There is no certainty about the time of self-explosion. It may be just out of the oven, or it may be 1~2 months after leaving the factory, or it will be 1~2 years after leaving the factory. . According to incomplete understanding, the probability of most manufacturers' products is 3%. The rate of self-destruction is about the same; the probability of individual manufacturers' products may be even higher. The root cause of the self-explosion of tempered glass is that the glass contains nickel sulfide and heterogeneous particle impurities. How are the impurities mixed in? It has not been fundamentally found out how nickel is mixed into the glass. The most likely source is the use of equipment. Various heat-resistant alloys used in various nickel-containing alloy parts and furnaces. For oil-fired furnaces, nickel-rich condensates are commonly reported in small furnaces. Thiophane undoubtedly comes from the sulfur content in the batch and in the fuel. When the temperature exceeds 1000 °C, nickel sulfide exists in the molten glass in the form of droplets, and the solidification temperature of these small droplets is 797 °C. 1 gram of nickel sulfide can produce about 1000 small stones with a diameter of 0.15mm. Nickel sulfide can occur at any time after the production is completed, so it cannot be completely eliminated. So far, there is no effective way to prevent it, which is called "cancer of the glass pedicle". "The cancer of the glass wall came from the famous architect Foster: That year, the floor-to-ceiling-height glass of the London Town Hall block designed by Foster + Partners broke. The glass was clad, and the contractor had to start checking all the internal glass. A spokesman for the Greater London City Council said that based on initial investigations, the problem was that the glass contained nickel sulphide, which means that the glass was contaminated with nickel during the construction process. , Nickel reacts chemically with the sulfide in the glass, causing cracks. Nickel sulfides are difficult to replace after self-explosion, and the processing cost is high. At the same time, it will be accompanied by large quality complaints and economic losses, resulting in dissatisfaction of the owners and even more serious other Consequences. Call it "glass pedicle cancer."

2. The rate of self-explosion of chemical glass and the reasons for self-explosion

1. Self-explosion rate The domestic self-explosion rate is not consistent among manufacturers, ranging from 3% to 0.3%. -Generally, the self-explosion rate is calculated by the number of pieces, without considering the area size and thickness of a single piece of glass, so it is not accurate enough and cannot be compared more scientifically. In order to measure the rate of self-destruction uniformly, the assumptions must be determined. Set the uniform-conditions: every 5~8 tons of glass contains one nickel sulfide enough to cause spontaneous explosion; the average area of ​​each piece of tempered glass is 1.8mm; the nickel sulfide is evenly distributed. Then it is calculated that the self-explosion rate of 6mm thick tempered glass is 0.64% ~ 0.54%, that is, the self-explosion rate of 6mm tempered glass is about 3%. ~ 5%. This is basically in line with the actual value of domestic high-level processing enterprises. Even if it is produced according to the standard, the self-explosion of tempered glass cannot be completely avoided. Large buildings can easily use hundreds of tons of glass, which means that the presence of nickel sulfide and heterophase impurities in the glass is very high, so although the tempered glass is hot-dipped, self-explosion is still inevitable.

2. Reasons for uncontrollable self-explosion of tempered glass-nickel sulfide (NS) and heterogeneous phase particles The source of uncontrollable self-explosion of tempered glass is not only nis particles in traditional understanding, but also many other heterogeneous phase particles. Crack initiation and propagation in glass is primarily due to residual stresses developed in the vicinity of the particles. This kind of stress can be divided into two categories, one is the phase transformation stress in the process of phase transformation and expansion, and the other is the residual stress caused by the mismatch of thermal expansion coefficients. Nickel sulfide ( nis ) and heterogeneous particles. The interior of the glass contains nickel sulfide impurities, which exist in the state of small crystals. Under normal circumstances, the glass will not be damaged. However, due to the reheating of the tempered glass, the phase state of the nickel sulfide impurities has been changed. Freezing, it may take years for them to recover to the state. Because the nickel sulfide impurities in the low temperature state will increase in volume, local stress concentration will be generated inside the glass, and the self-explosion of the tempered glass will occur. However, only relatively large impurities will cause self-explosion, and the self-explosion of tempered glass can only occur when the impurities are at the core of tensile stress. Nis is a kind of crystal, there are two crystal phases: high temperature phase a-nis and low temperature phase β-nis, the phase transition temperature is 379°C, when the glass is heated in the tempering furnace, because the heating temperature is much higher than the phase transition temperature, nis are all transformed into a phase. However, in the subsequent quenching process, the a-nis did not have time to transform into β-nis and was frozen in the tempered glass. At room temperature, a:nis is unstable and tends to gradually transform into β-nis. This transition is accompanied by a volume expansion of about 2 to 4%, which causes the glass to undergo a huge phase transition tensile stress, resulting in self-explosion. From the scanning electron microscope pictures of nis stones extracted from glass fragments after self-explosion, it can be seen that the surface is uneven and very rough. Heterogeneous phase particles cause the self-explosion of the tempered glass, which can be seen in the cross-sectional photos of the glass fragments at the source of the rupture, the first cracking trace and the boundary zone of the secondary cracking caused by the bow of a spherical tiny particle.

3. How to identify the self-explosion of tempered glass? First of all, check whether the detonation point (the cracks of tempered glass are radial and have starting points) is in the middle of the glass. , resulting in stress concentration and the gradual development of cracks; if the detonation point is in the middle of the glass, see if the detonation point has a pattern (butterfly spot) composed of two small polygons similar to two butterfly wings, if the two small polygons are carefully observed There should be small black particles (nickel sulfide stones) visible to the naked eye on the common side (the torso part of the butterfly), which can be judged to be self-explosion; otherwise, it should be destroyed by external force. The typical feature of glass self-explosion is butterfly spots. The glass fragments are distributed radially, and there are two glass blocks shaped like butterfly wings in the radial center, commonly known as "butterfly spots". nis stones are located at the interface of two butterfly spots*.

4. Theoretical discussion on the mechanism of self-explosion of tempered glass. Radial stress r≥a Tangential stress r≥a The stress at the interface between particles and glass is negative for heterogeneous particles in the glass matrix, and the temperature during the cooling process is negative, so the radial direction around the particles Stress is compression and tangential stress is tension. SEM images and edge extrusion morphology of spherical elemental silicon particles in the glass intermediate layer. The radial stress around the particles is pressure, and the tangential stress is tension, so the tangential stress is the root of crack initiation.