B7 and B7M Differences?

[robday2000]

I know that the B7M Bolts are harder but are there any chemical differences between the 2, is it the treatment that makes them harder?

Regards
Rob

 

[metengr]

robday2000;
No, there are no chemical composition differences between these Grades only heat treatment practice. By the way, ASTM A 193 Grade B7M bolting material is not harder in comparison to Grade B7. You need to review the ASTM material specification. The hardness requirements per Table 2 list the Grade B7 material (based on a diameter of 2.5" and under as an example) of 321 HB versus Grade B7M is 235 HB.