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  • 1.  Set screw hardness standards

    Posted 12-02-2022 11:47
    ASTM F880 and ISO 3506-3 both specify hardness for stainless steel cup point set screws. Both recognize this depends on whether the screw is machined or cold worked. But cold working does not occur at the cup point edges, which is where hardness is the most important. Has anyone had to research this?

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    John Merrill
    Principal Engineer
    EagleBurgmann
    Matthews NC
    7048407045
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    Journal of the Month


  • 2.  RE: Set screw hardness standards

    Posted 12-04-2022 11:25
    John,

    Before retirement I was responsible for an accredited (ISO 17025) fastener testing laboratory.  While we didn't see very many austenitic stainless steel set screws we did supply quite a few Hex Head Cap Screws and we did run into problems with meeting tensile strength (110,000 psi min).  The root cause of the issue was the grip length tolerance.  In order to meet that requirement, we needed to have wire that was strain hardened before forming or we needed to supply a fully threaded length screw (zero grip length).

    Moving to you issue, there appears to be an error in ASTM F880 in that it has the cold worked (rolled) threads at a lower strength that an annealed machined thread.  If the threads are rolled from pitch diameter wire there should be an increase in strength at the cup point.  Having said that I looked up ASTM F593 for proof.  Table 2 list the machined parts with a lower hardness (Designation A - 65-95 HRB, YS 20ksi, UTS 70ksi) that the cold worked (Designation CW B95-C32, YS 60ksi, UTS 95ksi) has.

    I hope this information has value for you.

    Sincerely,
    Michael B. Connelly, FASM, CQE

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    Michael Connelly
    Connelly Consulting
    Chicago IL
    (312) 972-8556
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    Journal of the Month


  • 3.  RE: Set screw hardness standards

    Posted 12-05-2022 09:49
    I had the same thoughts as Michael when I saw the post first, but did not respond as it did not make sense. The cup point of a cold rolled thread gets maximum deformation (work hardening) and should have the highest hardness. I would suggest confirm that the set screw threads are actually cold rolled by macro section checking the grain flow and measuring microhardness at the cup point tip.
    Hope the issue is resolved by now.

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    Prabir Chaudhury
    President
    Education and Consulting LLC
    Phoenix AZ
    (480) 577-0772
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    Journal of the Month


  • 4.  RE: Set screw hardness standards

    Posted 12-05-2022 10:03
    Maybe my use of the term cup point caused some confusion. I am referring not to the threads themselves but the tip of the fastener that engages another component. I am not sure how this feature is manufactured.


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    John Merrill
    Principal Engineer
    EagleBurgmann
    Matthews NC
    7048407045
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    Journal of the Month


  • 5.  RE: Set screw hardness standards

    Posted 12-05-2022 12:17

    John,

     

    If the screw is cold worked then you start with wire, that is upset and broached and then the threads are rolled.  The whole part is cold worked not just the threads.

     

    Regards,

     

    Michael B. Connelly, FASM, CQE




    Journal of the Month


  • 6.  RE: Set screw hardness standards

    Posted 12-06-2022 13:29
    By sectioning the screw and mounting it in bakelite, a hardness scan of the screw threads and cup tip should reveal whether the strength called for in the ASTM standard is met.  Make sure that the A370 hardness conversion table is used for austenitic stainless steels, and not for bcc carbon steels.

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    Christopher Hahin
    Engineer of Structural Materials & Bridge Investigations
    Illinois Department of Transportation
    Springfield IL
    (217) 522-4023
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    Journal of the Month