Several ASTM specifications are followed by an “M” that denotes it being an equivalent metric standard. While there is actually an A193M Grade B7 specification which is applicable to metric fasteners, this is not the case with ASTM A193 Grade B7M. This is a different grade of material than A193 Grade B7. So how exactly does A193 Grade B7M differ from A193 Grade B7?
There are some significant differences in both the mechanical properties and the testing requirements of A193B7M and A193B7. For instance, A193B7M has a minimum tensile strength of 100 ksi for diameters under 4”. The minimum tensile strength for A193B7 is 125 ksi for diameters under 2-1/2”, and 115 ksi for 2-1/2” – 4” diameter. Hardness is probably the most significant difference between A193B7M & A193B7. The maximum hardness of ASTM A193B7M is 235 HB or 99 HRB, while the maximum hardness of A193B7 is 321 HB or 35 HRC. The most notable difference from a manufacturing perspective is that when providing A193B7M bolts, every individual bolt or stud in the lot must be hardness tested. A193B7M fasteners will also require a different grade of heavy hex nut than A193B7, which would be ASTM A194 Grade 2HM.
While A193B7 is specified relatively often in the construction industry, A193B7M is a specification that is not seen as regularly. This makes it important to understand the differences between the two specifications so that the correct material is ultimately provided.
Is it acceptable if a spec requires B7M but construction replace it with B7 instead?
@Syed- B7 and B7M are very different grades. Any substitution would need to be approved by the engineer of record.
I am looking at a spec requiring ASTM A 193/ A193M stainless steel fasteners even though technically a much lower grade stainless fastener would suffice because these are not used in any high heat or high pressure application, not even close. I suspect the specification writers just saw the word “pressure” in the description and defaulted to fastener made for high pressure apps such as boiler manufacturing. I am finding these fasteners to be extremely expensive. What stainless fastener I should look for terms of meeting the spec? For instance, a B18.63 meets the spec requirement but are there less costly alteratives to B18.63 grade stainless that still meet ASTM A 193/ A 193M spec?
@Matt- Apologies, but we are unfamiliar with ASME B18.63. A193 graded stainless fasteners are fairly common and non-costly fasteners, but if you are not needing a graded fastener like that, you can usually buy non-graded SS304 or SS316 fasteners off the shelf that might be less costly.
do we need to carry wedge test for full threaded hex bolts of GrB7 ?
Already we have tested the test piece for UTS.
@Sanjay- Headed B7 bolts that are 1-1/2″ diameter and smaller are required to be full size wedge tested in addition to machine sample testing. Larger sizes are exempted from this additional test.
Usage:
A193B7M / A194-2HM – These are listed in NACE MR0175 and MR0103 for “Sour Service”, along with A320 L7M fasteners and A914-7M nuts. These standards are for materials resistant to stress corrosion cracking in refinery processes. Washers – not applicable. Scope states that it does not apply to items subject to only compression stresses.
Good day,
Great article and information.
Is there a big difference between torqueing values for B7 and B7M? Guidance is available for the B7 but not the B7M.
Torqueing value is 3370 Ibs.ft for the nonstandard flange with 2″ diameter and 36 nos
Appreciate the lubricant is important – in this case Molykote 1000
Thanks
@Robert- Torque is usually calculated based on yield strength, and since the yield for B7M is about 20% less that the yield for B7, the torque needed will follow suit. We do not have any experience with Molykote 1000, so we can’t say how the addition of that lubricant will affect the friction coefficient.