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.

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    @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.

    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 non-standard flange with 2″ diameter and 36 nos
    Appreciate the lubricant is important – in this case Molykote 1000


    @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.

    Estimado señor,
    Superviso a una contratista y ellos argumentan que los esparragos con especificación A193 B7M pueden ser utilizados con las tuercas ASTM A194 Grado 2H, y que los esparragos con especificación A193 B7 pueden ser utilizados con las tuercas ASTM A194 Grado 2HM, que consecuencias puede tener si se utilizan asi?
    Gracias por su atención.

    @Jorge- A193 B7 se usa normalmente con tuercas A194 2H, y A193 B7M se usa normalmente con tuercas A194 2HM. Los grados “M” están sujetos a un mayor nivel de escrutinio y siempre deben usarse juntos a menos que el ingeniero del proyecto apruebe las sustituciones.


    Are the ASTM A193 B7M bolts (nut/screw) magnetic material? From what I can find in ASTM A193, B7 is listed under ferritic steels, which to my knowledge is magnetic. However, I find some contradicting answers when searching around, can it be both depending on the heat treatment and consequent microstructure? Your advise would be much appreciated.

    @Solveig- Yes, A193 B7 and B7M material is magnetic. It is made from 4140 alloy steel which is a ferritic steel. There are demagnetization techniques, but normal, off the shelf B7 and B7M bolts will be magnetic.

    @Usama- In order to change B7 to B7M, you would need to temper the bolts to a lower hardness. However, the stamping requirement for B7M is different than B7, so the parts would need to be re-stamped and 100% hardness tested. Your heat treater can talk you through the specifics of the heat treat/temper process.

    Mr. Dane McKinnon,

    Good Evening sir,

    I am working in petroleum refinery. We use B7M stud/nuts for our Heat exchanger internal bolts (always submerged in hot process fluid). for external studs.nuts, we use B7 only. As we know B7M is having less strength but low hardness than B7, how it is helping us.
    I understand we could not tighten the internals bolts (B7M in our case) and it should withstand in extreme process condtion i.e. should not fail leading to internal leak.

    @Saddam- Your supplier should have material test reports and documentation to show how the material was manufactured. Regardless, the hardness testing should be completed after the final heat treatment.

    @Ader – ASTM A193 B7 is a heat treated, high strength alloy steel for use at high temperatures. ASTM A307 is a low carbon steel, low strength, general purpose bolt standard. We have strength and chemical summaries on the individual ASTM summary pages on our website.

    @Martin- For all intents and purposes, they are the same. ASME just re purposes the ASTM standard and adds the “S” to differentiate it. The differences can come if the item is being used in nuclear or special applications, then there might be some additional requirements, but we are less familiar with those.

    @Jonathan – for materials that are heat treated, the reason is that the heat treating has hard time penetrating all the way to the center of the steel, so the larger your material gets, so weaker the center section might be. Same as baking a cake – if it is large, the center never gets completely done.

    Dear Sir,

    I have a query. Which is category for Mushroom Head roofing bolt (without neck) to be manufactured?


    @Manickam- We manufacture carriage bolts and timber bolts, both which have a round, curved head. We are not familiar with a bolt specifically called a mushroom head. If you’d like to send in a drawing or photo, we’d be happy to look at it.

    can any one tell why the thread for stud bolts of 1inch and smaller have Course thread series (UNC) and above 1inch size have 8UN thread series ?

    @Karthikeyan- UNC is the most common thread pitch for smaller bolts, so that is why the small ones are UNC. As for why the larger bolts are 8tpi, the threads are more shallow than their UNC counterparts, which creates a larger thread root area. That allows for slightly (about 10%) more clamping force than the same diameter UNC threaded bolt. The downside is that the shallow threads are more susceptible to damage and thread stripping in some situations.

    @Osama- We are sorry, but we are not familiar enough with the requirements of sanitary lifting stations to know if B7M bolts are allowed. Apologies.

    @Neil- A193 B7M does not specifically cover washers, although I guess theoretically you could make them from the same material. I see no issues using the same bolting material to make washers for use here. That said, ASTM F436 washers are an off the shelf, very common washer that may provide the same benefit at a much lower cost.

    At wellhead platform , B7 bolts used instead of B7M , is there any big issue .Enviornment is a little bit corrosive.

    @Jayant- B7 and B7M bolts are chemically identical, so they would both have the same corrosion resistance, which is negligible. However B7 and B7M bolts re not mechanically identical. B7 bolts are stronger, but rated for lower temperatures than B7M. You will need to consult the project engineer and get their approval for the substitution.

    Dear Sir,

    May you help me about correlation Bolt Tensile Strength vs Pressure?
    Right now I want to have high pressure valve test using test flange 10″1500# (standard no. of bolts is 12 ea) and the test pressure is 5,575 Psi.
    At the moment I only have 6 bolts size 1-7/8″, it is permissible to test the valve with only 50% attached to the valve?
    Is there any calculation about it?



    @Ferry- Apologies, but we do not have any engineers on staff, and do not have the expertise to answer this. Sorry we could not help.

    Dear Mr McKinnon
    May you help me about difference between stud bolt & machine bolts , in fact about Mechanical properties , I think that the stud bolts will be used in piping components without washer , but the machine bolt use for structure with washer. At least Can We say the stud bolts are stronger than machine bolts?
    Thanks a lot,

    @Ashkiani- The only difference is that the machine bolt has a head, and the stud bolt does not. Mechanically, assuming that they are the same grade, they will be identical. Machine bolts sometimes fare better during wedge tests due to the forged head, but we would not say that one is stronger than the other. It will depend on how they are being used and what the engineer designed for.

    Dear Sir
    I have two question for stud bolt & machine bolt.
    1- What is the technical difference between stud bolt & machine bolt?
    2- Can we use the washer for stud bolts ? why no / why yes
    We know that usually the stud bolt will use for flanges, valves,… fastener and machine bolt for structure.
    Best Regards,

    @Ashkiani- A stud bolt is a threaded stud with no head, whereas a machine bolt has a forged head on one end. Washers can, and are sometimes used with both stud bolts and machine bolts. It is up to the designer or engineer if they are required or not.

    @Oktay- ASTM A193 does not make reference to any washers, but F436 is the normally provided washer. A mild steel washer is not hardened and will likely not perform as well with B7 bolts or studs.

    what is equivalent grade for UNI5739 class 8.8, from following.
    A193 B7, A193 B7M, A 193 B8 Class1 or a193 B8 Class2

    @Vijay- Of those, A193 B7 is the closest to grade 8.8. It is not exact, but the closest of the four grades you listed.

    We have a valve with Grade B7 bolts that need to be change out, we will need longer bolts when going back together. Will Grade 8 bolts work as a replacement? If Grade 8 will not work will Grade B7 threaded studs work as a replacement for the Grade B7 bolts?

    @Joe- Grade 8 bolts might work, but you’d really need to get that approved by an engineer. There are some differences between the two. As for the studs, same answer, make sure an engineer is ok with the substitution as there may be pros and cons to making the switch.

    We have B7M thread rod and we cut it to some required short length stud bolts did now it is again required heat treatment. While as per clause 6.1.5 it is written that after all type of cutting heat treatment required

    @Waqas- You are correct, no further processing is permitted after final heat treatment. Even a simple cutting process will require an additional heat treat process.

    @Vivian- B7M is an alloy steel grade, not a stainless steel like 304/316, so it does not fall under any of them.

    @Ningxu- The difference between A194 2H and 2HM nuts is that the 2HM nuts are 100% hardness tested, and are slightly lower in strength than their 2H counterparts. 2HM nuts are also tempered at a much higher temperature than 2H nuts, so they would perform better in high temperature applications. However, we do not have any engineers on staff, so we cannot speak to all the possible risks, but what we can say is that 2HM nuts are quite specialized, and I would assume there is a good reason that they were originally specified.

    @Maroof- A193 B7M is an alloy steel bolting standard, whereas A153 is a hot dip galvanizing standard, so they are two, very different specifications.

    What is the difference between B7M and B7M comply to nace mr 0175. Is it just the hardness we are talking abt?

    @Gerard – B7M is a lower strength version of B7 and requires 100% hardness testing. we are not familiar with NACE MR 0175, so we cannot comment on that.

    Dear Mr. Mc Kinnon / Mr. Monlux,

    Is A193 B7 similar with ASTM A325 ?
    What if I substitute ASTM A325 into A193 B7 on my project, are both same ?

    @Ari- There are some similarities in tensile and yield, but they are not identical. You can find summaries for both ASTM standards on our website. Any substitution should be run by an engineer.

    @Jugal – I am sorry, but we are unable to answer that question. You may need to contact an engineer or metallurgist.

    What is difference between BS 3410 and F 436 washers – secondly in either case with A193/A194 2H which dimension table is recommended for Washers.

    @Tauseef- We are not familiar with BS 3410 washers, so I cannot compare those. F436 washer are made per ASME B18.21.1 Type A narrow dimensions, and those are what we supply with A193/A194 fasteners.

    Hi, I would like to clarify if the commonly referred bolt classes (class 1,class 2) is of the same meaning segregating b7 from b7m?
    i.e b7 is of class 2 , b7m is of class 1

    @Carlyn- No, class 1/class 2 are entirely different from B7/B7M. Class 1/2 are terms used with stainless steels like B8 or B8M. Class 1 refers to lower strength materials whereas class 2 refers to higher strength, strain hardened material. B7 and B7M are two separate grades within A193 that are made from alloy steel.

    Is it possible to use A193 Gr. B7M bolt on an SS316L flange. If not which is the suitable bolt material( A193 Gr. B16 or A193 Gr. B8M). The design temperature of the tank is 85 deg C only.

    For washers which grade of material should be used with B7/2H ( Bolt/Nut) and same for B7M/2HM.

    @Tauseef- F436-1 washers are the recommended washer for A193 grades B7 and B7M, if washers are needed.

    What standard is the nut compatability chart listed on your website pulled from? I don’t see which grade nuts go with which grade bolts anywhere is SA-193, SA-194, A-962, … Thanks.

    @Marc- The main nut compatibility chart came from ASTM A563. The A194 nut chart we created based on common industry practices.

    @Aftab- ASME B31.1 defines allowable stress for B7 up to and including 1000F, although the allowable stress begins to decline at 750F.


    @Francisco- Sí, si los dos pernos están hechos de 4140, se podía teóricamente calentar convite / temperamento del grado 8 hasta A193 B7.

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

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

    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?

    @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.

    @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.

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

    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.??

    @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.

    @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.

    Although NACE MR0175 does not implicitly state, does it allow the use of B7 bolts for unexposed bolting?

    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

    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?

    @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.

    @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.

    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

    @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.

    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?

    @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.

    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

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