Do A193 Grade B8 Class 2 bolts require a class 2 nut?

ASTM A193 Grades B8 and B8M Class 2 are grades of stainless steel bolts that are strain hardened and therefore have higher yield and tensile strengths than A193 Grades B8 and B8M Class 1 bolts. However, the ASTM A193 specification does not indicate a nut requirement used with Class 2 bolts, other than simply A194 Grade 8 or 8M.

If strain hardened nuts are required, there is a supplementary requirement S1 under ASTM A194 titled “Strain-Hardened Austenitic Steel Nuts.” Strain hardened Grade 8, 8C, 8T, 8M, 8F, 8P, 8N, or 8MN may be specified. Nuts made to this supplementary requirement are higher in strength and marked with the grade symbol underlined.

The industry standard is to typically provide “regular” A194 Grade 8 or 8M nuts with Class 2 bolts. However, the ultimate determination of nut compatibility should be made the Engineer of Record.  If you are purchasing Class 2 bolts, check the project specifications to determine if strain hardened nuts are required with Class 2 bolts. If strain hardened nuts are required, A194 Supplementary Requirement S1 must be specified on the order.

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14 comments

    Dear All

    Regarding ASTM A194 Grade 8M with S1 supplementary requirements, is it possible to machine 2-3/4 8UN Nuts?
    It seems to me impossible to reach the requested values by the ASTM from Cold Drawn bars due to the size. Lone solution Cold forged but possible for 2-3/4 8UN nuts?
    Thank you in advance for your help.
    Best Regards
    Thierry GEORGE

    @Thierry- The sizes listed in A194 S1 only go as large as 1-1/2, and since the minimum mechanicals decline as the nuts get larger, one would not expect a nut that large to be able to meet the minimum proof load required for smaller nuts. If you were to extrapolate out the declining proof load values at the same rate as they decline for nuts up to 1-1/2, we’d expect that the S1 value for a 2-3/4 nut would be about equal to the non S1 value at that diameter anyway.

    @Ashwaria- We are not sure if this is a question or statement, but we are happy to quote if you have some needs along these lines.

    Good day, I would like to ask the question if regular maintenance needs to be performed , is it good practice to combine a 316 type bolt with a 304 type nut and the other way around?
    We’ve experienced that when using a 316 nut and bolt that it would easily seize?
    We did use a torque wrench, but when we need to loosen the nuts , it seized…
    Has anyone else experienced this?

    @Monique- We have heard of users mixing 304/316 stainless in order to mitigate the galling effects. However we don’t have any published evidence to back it up, so are unsure of how well it works, and if different applications have differing results. Using a moly or wax based anti-seize might help as well.

    I have a question related to this specific topic. As you’ve noted ASTM A193 does not specify this requirement. However, ASME B16.5 specifies that low strength bolting is limited in flange ratings no higher than ANSI 300#, while intermediate strength bolting can be used in any flange rating. A193 B8M Cl. 2 studs qualify as intermediate strength and the code states that nuts are to be per A194 and of “comparable material”. To me that means that the nuts are also supposed to be strain hardened just like the studs. So by the language of the B16.5 code, how do you interpret the requirement?

    @Walter- You are correct that this is quite vague, as is the A193/A194 standard when it comes to bolt/nut combination recommendations. Commonly a standard nut of the same alloy is used with a class 2 bolt unless a strain hardened nut is requested. In some cases, depending on the diameter of the bolt/nut, the proofload value of a standard nut will exceed the yield strength of the class 2 bolt, so in those cases it may be preferable and cheaper to use the more common nut. Perhaps that is why the authors of the A193/4 standard left it vague.

    Dane – Wouldn’t you say in most cases your A193 B8-CL2 stud sizes are 1″ or smaller ? If so, the standard A194 Gr 8 nut proof load (80 ksi) would not exceed the yield strength of B8-CL2 stud. It would seem to me that strain-hardened Gr 8 nuts should be required for all B8-CL2 studs 1″ or smaller. Not sure why neither standard recognizes this.

    @Josh- Yes, I would say that most of the time we are supplying class 2 studs they are in the smaller size range. We are also unsure why the A193 standard does not specifically require any specific nut (other than one from A194), instead leaving that up to the purchaser and supplier.

    Hi! I have a customer requiring some 1-8 X 5″ SS Bolts grade A193 B8M. Is there another grade of SS stronger than this B8M Class? They are now using the B8M and they are “stretching”. Can you quote on what you do have? Thank you.

    F.G. Wilcox, Inc.

    @Carol- We are able to supply A193 B8M class 2 bolts, which are significantly stronger. One of our salespeople will be contacting you shortly.

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