Why are you worried about minimum hardness? The material specification provides a maximum hardness only to ensure sufficient tempering for sufficient low temperature fracture toughness in service. By the way, there is no correct or specified minimum hardness value if the material meets the chemical requirements of ASTM A 320 L7M.
Use the minimum required strength levels as provided in the material speicfication for service application.
Thank you for your reply but I have been told in the ASTM A320 L7M standard there is a min hardness on the bolts of 200-235HB I just wondered if it is correct as they have told me it is in the ASTM A320 L7M latest spec in one of the sections i think its 6,2 not sure. I have not got the spec to hand so wondered if anyone was familiar with the spec. I know it has to meet a certain mechanical req also but did not know about there being a min hardness.
Upon further review, there is a Note in the SA 320 specification, 2007 edition for Table 1 that states under the maximum hardness for Grade L7M;
(B)To meet the tensile requirements, the Brinell hardness shall not be less than 200 HB or 93 HRB.
Do you have the material test report(s) for the batch or lot of bolts? If so, review the tensile results for compliance with the minimum strength requirement.
I also would question the hardness test results in terms of how it was performed, was a qualified procedure used?
I agree with metengr, how was the Brinell hardness number obtained? Brinell is usually a 10 mm ball and 3000 kgf load, which may be too large for your particular bolt.
L7M is a derivative of A193 B7M ; "M" indcating an upper hardness limit for sulfide stress cracking ( max BHN 235 or 242 depending on how the wind is blowing).
