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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4 comments

    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.

    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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