What is the difference between A193 Grade B7 & A193 Grade B7M?

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.

Written February 4, 2013 by
Mike MonluxMike Monlux

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31 comments

  1. Dear sir,

    May you help me about material specifications…
    I have a problem about material MS-279. don’t know refer to what material it is.

    Thanks for your help

  2. Dear Sir,

    May I know why there is a restriction of size(length) of A193 B7M up to 64 as per ASME part 2 D. In another way, is it possible to have A193 B7M for length more than 64MM?

    1. @Leonard – We are not familiar with ASME 2D so cannot speak to why there is any restriction, but ASTM does not limit the available lengths of A193 B7M, so yes, could could get B7M bolts or studs in lengths longer than 64mm.

  3. Hello,
    While inspecting studs for assembly,I noticed on the B7M stud, the B7M was underlined. Why is this line not mentioned in numerous doccuments? It seems to be a requirement in ASTM 193.
    Thank You
    Bob Hale
    QC Inspector

    1. @Robert Hale – The underline on B7M is to indicate that the bolts/studs were 100% hardness tested in compliance with the specification. The line was added several years ago in order to be able to distinguish product made to the B7M grade from many years ago when the 100% hardness testing was not a requirement. I do not know why the documents you reference do not mention the line, perhaps they have not been updated since the change was made.

    1. @Rodrigo – per section 16 of A193, the line under B7M is to indicate 100% hardness testing and to differentiate from older version of A193 where 100% hardness testing was not a requirement. So more recently produced B7M bolts and studs will have a line under the stamp and will be 100% hardness tested.

  4. Dear Mike,

    I understand that there is a 100% hardness testing requirement for B7M bolting. Why is this requirement only appied to an order if the customer specifies Supplimentary requirement S3.1? If S3.1 is not specified in an order, and the bolting manufacturer uses an alternate to 100% indentation hardness testing, shall the manufacture proove the use of this alternate method to certify to B7M?

    1. @John – A193 section 9.2.2 allows the use of electromagnetic hardness testing as an acceptable alternative. If the purchaser does not specify 3.1, then the electromagnetic method is acceptable without additional proof. If the purchaser requires indentation testing, he must specify 3.1.

  5. Dear sir/madam,
    please kindly reply below questions:
    1-can galvanize hex bolt A-193 GR.B7
    2-can galvanize hex nut A-194 GR.2H

    best regards
    sohrabmohammadi

    1. @T.Mathew – There are two ASTM standards for stainless bolts, ASTM A193 B8M and F593 group 2. The corresponding nuts standards are A194 grade 8M and F594 group 2. There are no ASTM standards for stainless washers. We have summaries for both ASTM standards on our website.

  6. Customer specs require the use of A-193 grade B7M studs (not for cryogenic service) in 316 SS piping for produced water LACT units. While no other indication is made, would these studs have to be coated, zinc plated, etc.??

    1. @E.Noble – I am sorry, but we cannot make any specific recommendations regarding whether or not you would need a coating. You will need to consult with the project engineer.Apologies.

    1. @Mehul – We do not heat treat in house, so you will need to contact a heat treat facility to get specifics about their process.

    1. @Fabiola – B7M is slightly lower in strength than B7 and requires 100% hardness testing. It is usually used in higher temperature applications. I cannot recommend one or the other for your application, you would need to consult with the project engineer.

  7. B7 and B7M Bolts have their own equivalent nuts which is 2H and 2HM respectively, is it allowoed to interchange these nuts? If washers were to be required what would it be?

    1. @Richard – 2H and 2HM nuts are not the same strength (2HM are lower strength, but rated for higher temperature), so I would recommend against substituting one for the other. Washers are typically ASTM F436.

  8. If B7M bolts requires 100% hardness test as per A193, is the nuts and washers automatically the same requirements?

    1. @Richard – Yes, the A194 2HM nuts also require 100% hardness testing. The F436 washers do not require 100% testing.

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