Jinan LIJIANG Glass hopes that with the help of professional analysis articles, it can help readers, especially some practitioners in the glass deep processing industry, to understand tempered glass, clarify the reasons for its self-explosion, and thus understand the selection and application of glass curtain wall projects.
1. Causes and solutions of self-explosion of tempered glass
1.1 Self-destruction and its classification
The self-explosion of tempered glass can be expressed 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: one: self-explosion caused by visible defects in the glass, such as stones, sand, bubbles (the entry "bubble" is provided by the industry encyclopedia), inclusions, gaps, scratches, explosions Bian et al; 2: It is a self-explosion caused by the expansion of nickel sulfide (NiS) impurities in the glass. These are two different types of self-destruction, which should be classified, treated differently, and dealt with and handled in different ways. The former is generally visible and relatively easy to detect, so it is controllable in production. The latter is mainly caused by the volume expansion of tiny nickel sulfide particles in the glass, which cannot be visually inspected and therefore uncontrollable.
In actual operation and processing, the former can generally be eliminated before installation, while the latter continues to exist because it cannot be inspected, which becomes the main factor for the self-explosion of tempered glass in use. Nickel sulfide is 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 owner and even more serious consequences. Therefore, the spontaneous explosion caused by nickel sulfide is the focus of our discussion.
1.2 Self-explosion mechanism of tempered glass
The expansion of nickel sulfide inside the tempered glass is the main reason for the self-explosion of the tempered glass. After the glass is tempered, the surface layer forms compressive stress. The inner core layer is under tensile stress, and the compressive stress and tensile stress together form a balanced body.
The glass itself is a brittle material, resistant to pressure but not tensile, so most of the breaking of glass is caused by tensile stress. When the nickel sulfide crystal in the tempered glass undergoes a phase change, its volume expands, and the expansion of the nickel sulfide in the tensile stress layer of the glass plate causes greater tensile stress inside the tempered glass. When the tensile stress exceeds the limit that the glass itself can bear, It will cause the tempered glass to explode.
Research has proved that: the main glass material quartz sand or sandstone is brought into nickel, and the fuel and auxiliary materials are brought into sulfur, which is burned and melted in a high-temperature furnace at 1400 ° C ~ 1500 ° C to form nickel sulfide.
When the temperature exceeds 1000℃, nickel sulfide is randomly distributed in the molten glass in the form of droplets. When the temperature dropped to 797°C, these small droplets crystallized and solidified, and the nickel sulfide was in the α-NiS crystal phase (hexagonal crystal) in a high-temperature state. When the temperature continued to drop to 379 °C, the crystal phase transitioned to β-NiS (trigonal system) in a low-temperature state, accompanied by a volume expansion of 2.38%. The speed of this transformation process depends not only on the percentage content of different compositions (including Ni7S6, NiS, and NiS1.01) in the nickel sulfide particles but also on the surrounding temperature.
If the nickel sulfide phase transformation is not completely transformed, even under the temperature conditions of natural storage and normal use, this process continues, but the speed is very low.
When the glass is tempered and heated, the inner core temperature of the glass is about 620℃, and all the nickel sulfide is in the high-temperature α-NiS phase. Subsequently, the glass enters the air grid to be quenched, and the nickel sulfide in the glass undergoes a phase transition at 379 °C. Different from the float annealing kiln, the tempering quenching time is very short, and it is too late to transform into the low-temperature β-NiS, and the high-temperature nickel sulfide α phase is "frozen" in the glass. Rapid quenching enables the glass to be tempered, forming a unified stress balance body with external pressure and internal tension.
In tempered glass, the phase transformation of nickel sulfide continues at a low speed, the volume expands and expands continuously, and the force on the surrounding glass increases accordingly. The core of the tempered glass plate itself is a tensile stress layer. When the nickel sulfide in the tensile stress layer undergoes a phase change, the volume expansion also forms tensile stress. The superposition of these two tensile stresses is enough to cause the rupture of the tempered glass, that is, self-explosion.
Further experiments show that for tempered glass with a surface compressive stress of 100MPa, the internal tensile stress is about 45MPa. At this time, any nickel sulfide with a diameter greater than 0.06mm in the tensile stress layer can cause self-explosion. In addition, according to the analysis of the statistical results of self-explosion research, more than 95% of the self-explosions are caused by nickel sulfide with a particle size distribution between 0.04mm and 0.65mm. According to the fracture mechanics of the material, the average particle size of the spontaneous explosion caused by nickel sulfide is calculated to be 0.2 mm. Therefore, glass processing industry practitioners in some developed countries unanimously believe that nickel sulfide is the main reason for the self-explosion of tempered glass. There are other factors in the self-explosion of tempered glass: unreasonable glass slotting and drilling, poor quality of the original glass, uneven thickness such as patterned glass, uneven stress distribution such as curved tempered glass, and regional tempered glass.
Jinan LIJIANG Glass surveyed the safety status of existing curtain walls in 10 overseas cities that have sold glass deep processing equipment. The survey samples were selected based on the self-inspection and self-inspection of 10 cities. In this investigation, 437 pieces of curtain wall glass were damaged. There are 17 items in the survey of all-glass curtain walls, of which 10 items found that large glass was broken, a total of 68 pieces, 3 pieces of glass rib were broken, and many glass curtain walls were found to have no rib glass. Insulating glass (the entry "insulating glass" is provided by Google) leaked 180 pieces of air, and the phenomenon of coating glass peeling is also more common in individual cities. In the survey, 9 curtain wall projects with important hidden dangers were found, accounting for 9.38% of the total number of surveyed projects. If the self-explosion and rupture of tempered glass are removed, the proportion will drop to 2.3%. The use of tempered glass for curtain wall doors and windows has led to high glass breakage accidents in glass curtain walls and doors and windows. It is urgent to change this situation.
According to the analysis of glass breakage accidents of curtain walls and doors and windows, this article suggests that anti-scattering glass should be used for curtain walls and doors, and windows. Self-explosion of tempered glass and its classification 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.
3. Self-explosion can be divided into two types according to different causes:
One is the self-explosion caused by visible defects in the glass, such as stones, sand, air bubbles, inclusions, notches, scratches, burnt edges, etc.;
The second is the self-explosion of tempered glass caused by nickel sulfide (NIS) impurities and heterogeneous phase particles in the glass.
BALLANTYNE first proposed the nickel sulfide mechanism of self-explosion of tempered glass in 1961. Through the study of 250 cases of self-explosion, BORDEAUX and KASPERr found that the diameter of nickel sulfide caused by self-explosion was between 0.04 and 0.65mm, and the average particle size was 0.2mm. Newly discovered heterogeneous phase particles cause self-explosion of tempered glass.
These are two different types of self-destruction, which should be classified, treated differently, and dealt with and handled in different ways. The former is generally visible and relatively easy to detect, so it is controllable in production. The latter is mainly caused by the volume expansion of tiny nickel sulfide particles in the glass, which cannot be visually inspected and therefore uncontrollable. In actual operation and handling, the former can generally be removed before installation, while the latter continues to exist because it cannot be inspected, becoming the main factor for the self-explosion of tempered glass in use.
Figure 1 The tempered glass self-explosion 1
4. The characteristics of uncontrollable tempered glass self-explosion
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 may be 1-2 years after leaving the factory...
According to incomplete understanding, the probability of most manufacturers' products is a self-explosion rate of about 3‰; the probability of individual manufacturers' products may be even higher. The fundamental reason for the self-explosion of tempered glass is that the glass contains nickel sulfide and heterogeneous particles. How the impurities are mixed has not yet been fundamentally found out. How nickel is mixed into the glass is the most likely source. A variety of heat-resistant alloys are used in nickel-containing alloy parts and furnaces. For oil-fired furnaces, nickel-rich condensates have been reported to be found in small furnaces. Sulfur undoubtedly comes from the sulfur-containing components in the batch and 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 eliminated. So far, there is no effective prevention method called "cancer of glass curtain wall".
"The cancer of the glass curtain wall" comes from the mouth of the famous architect Foster: That year, several pieces of glass from the floor to the ceiling height of the London City Hall designed by Foster + Partners broke. The town hall, which is close to Tower Bridge, is entirely clad in glass, and contractors had to set about inspecting all the interior glass. A spokesman 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 elements during the construction process, and the nickel and the sulfide in the glass reacted chemically, causing cracks. Nickel sulfide is 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 owner and even more serious consequences. Call it "the cancer of the glass curtain wall".
Figure 2 "The cancer of the glass curtain wall" 1
5. Self-explosion rate of tempered glass and reasons for self-explosion
Self-explosion rate | At present, the self-explosion rate of developed and developing countries 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. To measure the self-destruction rate uniformly, a uniform assumption must be determined. Set uniform conditions: every 5-8 tons of glass contains a nickel sulfide enough to cause self-explosion; the area of each piece of tempered glass is 1.8 square meters and the nickel sulfide is evenly distributed. Then the calculated self-explosion rate of 6mm thick tempered glass is 0.64% to 0.54%, that is, the self-explosion rate of 6mm tempered glass is about 3‰ to 5‰. This is consistent with the actual value of high-level glass processing enterprises.
Even if it is completely 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 there is a high probability of nickel sulfide and heterophase impurities in the glass. Therefore, although the tempered glass is hot-dipped, self-explosion is still inevitable.
Figure 3 The self-explosion of tempered glass 1
6. Reasons for uncontrollable self-explosion of tempered glass
The source of uncontrollable self-explosion of tempered glass is not only nis particles in traditional understanding but also many other heterogeneous particles. The initiation and propagation of cracks in glass (the entry "crack" is provided by the Encyclopedia of Industry) are mainly caused by residual stress generated in the vicinity of the particles. This type of stress can be divided into two categories, one is the phase transformation stress during the phase transformation expansion process, and the other is the residual stress caused by the mismatch of thermal expansion coefficients. Nickel sulfide (nis) and heterogeneous particles. 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. The high-temperature α state of nickel sulfide occurs when the glass is quenched. After being frozen, it may take several years for them to recover to the β state. Because the nickel sulfide impurities in the low-temperature β state will increase in volume and generate local stress concentration inside the glass, 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 α-nis and low-temperature phase β-nis, the phase transition temperature is 379 ℃, when the glass is heated in the tempering furnace because the heating temperature is much higher than the phase transition temperature, nis All transformed into alpha phase. However, during the subsequent quenching process, the α-nis did not have time to transform into β-nis and was thus frozen in the tempered glass. At room temperature, α-nis is unstable and tends to gradually transform into β-nis. This transition is accompanied by a volume expansion of about 2 to 4%, subjecting the glass to a huge phase transition tensile stress, leading to self-explosion. It can be seen from the scanning electron microscope pictures of nis stones extracted from glass fragments after self-explosion that their surfaces are undulating and very rough.
The self-explosion of the tempered glass caused by the heterogeneous phase particles can be seen in the cross-sectional photos of the glass fragments at the source of the rupture, and the boundary zone between the first cracking mark and the secondary cracking caused by a spherical tiny particle.
7. How to identify the self-explosion of tempered glass
First of all, look at whether the initiation point (the cracks in the tempered glass are radial and have starting points) is in the middle of the glass, such as at the edge of the glass, generally because the glass has not been chamfered and edged or the glass edge is damaged, resulting in stress concentration and gradual development of cracks If the detonation point is in the middle of the glass, see if the detonation point has a pattern similar to two butterfly wings composed of two small polygons (butterfly spots), if so, carefully observe the common edge of the two small polygons (the torso of the butterfly) There should be small black particles (nickel sulfide stones) visible to the naked eye, which can be judged to be self-explosion; otherwise, it should be destroyed by an 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 center of the radiation, commonly known as "butterfly spots". Nis stones are located on the interface of two "butterfly spots".
Radial stress r≥a Tangential stress r≥a The stress at the interface between the particle and the glass For heterogeneous particles in the glass matrix, the temperature difference during the cooling process is negative, so the radial stress around the particle is pressure, and the tangential stress is pulling.
Scanning electron microscope image and edge extrusion of spherical elemental silicon particles in the glass intermediate layer (the entry "extrusion" is provided by the Encyclopedia of the Industry), the radial stress around the particles is pressure, and the tangential stress is tension, so the tangential stress is the origin of the crack initiation.
8. Theoretical discussion on the mechanism of self-explosion of tempered glass
8.1 The self-explosion of tempered glass is an important problem that needs to be solved urgently in the current safety of glass curtain walls.
But for the concept of safety glass, the traditional concept is that (full) tempered glass belongs to safety glass. In addition to the high strength, the main reason is that when the (full) tempered glass is broken, the whole glass will be broken into small honeycomb-shaped obtuse-angle particles, which are not easy to hurt people. Through this investigation and the practice of many accidents, this concept has been questioned, and it is necessary to discuss the use of safety glass for glass curtain walls in high-rise buildings (the entry "high-rise buildings" is provided by the Encyclopedia of Industry). For the use of safety glass in the glass curtain wall of high-rise buildings, the main safety concern is that the glass breaks and falls and hurts people. There should be three requirements here:
First, the glass has sufficient strength to withstand the design load without damage.
Second, if the glass is broken, it must have anti-collision and scattering properties, so that it will not fall and scatter when it is in a broken state.
The third is sufficient fracture toughness k1c.
8.2 Tempered glass has two safety factors
High strength and its failure form are obtuse-angle small particles. However, it does not have anti-shattering and scattering properties, which is a key safety factor for high-rise building glass curtain walls, and the resulting unsafe consequences.