Hardness follows strength and heat treatment, so it catches a lot that was tempered wrong. It also sets ceilings: B7 and B16 may not exceed 321 HBW (35 HRC). Grades for sour (hydrogen sulfide) service are tempered softer and capped lower: B7M and L7M studs at 235 HBW (99 HRB), 2HM and 7M nuts at 159 to 235 HBW. One over-hard part can crack in sour service, so the specifications require every piece of these grades to be hardness tested, not a sample.
Hardness tracks strength but does not replace a tensile test: where the two disagree on minimum strength, the tensile test decides (ASTM A962).
Read the full hardness testing page →
model dataRockwell B scale (1/16 in. ball, 10 kgf preload, 100 kgf total) and Brinell (10 mm tungsten-carbide ball, 3000 kgf, held 10 to 15 s) from ASTM A370. B7M limits, 100% testing and testing on the stud's end from ASTM A193 Table 2 and section 9.2.2; Brinell on bolts only over 1-1/2 in. from ASTM F606. The testers and microscope are generic, the readings are for illustration, and the depths in the Rockwell close-up are exaggerated.
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