ASTM A194 Gr7 vs A194 Gr4 heavy hex nuts


While A194 Grade 4 and A194 Grade 7 nuts are mechanically identical and have the same intended applications, the only difference is the chemical values.
Grade 4 was created to be a low-cost alternative to Grade 7 by using a lower cost raw material. Unfortunately, this did not work out. The idea was that the material used to make Grade 4 was cheaper, and therefore the nuts would be cheaper.
In practice that was not the case. Since the steel used to manufacture Grade 7 nuts is mass produced and the material used to make Grade 4 was not, Grade 7 nuts remained cheaper, thus defeating the entire reason for Grade 4 to exist.
As far back as 2014, the ASTM Committee that covers the A194 specification planned to phase out Grade 4 and explicitly stated the substitutability of Grade 7 for Grade 4. In 2017, Grade 4 nuts were removed from the A194 specification.



ASTM A194 Nuts


Difference in Chemical Requirements

Grade C % Mn % Cr % Mo %
4 .40-.50 .70-.90 ... .20-.30
7 .37-.49 .65-1.10 .75-1.20 .15-.25

C-Carbon 
Mn-Manganese 
Cr-Chromium 
Mo-Molybdenum 

Source.. Portland Bolt