What is heat soak testing?
All float glass contains some level of imperfection. One type of imperfection is nickel sulfide (NiS) inclusion. Most NiS inclusions are stable and don’t cause any problems. There is, however, the potential for NiS inclusions that may cause spontaneous breakage in tempered glass without any load or thermal stress being applied.
Heat soak testing is a destructive test. Tempered Glass is put into a heat Soak Oven and brought to and held at a temperature of 555 degrees Farenheit for two hours. Most glass containing Nickel Sulphide will shatter during this process and thus be eliminated from the glass project. This causes glass containing NiS inclusions to break in the heat soak chamber, thus reducing the risk of potential field breakage.
Does heat soak zeros the risk completely?
Unfortunately, the heat-soaking process is not 100 percent effective, adds cost and carries the risk of reducing the compressive stress in tempered glass.
Most of float glass producers recommends heat soak process for fully toughened float glasses.
Heat-strengthened glass has a much lower potential incidence of spontaneous breakage than tempered glass. For applications where additional glass strength is required due to thermal stress, and safety glass is not mandated, Guardian recommends heat-strengthened or laminated glass to reduce the potential for spontaneous breakage.
Nickel Sulfide (NiS) inclusions:
While extremely rare, nickel sulfide inclusions are another potential cause of breakage.
Nickel sulfide (NiS) inclusions can occur if nickel-rich contaminants are present during the manufacturing process. These contaminants, such as stainless steel, can combine with sulphur to form NiS inclusions.
When glass is heat treated to obtain fully tempered glass, NiS inclusions change size from what is known as a low-temperature (LT) structure to a high-temperature (HT), crystalline structure. When cooled quickly, the NiS particle is unable to change completely back to its original form (LT).
Over a certain period of time, NiS will slowly convert to its (LT) form but with an increase in volume of about 2 to 4 percent. That increase in size can cause breakage. Float glass manufacturers work extremely hard to avoid any nickel-based contaminants in the batch mix. Instances of NiS inclusions are very unusual.
Design professionals can reduce the risk of breakage due to inclusions by specifying heat-strengthened glass, heat-soaking for fully tempered glass or laminated glass.
It has been concluded that the incident of critical NiS inclusions is, on average:
1 inclusion in 11,000lbs of glass for ordinary tempered glass EN12150 (nearest equivalent ASTM C1048)
1 inclusion in 882,000lbs of glass for heat soak tempered glass to EN 14179.
Sources:
www.guardianglass.com;
www.careyglass.com/
