1. Self-explosion of tempered glass and its classification

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

2. The characteristics of uncontrollable tempered glass self-explosion The cause of self-explosion of tempered glass is unclear, and the responsibility is difficult to understand. The self-detonation time is not certain. It may be just out of the oven, or it may be 1 to 2 months after leaving the factory, or it may only be 1 to 2 years after leaving the factory. The time that causes more self-explosion of tempered glass may be 4 to 5 years after the completion of the production of the product. . According to incomplete understanding, the probability of most manufacturers' products is 3%. The spontaneous explosion rate of the left and right; the probability of individual manufacturers’ products may be even higher. The root cause of the spontaneous explosion of tempered glass is because the glass contains nickel sulfide and heterogeneous particulate impurities. How did the impurities get mixed in? It has not been fundamentally found out, how nickel is mixed into the glass, and the most likely source is the equipment used Various nickel-containing alloy parts and furnaces. Various heat-resistant alloys used on them. For oil-burning furnaces, Pu reported that nickel-rich condensates were found in small furnaces. Sulfur is undoubtedly derived from the sulfur components in the batch materials and fuels. 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. One gram of nickel sulfide can produce about 1,000 small stones with a diameter of 0.15 mm. Nickel sulfide can occur at any time after the production is completed, so it is still not completely eradicated. There is no effective way to prevent it as "cancer of the glass pedicle wall". "The cancer of the glass wall came from the mouth of the famous architect Foster: That year, the glass from floor to ceiling of the London City Hall block designed by Ster & Partners broke. This city hall is close to the Tower Bridge, and it is completely used. The glass was cladding, and the contractor had to check all the internal glass. A spokesperson for the Greater London City Council said that according to preliminary investigations, the problem was that the glass contained nickel sulfide, that is, the glass was contaminated with nickel during the construction process. , Nickel and the sulfide in the glass chemically react, causing cracks. Nickel sulfide is difficult to replace after self-detonation, and the processing cost is high. At the same time, it will be accompanied by large quality complaints and economic losses, causing dissatisfaction and even more serious other Consequences. Call it the "cancer of the glass pedicle wall."

2. The spontaneous explosion rate of chemical glass and the cause of the spontaneous explosion

1. Spontaneous explosion rate The domestic spontaneous explosion rate of various manufacturers is not consistent, ranging from 3% to 0.3%. -The general self-explosion rate is calculated based on 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 with each other more scientifically. In order to uniformly measure the spontaneous explosion rate, it is necessary to determine the unified assumption. A unified condition is set: every 5-8 tons of glass contains a nickel sulfide that is sufficient to cause spontaneous explosion; the average area of ​​each piece of tempered glass is 1.8mm; the nickel sulfide is evenly distributed. 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 heterogeneous impurities in the glass is very high, so even though the tempered glass is hot-dipped, self-explosion is still inevitable.

2. Reasons for uncontrollable spontaneous explosion of tempered glass-nickel sulfide (NS) and heterogeneous particles The source of uncontrollable spontaneous explosion of tempered glass is not only the traditional nis particles, but also many other heterogeneous particles. The initiation and propagation of cracks in glass are mainly due to the residual stress generated near the particles. This type of stress can be divided into two types, one is the phase change stress during the phase change expansion process, and the other is the residual stress caused by the mismatch of the thermal expansion coefficient. Nickel sulfide (nis) and heterogeneous particles. The inside of the glass contains nickel sulfide impurities, which exist in the state of small crystals. Under normal circumstances, it will not cause glass damage. However, due to the reheating of the tempered glass, the phase state of nickel sulfide impurities is changed. The high temperature state of nickel sulfide is destroyed when the glass is quenched. Freezing, it may take years for them to return to the state, due to the low temperature state nickel sulfide impurities will produce volume increase, and local stress concentration inside the glass, then the tempered glass will explode. However, only relatively large impurities will cause spontaneous explosion, and only when the impurity is at the core of tensile stress can the tempered glass explode spontaneously. nis is a kind of crystal. There are two kinds of 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, the heating temperature is much higher than the phase transition temperature. All nis is transformed into a phase. However, in the subsequent quenching process, a-nis was too late to transform into β-nis, which was frozen in the tempered glass. At room temperature, a: nis is unstable and has a tendency to gradually change to β-nis. This transformation is accompanied by a volume expansion of about 2 to 4%, which causes the glass to withstand a huge phase change tensile stress, which leads to self-explosion. From the scanning electron micrograph of the nis stone extracted from the glass fragments after the self-explosion, it can be seen that its surface is undulating and very rough. Heterogeneous phase particles cause the tempered glass to explode spontaneously, which can be seen in the cross-sectional photos of the glass fragments at the source of the fracture. The boundary zone between the first cracking trace and the second fragmentation bowed by a spherical minute particle can be seen.

3. How to identify the spontaneous explosion of tempered glass. First look at whether the detonation point (the cracks of tempered glass are radial and all have starting points) are in the middle of the glass, such as at the edge of the glass, usually because the glass has not been chamfered and edged or the glass edge is damaged. , Resulting in stress concentration and cracks gradually developing; if the initiation point is in the middle of the glass, see if the initiation point is composed of two small polygons that resemble two butterfly wings (butterfly spots), if there are two small polygons carefully observed The common edge (the torso of the butterfly) should have small black particles (nickel sulfide stones) visible to the naked eye, which can be judged to be self-explosive; otherwise, it should be destroyed by external force. The typical feature of glass spontaneous explosion is butterfly spots. The glass fragments are distributed radially, and there are two glass blocks shaped like butterfly wings in the center of radiation, commonly known as "butterfly spots". The nis stone is located on the interface of the two butterfly spots*.

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