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 carbide 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 the letters “SH” after the grade marking on the fastener and an underline for both A193 and A320 fasteners. The marking typically appears on the heads of bolts or the ends of threaded studs. For both A193 and A320, the marking requirement historically was only the underline, the “SH” lettering was added in approximately 2006.

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 ,


    In what cases will a A-194 Class 2 hex nut be used with the A-193 hex bolt? Is it even necessary to use class 2 hex nuts?

    @Dan- it is up to the buyer to request which nut they need, and ideally before that the designing engineer will specify. The A193/A320 bolting specifications do not mandate any specific nut other than one of the nuts within A194. Side note – the term class 2 is not used with A194 gr.8/8M nuts. Instead the strain hardened grades are designated by calling out supplement S1.

    @BK- Those grades are not mechanically identical, so any substitution should be approved by the project engineer.

    @Anil- those material call outs are not exactly identical, so any substitutions should be approved by the project engineer.

    Dear Sir,

    I have “A-193 B8″ bolts of dia 3/4”. I understand that “A-193 B8” bolt means “A-193 B8 Class 1”.

    Please confirm.

    @Ijaz- that is correct. Class 2 is typically called out as A193 B8 class 2, whereas the wording “class 1” is usually omitted and just referenced as A193 B8.

    B8M CL2 bolts can we confirm to Nace MR0175??
    As per NACE HRC value is limited to 22. And hardness value for B8M CL2 is more than HRC 22.

    @Joel- We are not familiar enough with NACE standards to know the specifics of what is permitted and what is not. There is no minimum hardness requirement for A193 B8M cl.2 bolts, so you may be able to find one that falls below 22 HRC, but you would need to check the MTRs on existing stock or contract to have the bolts manufactured in such a way that they conform to your requirement.


    We have received a B8M Class 2 Bolt Shipment from our supplier. What is the proper way to have it tested? Our supplier says that the core hardness would be less than that of surface hardness. Is that true?

    @Mike- Yes, the core hardness may be lower than the surface, that is common in strain hardened stainless steels. The best way to test would be to have the tensile and yield tested per ASTM F606, as well as the mid radius hardness and see if those values conform to the A193 standard.

    How is A193 B8MCL2 different to A320 B8MCL2 in terms of its application when chemistry & mech propr are same

    @Jayendra- A193 is typically used in high temperature applications, whereas A320 is for low temperatures. However, the B8M cl.2 grades of these two standards are identical, they can be used interchangeably.

    @Mora- The class 2 version of B8 will be stronger, but slightly less ductile. You would need to have the project engineer approve any substitutions.

    @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.

    The reason why ASTM A-193 B8M Class 2 is most likely not available is the lack of machinery to cold draw the wire to make the blank to the strain hardened tensile requirement.

    @Paul- The reason is that cold drawing/strain hardening only penetrates a fraction of an inch below the surface. As the diameter of the bar increases, the center of the bar, which is not affected by the cold drawing, is left in it’s original condition. Therefore, as the bar diameter increases, it is harder and harder to get the steel to pass the elevated mechanical requirements of class 2. It is sometimes possible to find strain hardened, class 2 material larger than 1-1/2, but it is rare.

    @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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