Superalloys and exotic metals are used to make fasteners that withstand high temperatures. Austenitic alloys are the primary types for use at elevated temperatures. They includenonheat-treated alloys such as the SAE 30300 series of stainless steels, as well as Hastelloy, Inconel, and Monel, and precipitation-hardening alloys, such as AMS 5725, and 17-7PH.
Hot-heading techniques have produced fasteners that have physical properties satisfactory for use at 1,500°F.
Mechanical fasteners made from refractory metal are used for short-term (usually only a few hours) exposure to temperatures up to 3,000°F. Columbium fasteners are usable in the 2,000 to 2,600°F range and tantalum is preferred for the 2,800 to 3,600°F bracket. Tungsten is the only fastener material that survives above 3,600°F, despite its susceptibility to severe oxidation if uncoated. Over a period of time, existing coatings can deteriorate, which can lead to catastrophic oxidation of refractory metals at these temperatures. At cryogenic temperatures, threaded fasteners made from nickel and iron-based alloys have excellent mechanical properties.
