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What is heat soaked testing?

Tempered Glass Heat Soaked Testing

A rare but nonetheless undesirable property of tempered glass is spontaneous breakage: tiny inclusions of nickel sulfide, invisible to the naked eye, expand in volume over time and can cause the pane to break unexpectedly even years after installation. One reliable means of detecting nickel sulfide inclusions is the heat soaked test, in which the panes are warmed to about 290℃. During a soak time of about four hours in the heat soak oven, the tempered glass panes will nickel sulfide inclusion will probably break and hence never be installed in the first place.

Table of Contents

What is heat soaked tempered glass? Why is heat soaked test treatment necessary?

Homogenized tempered glass, also known as heat-soaked tempered glass, refers to soda-lime-silica tempered glass (HST) that has been processed under specific conditions. Tempered glass is widely used as a safety glass in various fields, especially in high-rise buildings. However, tempered glass has a self-explosion problem, and after self-explosion, the tempered glass breaks into a large number of blunt cornered pieces that can be dangerous and even fatal to people due to the effect of gravity acceleration, even if the particles are small. By homogenizing the tempered glass and causing it to explode before leaving the factory, the self-explosion rate of tempered glass during use can be greatly reduced.

What is tempered glass self-explosion, and what are the main reasons?

Tempered glass self-explosion refers to the automatic explosion that occurs without external forces, leading to significant accidents. According to observations and tests of 17,760 pieces of tempered glass on glass curtain walls in Australia over the past 12 years, there were 306 cases of self-explosion, with a self-explosion rate of 1.72%. Self-explosion is one of the inherent characteristics of tempered glass, which can occur during processing, storage, transportation, installation, and use.

The reasons for tempered glass self-explosion are multifaceted, mainly due to three aspects: excessive internal stress, uneven stress, and defects in the glass.

The impact of glass quality defects on tempered glass self-explosion

The quality defects of the glass raw material have a decisive impact on tempered glass self-explosion, but the degree of impact and mechanism of different defects are different, as follows:

①There are stones and impurities in the glass: Impurities in the glass are the weak points of tempered glass and the locations of stress concentration. If a stone is in the tensile stress zone of the tempered glass, it is an important factor leading to explosion. Stones exist in the glass with a different expansion coefficient than the glass body. The stress concentration around the stone area after glass tempering increases exponentially. When the expansion coefficient of the stone is smaller than that of the glass, the tangential stress around the stone is in a tensile state. The crack extension associated with the stone is prone to occur.

②There are scratches, burst marks, and deep burst edges on the glass surface caused by processing or improper operation, which can easily cause stress concentration or lead to tempered glass self-explosion.

③The crystallized sulfide in the glass is the main cause of tempered glass self-explosion. After the glass is tempered, the surface layer forms compressive stress, and the internal core layer is under tensile stress. Compressive stress and tensile stress together form an equilibrium body. When the sulfide crystal in the tempered glass undergoes a phase change, its volume expands. The expansion of the sulfide under tensile stress in the core layer of the glass increases the tensile stress inside the tempered glass. When the tensile stress exceeds the limit that the glass can withstand, the tempered glass will self-explode.

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What is the impact of uneven stress distribution and displacement on the spontaneous breakage of tempered glass?

Tempered glass has a tendency to spontaneously break due to uneven and asymmetrical temperature gradients along its thickness direction during heating or cooling, which can cause “wind blast” during rapid cooling. If the tensile stress zone is shifted to one side or to the surface of the product, tempered glass can form spontaneous breakage.

How does the degree of tempering affect the spontaneous breakage of tempered glass?

When the degree of tempering of glass is too strong, the stress generated in the glass exceeds the ultimate strength of the glass, leading to spontaneous breakage. It is generally believed that the limit of glass tempering technology is a tempering degree of 9N/cm. If it exceeds this limit, the internal strength of the glass is lower than the surface strength, and the fracture starts from the inside and occurs under a load lower than the allowable load of tempered glass, especially when there are defects in the tensile stress layer, making it more prone to stress concentration and spontaneous breakage.

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How can we prevent and reduce the spontaneous breakage of tempered glass?

To prevent and reduce the spontaneous breakage of tempered glass, we should consider and pay attention to factors such as the original glass, tempering production environment, and production process system. Common measures to prevent and reduce the spontaneous breakage of tempered glass mainly include using high-quality original glass, reducing stress values, ensuring even stress distribution, and homogeneous treatment.

What is the impact of reducing stress values on preventing spontaneous breakage of tempered glass?

The stress distribution in tempered glass is such that the two surfaces of the tempered glass are in a compressive stress state, while the plate core layer is in a tensile stress state. The stress distribution along the glass thickness is similar to a parabola, with the center of the glass thickness being the vertex of the parabola, where the tensile stress is the greatest. The stress near the two surfaces of the glass is in a compressive stress state, and the zero stress surface is approximately located at 1/3 of the glass thickness. By analyzing the physical process of rapid cooling of tempered glass, it can be known that the maximum tensile stress inside the glass is roughly proportional to the surface tension of tempered glass, i.e., the tensile stress is about 1/3 to 1/2 of the compressive stress. Domestic manufacturers generally set the surface tension of tempered glass at around 100MPa, which may be higher in actual situations. The tensile stress of tempered glass itself is 32-46MPa, and the tensile strength of glass is 59-62MPa. As long as the stress generated by sulfide expansion is 30MPa, it is enough to cause spontaneous breakage. If the surface stress is reduced, the inherent tensile stress of tempered glass will be correspondingly reduced, which can help reduce the occurrence of spontaneous breakage.

The US ASTM C1048 specifies that the surface stress range of tempered glass should be greater than 69MPa, and that of semi-tempered (heat-strengthened) glass should be 24-52MPa. BG 17841 specifies that the stress range of semi-tempered glass is 24MPa < 69MPa. The Chinese GB 15763.2-2005 “Safety Glass for Buildings Part 2: Tempered Glass” requires that the surface stress should not be less than 90MPa, which reduces the previous standard of 95MPa by 5MPa, and is beneficial to reduce spontaneous breakage.

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How to maintain stress uniformity to prevent the impact of tempered glass self-explosion?

The uneven stress in tempered glass can significantly increase the rate of self-explosion to an alarming degree. Self-explosion caused by stress unevenness can sometimes be very concentrated, especially for a specific batch of bent tempered glass, with a shockingly high self-explosion rate and the possibility of continuous self-explosion. The main reasons for this are local stress unevenness and the displacement of tensile layers in the thickness direction, as well as the quality of the glass substrate itself. Uneven stress can greatly reduce the strength of the glass, effectively increasing the internal tensile stress and hence the self-explosion rate. If the stress distribution of tempered glass can be made uniform, the self-explosion rate can be effectively reduced.

What is the mechanism by which sulfur inclusion causes tempered glass self-explosion?

The expansion of sulfur inclusions inside tempered glass is the main cause of tempered glass self-explosion. The main material for glass production, such as quartz sand or sandstone, contains sulfur inclusions. During the high-temperature melting process at 1400-1500°C, the fuel and auxiliary materials bring in sulfur, which reacts with the iron in the glass substrate to form sulfur inclusions. When the temperature exceeds 1000°C, these small liquid droplets randomly distribute in the molten glass. When the temperature drops to 797°C, these droplets crystallize and solidify, forming the high-temperature a-NiS phase (hexagonal crystal). When the temperature continues to drop to 379°C, a phase transition occurs, resulting in the low-temperature y-NiS phase (trigonal crystal system) with a volume expansion of 2.38%. The speed of this transition depends on the content of different components (including Ni7S6 and NiS) in the sulfur inclusion particles and the surrounding temperature. If the phase transition of sulfur inclusion is not completely converted, this process will continue, albeit at a slow rate, even under natural storage and normal usage temperature conditions.

During the heating process of glass tempering, the core temperature inside the glass is about 620°C, and all sulfur inclusions are in the high-temperature a-NiS phase. Afterwards, the glass is rapidly cooled by forced convection, and the sulfur inclusion in the glass undergoes a phase transition at 379°C. Unlike float glass annealing kilns, the rapid cooling time of tempering is short, and there is not enough time for the sulfur inclusion to transform into the low-temperature y-NiS phase, but rather, it is “frozen” in the high-temperature a-NiS phase in the glass. Rapid cooling allows the glass to be tempered, forming a stress equilibrium body with external compression and internal tension. In tempered glass, the sulfur inclusion undergoes a slow and continuous phase transition, with volume expansion and expansion, increasing the force acting on the surrounding glass. The glass substrate itself is already under tensile stress, and the volume expansion of sulfur inclusion within the tensile stress layer also forms tensile stress. When these two types of stress are superimposed, it is enough to trigger the rupture or self-explosion of tempered glass.

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What is the principle of homogenization treatment? What are the technical difficulties?

Homogenization treatment heats tempered glass to 290°C and holds it for a certain period of time to promote the rapid completion of phase transformation of nickel sulfide in tempered glass, which artificially induces the glass to break in the homogenization furnace in the factory before it may explode during use, reducing the risk of tempered glass self-explosion after installation.

In principle, homogenization treatment is neither complicated nor difficult, but in practice, it is not easy to achieve this process indicator. Research has shown that there are multiple specific chemical structures of nickel sulfide in glass, such as Ni7S6, NiS, NiS2, etc., not only with different proportions of each component but also possibly doped with other elements. Their phase transition speed is highly dependent on temperature. Studies have shown that the phase transition 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 undergoes the same temperature regime. Otherwise, on the one hand, glass with a low temperature may not fully transform nickel sulfide due to insufficient holding time, weakening the effect of thermal immersion; on the other hand, when the glass temperature is too high, it may even cause nickel sulfide to undergo reverse phase transition, resulting in even greater hidden dangers. Both of these situations can lead to homogenization treatment being ineffective or even counterproductive. Therefore, the control of heating time, temperature, and temperature uniformity of the homogenization furnace will determine the properties and functions of the homogenized tempered glass.

How to carry out homogenization treatment?

The homogenization treatment process includes three stages: heating, holding, and cooling.

(1) The heating stage begins at the ambient temperature where all glass is located and ends at the moment when the surface temperature of the last piece of glass reaches 280°C. The furnace temperature may exceed 320°C, but the glass surface temperature should not exceed 320°C, and the time when the glass surface temperature exceeds 300°C should be minimized.

(2) The holding stage begins when the surface temperature of all glass reaches 280°C, and the holding time is at least 2 hours. During the entire holding stage, the glass surface temperature should be maintained within the range of 290°C ± 10°C.

(3)The cooling stage begins when the last piece of glass that reaches 280°C completes a 2-hour holding and the glass temperature drops to the ambient temperature in this stage. The cooling rate should be controlled to minimize the damage caused by thermal stress to the glass when the furnace temperature drops to 70°C, the end of the cooling stage can be considered.

What are the requirements for homogenization treatment in a homogenization furnace?

The homogenization furnace uses convection heating. The hot air flow should be parallel to the glass surface and flow smoothly between each piece of glass, and should not be hindered by glass breakage. Measures should be taken during the homogenization treatment of curved tempered glass to prevent irregular glass shapes from obstructing smooth airflow.
The inlet and outlet of the air should not be hindered by glass breakage either.
What are the requirements for glass support during homogenization treatment?

The glass can be supported vertically and should not be fixed or clamped by external force, and should be in a state of free support.
The vertical support can be absolutely vertical or at an angle less than 15° from the absolute vertical.
Glass should not come into contact with other glass pieces.
What are the requirements for glass spacing during homogenization treatment?
The spacing between the glass should allow unobstructed airflow, and the spacing material should not block the airflow. In general, the minimum recommended spacing between glass pieces is 20mm. When there are significant differences in glass size or when glass with holes and/or grooves are placed on the same bracket to prevent glass breakage, the spacing between glass pieces should be increased.

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