You mean the allowable stress values? I checked, ASME SA-193 Grade B7 and B7M bolting material allowable stress values at various design temperatures and similar diameter ranges (at or below 2.5"),

the Grade B7 bolting material has a higher allowable stress value (up to 5 Ksi difference) up until 800 deg F, beyond this temperature both Grades have similar stress values.

Ref – ASME B&PV Code, Section II, Part D, Table 3

Thanks metengr for the response. I had found some internet information that indicated the general differences in allowable stress levels of the two grades of bolts. If I were to have two mated 300# flanges at 150 Degrees F, what impact would Grade B7M over B7 bolts have on the pressure rating of 707 psig?

You need to determine the actual service stress in each stud and compare that with the allowable stress value at 150 deg F. Considering how close the allowables are for these materials, off-hand I would say that you should see no real difference in using B7 and B7M. However, you need to substantiate this with an engineering analysis. You know the pressure, now calculate the load on each stud. Knowing the cross sectional area of each stud you should be able to calculate the tensile stress.

Thanks again metengr. I can take it from here.

B7M bolts can be used without any analysis on standard flanges. These are intermediate strength per B16.5 Table 1B.

Yes indeed, if you read the notes pertaining to Table 1B in ASME B16.5, the intermediate strength bolting materials can be used for all listed materials and gaskets PROVIDED it has been verified that a sealed joint can be maintained under working pressure and temperature. So, you can do one of two things, either install the intermediate strength studs and hope for the best in terms of a leak tight joint or perform an engineering analysis to verify that a leak tight joint would prevail by determining the correct preload.

The former is correct since the latter has been verified a million times over. If you were experienced in flange design you would already know this.

Met the same question last week. After some research then found that B7M is mainly used in the potentially corrosive service. The carbon in B7M is a little lower than that in B7, so the stress corrosion crack could be avoided by using B7M. Certainly, the UTS and Yield are lower due to the low carbon trait in B7M.

Hi,

B7M bolting is specified above B7 if applied in piping subject to Sour Service (NACE MR0175) if the piping is insulated. It's a modified grade.

Regards,
FKE

B7M bolts are specifically for applications where bolting could be exposed to the H2S-containing environment.  B7M bolts and 2HM or 2M nuts are tempered and hardness tested versions of the B7 and 2H grades.  HRC 22 is the maximum allowable hardness.