If bolts are going to operate at high temperatures (e.g., near furnaces, boilers, or heating systems), don’t choose them “the usual way.” As temperature rises, metal becomes less resistant to loads, and during long operation so-called creep can occur—very slow deformation under constant stress. That’s why the 8.8/10.9/12.9 strength class from “room-temperature” tables doesn’t tell the whole story when things get hot.

What typically works? Heat-resistant/heat-strength steels, sometimes austenitic stainless steel, and for really tough conditions—nickel alloys. The right choice depends not only on temperature, but also on the environment: steam, moisture, flue gases, or chemicals (oxidation can really take its toll).

What to watch out for: zinc plating and some coatings make little sense at high temperatures, joints can loosen due to thermal expansion, and threads may seize or “bake on” (an anti-seize paste helps here). And importantly: choose the whole set—bolt, nut, and washer should be compatible in terms of material.