Is it okay to use a mild steel nut with a high tensile bolt?
Nut thickness standards have been drawn up on the basis that the bolt will always sustain tensile fracture before the nut will strip. If the bolt breaks on tightening, it is obvious that a replacement is required. Thread stripping tends to be gradual in nature. If the thread stripping mode can occur, assemblies may enter into service which are partially failed, this may have disastrous consequences. Hence, the potential of thread stripping of both the internal and external threads must be avoided if a reliable design is to be achieved. When specifying nuts and bolts it must always be ensured that the appropriate grade of nut is matched to the bolt grade.
The standard strength grade (or Property Class as it is known in the standards) for many industries is 8.8. On the head of the bolt, 8.8 should be marked together with a mark to indicate the manufacturer. The Property Class of the nut matched to a 8.8 bolt is a grade 8. The nut should be marked with a 8, a manufacturer's identification symbol shall be at the manufacturer's discretion.
Higher tensile bolts such as property class 10.9 and 12.9 have matching nuts 10 and 12 respectively. In general, nuts of a higher property class can replace nuts of lower property class (because as explained above, the 'weakest link' is required to be the tensile fracture of the bolt).