The following four conditions may warrant fastener replacement:
1. Elevated temperatures, like welding, could affect heat treatment and/or plating.
Other cautions include:
- Zinc plating starts burning off at 140 degrees F and completely burns off at 250 degrees F.
- Cadmium plating: Completely burns off at 400 degrees F.
- Black finish (thermo-black from heat treat): Exceeding 400 degrees F will destroy heat treatment.
2. Excessive or undue stresses. If one fastener of a group breaks, like one stud out of four on a wheel of a car, it can overstress the remaining fasteners, resulting in their failure.
3. Corrosion. Rust can create stress points that lead to failure and, obviously, if left long enough, can weaken or destroy fasteners.
4. Critical applications. When in doubt, replace these fasteners rather than risk their failure.