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.