In flange fastener selection, a common practice is that the nut's strength grade is typically one level lower than that of the mating bolt or stud. This is not a cost-cutting measure, but rather a mature and essential safety design in engineering.
First, this design ensures that potential failures are easier to detect. In connections for piping or pressure vessels, an unexpected bolt fracture can occur without warning, potentially leading to sudden flange separation and media leakage, which may cause severe accidents. By designing the nut with a slightly lower strength, it will undergo thread deformation or stripping first under extreme overload or thermal expansion. This type of damage is easily noticeable during tightening or disassembly, effectively preventing sudden catastrophic failures.
Second, this approach significantly reduces maintenance difficulty and costs. Nuts are compact and easy to replace. In the event of a minor overload, only the damaged nut needs to be replaced, and the bolt can remain in service. If the bolt were to fail first, extracting the broken shank would be highly difficult and could even damage the flange, resulting in exponentially higher repair costs.
Finally, this combination fully complies with national standards and mechanical principles. Relevant fastener standards actually recommend this higher-to-lower strength matching principle. Appropriately reducing the nut's hardness does not compromise the overall load-bearing capacity of the connection. Instead, it effectively prevents thread galling and allows the joint to better absorb external shocks.
Therefore, in industrial connections, specifying a nut with a slightly lower strength grade than the bolt is a strategic choice to achieve higher system safety at a lower cost, making connection failures controllable, visible, and easy to repair.
