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ASTM F-15 Alloy, commonly known as Kovar, is a controlled expansion alloy consisting of 29% Nickel, 17% Cobalt, and balance Iron. Kovar is produced to ensure good properties for machining and deep drawing as well as porosity free product. Its expansion characteristics match both borosilicate (Pyrex) glasses and alumina ceramics. It is one of the most popular of the controlled expansion alloys for hermetic sealing.