Is there a way to visually distinguish the difference between a Class 1 and Class 2 stainless steel bolt?

Yes there is. A193 and A320 grades B8 and B8M are assumed to be Class 1 unless otherwise specified. Class 1 fasteners are carbon solution treated, while class 2 fasteners are carbide solution treated and strain hardened. The work hardening that occurs in the manufacturing of Class 2 fasteners increases their strength. Class 2 fasteners are differentiated by a line under the grade symbol which appears on the heads of bolts or the ends of threaded rods.

ASTM Specification Stainless
Application Marking
Class 1
Class 2
A193 Grade B8 Type 304 High temperature / High pressure B8 B8SH
A320 Grade B8 Type 304 Low temperature B8 B8
A193 Grade B8M Type 316 High temperature / High pressure B8M B8MSH
A320 Grade B8M Type 316 Low temperature B8M B8M
Written ,


    @Shyeara- Stainless steels have a tendency to gall when under pressure, so we have elected not to provide stainless torque values since they would be unreliable. That said, I would assume the torque for class 2 bolts would be higher than that of class 1, but you would need to talk to the project engineer to determine the proper amount.

    @Ramesh – A193 B8 class 2 normally stops at 1-1/2″, although occasionally it can be found larger. As for what ASME B31.3 recommends, we are uncertain.

    @Mike- A320 B8 cl.2 (SH) should be an appropriate call out for low temperature service down to -200C. However, since we do not have any engineers on staff, we have to stop short of making a recommendation.

    I need to use a 304 SS bolt in place of an A325 HDG bolt. Would A193 B8 Class 2 be the most appropriate substitution to maintain the strength of the connection?

    @David- A193 B8 Class 2 would be closer in strength to A325 than class 1, but it is not identical (it varies by diameter). Additionally, galling can often be an issue for stainless steel, especially in high tension applications, so caution should be exercised when substituting.

    Hi, through visual inspection is it possible to segregate A193 B8 Cl1 from A320 B8 Cl1? The same question for A193 B8 Cl2 from A320 B8 Cl2 or A193 B8M Cl2 from A320 B8M Cl2. Our concern is that for a certain material class both A320 & A193 have got similar grade marking and to avoid any confusion at job site please advise how we can distinguish A193 & A320 (same class). Thanks for your insight and guidelines.

    @Shahram- The reason that the head markings for the above grades of A193 and A320 are the same is that the bolts are 100% identical. They are made from the same steel, and to the same mechanical properties. There is no need to segregate, as all the bolts will meet the requirements of both A193 and A320.

    @BoSang- Yes, A193 B8M class 2 requires an underline. The marking requirements can be found in table 5 within the ASTM A193 standard.


    We have 5/8 SS studs stamped with B8M however it is not underlined. Speaking to the suppliers (Australia) they have said (ASSUMED) that all class B8M studs are class 2 because class 1 is pretty much just a standard stainless rod and no one uses them. The Australian standard doesn’t specify the requirement for markings on bolts and requires the manufacturers specifications to be followed which I don’t have. have you ever heard of Class 2 – ‘B8M’ being written without the underline?

    @Tom- No, all B8M studs are not class 2. Class 1 is by far the more common of the two. Class 2 are strain hardened and much less common/more expensive. The old requirement for class 2 was for a simple underline, but as of a couple years ago, the marking mas changed to B8MSH with an underline. If your product is nor marked properly, I’d say it does nor meet the standard for class 2.

    @Chris- The only difference as far as I can tell, is the additional chemicals you mention. We are not certain as to what these additional chemicals will do to the overall performance of the material.

    Hello @Gary/@Dane,,B8 class 1 & 2 is available in ASTM grades both 193 & 320 ??
    I mean it is used as per P-T requirements???

    @Srivinayak- Yes, A193 and A320 are available in grade B8 in both class 1 and class 2. We are unsure of what P-T requirements are.

    @Noya- ASTM A193 B8M class 2 is stronger, but slightly less ductile than class 1. I wouldn’t think the substitution should be cause for alarm, but to be sure, the project engineer should be asked to approve it.

    The allowable stress value is not mensioned about B8M Cl.2 for temperature. over 750 F in Table A-10, ASME B31.1. It is regarded that the Cl.2 is not recommended to use for high temperature. If you want to use those bolts, you’d better to check the service temperature.

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