Why Are A194 Grade 7 Nuts Not Recommended With High Strength Bolts?
I have noticed in specifications from multiple organizations that ASTM A193 Grade B7 bolts are selected along with ASTM A194 Grade 2H nuts. I know that from ASME B16.5 Flanges and Flanged Fittings, the recommended high strength bolt is ASTM A193 Grade B7. However I cannot find in ASTM A193, A194, ASME B16.5, B18.2.1, B18.2.2, B31.1 or any other code or standard where it recommends using A194 Gr. 2H nuts with this bolt. Why would A194 Grade 7 nuts not be recommended? They have all of the same or better mechanical properties as Grade 2H and they are the same material composition as the A193 Gr. B7 bolts.
You are correct that none of those publications specifically pair a grade of nut with a grade of bolt, leaving it open to a bit of interpretation. The reason that A194 grade 2H nuts are used more often with A193 grade B7 bolts is simply cost. Both A194 2H and A194 grade 7 nuts have the same minimum mechanical requirements, they differ only in chemistry. The industry has chosen 2H nuts to be the primary nut for B7 bolts, as well as allows them to be substituted for A563 grade C and DH nuts for use with ASTM A325 and A490 structural bolts. The result is that, in addition to be a less costly nut to manufacture, they are much more widely mass produced, leading to economies of scale. That said, there is no reason that a specific engineer cannot require grade 7 nuts on a particular job if he feels that they will perform better in that application. The cost will be higher and the lead time may be slower due to availability, but the A194 grade 7 nuts would be acceptable for use with A193 B7 bolts or studs.