ASTM A563 Grade A heavy hex nuts are made from carbon steel. ASTM A563 dimension specifications are governed by ASME B18.2.2. Grade markings are not required for A563 Grade A heavy hex nuts, though in some cases ASTM A563 Grade A nuts may be marked with the letter A. ASTM A563 covers standard specifications for carbon and alloy steel nuts. Heavy hex nuts provide increased bearing surface area over finished nuts.