Difference between SA 193 Grade B7 and SA 193 Grade B7M

There are many difference between grade B7 and B7M as follow:

  1. 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.
  2.  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.
  3. When  manufacturing B7M each bolt must be hardness tested.
  4. Different grade of heavy hex nut is used with corresponding bolt grade. grade 2H with B7 and grade 2HM with  B7M.
  5. 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