SAE J429 Mechanical and Material Requirements for Externally Threaded Fasteners has a requirement for the core hardness of Grade 8 fasteners to be HRC 33-39.
SAE J995 Mechanical and Material Requirements for Steel Nuts has a requirement for the hardness of nuts to be as follows:
size 5/8 to 1 inch: 26-34 HRC
size 1 to 1.5 inch: 26-36 HRC
You can obtain these standards from SAE using the following link: