Grades O, A and B steel nuts are all designed to have no markings on the outside of the nut. Grade C nuts have three dashes on the outside, C3 nuts have the same dashes with a 3 imprinted on the outside as well. Grades D, DH, DH3, 1, 2, 2H, 2HM, 4, 7 and 7M steel nuts all feature letter and number imprints corresponding to their grade on the outside of the nut.
Steel nuts can be constructed out of different varieties of steel. Grades O, A, B and 1 are all constructed out of carbon steel. Medium carbon steels are employed in the construction of grades 2, 2H and 2HM steel nuts. Grades C3 and DH3 steel nuts are made out of corrosion-resistant steel. Carbon steel alloys are used to make certain grades of steel nuts, including grades 4, 7 and 7M steel nuts.
Proof load and yield strength are terms often used interchangeably. Grade O steel nuts offer 69,000-psi (pounds per square inch) proof load. Grade A nuts offer 90,000-psi proof load, grade B nuts offer either 105,000- or 120,000-psi proof load and grade 3 as well as grade C3 nuts offer 144,000-psi proof load. Grades D, 2, 2HM and 7M nuts offer 150,000-psi proof load while grades DH3, 2H, 4 and 7 offer 175,000-psi proof load. Grade 1 steel nuts offer 130,000-psi proof load.
The Rockwell scale measures hardness based on the depth of the impression that is left in a material when a load is applied. High numbers indicate harder materials on this scale and letters indicate the type of material (steel, diamond) used to make the impression. Grades O, A and B steel nuts offer C32 maximum Rockwell hardness levels. Maximum Rockwell hardness values on grades C, C3, D, DH, DH3, 4 and 7 steel nuts is C38. The Rockwell hardness values for grades 2HM and 7M is 237.