How should the hardness of studs and nuts be matched in a wet hydrogen sulfide environment?
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Section 5.4.4 of SHT3193-2017, \"Guidelines for the Design of Equipment Used in Hydrogen Sulfide-Containing Environments in the Petrochemical Industry,\" stipulates that the hardness of bolts should be less than or equal to 22HRC (237HBW). According to GB/T17394.4, this value should be converted to HB227. If bolts made of 35CrMo as specified in the drawings are used, their hardness (HB269–321) exceeds the required level; therefore, they need to be softened to some extent. As a result, their hardness becomes lower than that of the 30CrMo nuts used with them (HB234–285). Is it then necessary to soften the nut appropriately so that its hardness is lower than that of the stud? If so, why isn’t this mentioned in the standards?
150.2 It is specified that nuts need to have their tempering temperature adjusted in order to match the studs
For NBT47020, it is specified that the hardness of the nut should be 20–30 HB lower than that of the corresponding stud
The above are general situations; in special cases, the hardness of the nut may need to be higher than that of the stud. For example, when the nut is welded directly to the component, it is more reasonable and cost-effective to replace the stud rather than the nut. This approach is not recommended for use in extreme or highly aggressive media, or other special types of media; such considerations may be necessary when dealing with structural components. (These are just personal opinions; any corrections or criticisms are welcome.)
Furthermore, the hardness of the stud and its mechanical properties are matched with each other; if the hardness of the stud is reduced, its mechanical properties should also be adjusted accordingly