Heat Strengthened glass is a semi tempered which is two times stronger than Annealed glass. It has higher mechanical and thermal strength when compared to annealed glass having the same width and thickness. Heat strengthened glass is more difficult to break than annealed glass but it breaks into large fragment which might be injurious and risky to handle.
In this type of glass the basic float glass is heated to a very high temperature and then cooling is done very slowly. The cooling is slower than Tempered glass but faster than Annealed glass which results in less internal stress in Heat strengthened glass .This type of glass retains most of the properties of normal float glass and its breakage pattern is very similar to Annealed Glass but it is not prone to spontaneous breakage as is the case with tempered glass.
All holes have to be drilled and sizing is to be done before the semi tempering of glass.
Heat Strengthened Glass is used in the general glazing of the building as it can withstand high wind loads, thermal loads and mechanical loads. However it is not used in any safety glazing applications.