Issues regarding the material compatibility of bolts and nuts
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Today, I came across the requirements regarding the materials for fully threaded studs and special nuts in the standards of the European system and the American system as outlined in (HG/T 20592~20635-2009) “Steel Pipe Flanges, Gaskets, and Fasteners” – specifically in Table 5.0.5 of the European system standard HG/T20613 and Table 5.0.4 of the American system standard HG/T20634. According to Table 5.0.5 of HG/T20613, the material for fully threaded studs should be 35CrMo, while that for special nuts should be 30CrMo; whereas Table 5.0.4 of HG/T20634 requires that the material for fully threaded studs be 35CrMo, with special nuts also made of 35CrMo. I have doubts regarding this: whether the materials of the studs and nuts need to be the same or it is acceptable for there to be a difference in their material strength. I would appreciate some guidance from those with more experience.
There are requirements regarding differences in strength between bolts and nuts; generally, the strength of the nut is slightly lower than that of the bolt
May I ask: Should hex head bolts of grade C (GB/T5780-2000) be used with hex nuts of grade C (GB/T41-2000)?
Generally, two different materials are used for bolts and nuts; the material of the nut is usually slightly lower in quality than that of the bolt. The hardness of the bolt should be higher than that of the nut. If both the bolt and the nut have the same hardness, it becomes necessary to ensure a difference in strength – typically, the nut’s strength is lower than that of the bolt. This is done primarily to prevent binding between them, as well as to facilitate replacement.