There are many difference between grade B7 and B7M as follow:
- B7 has higher tensile strength; for instance, for 4 in diameter bolt the minimum tensile strength for the grade B7 bolt is 115 ksi as comapred to 100 ksi for B7M.
- B7 hsa higher hardness. The maximum hardness of B7 is 321 HB, while the maximum hardness of B7M is 235 HB as indicated in ASME II-A.
- When manufacturing B7M each bolt must be hardness tested.
- Different grade of heavy hex nut is used with corresponding bolt grade. grade 2H with B7 and grade 2HM with B7M.
- According to NACE MR0175, Grade B7M shall be used for bolting that can be exposed directly to a sour environment, or that is buried, insulated, equipped with flange protectors or otherwise denied direct atmospheric exposure