What is the difference between A194 Grade 4 and A194 Grade 7 heavy hex nuts?

While A194 Grade 4 and A194 Grade 7 nuts are mechanically identical and have the same intended applications, the only difference is the chemical values (aka steel used to manufacture). Grade 4 was created to be a low-cost alternative to Grade 7 by using a lower cost raw material. Unfortunately, this did not work out. The idea was that the material used to make Grade 4 was cheaper, and therefore the nuts would be cheaper. In practice that was not the case. Since the steel used to manufacture Grade 7 nuts is mass produced and the material used to make Grade 4 was not, Grade 7 nuts remained cheaper, thus defeating the entire reason for Grade 4 to exist. As far back as 2014, the ASTM Committee that covers the A194 specification planned to phase out Grade 4 and explicitly stated the substitutability of Grade 7 for Grade 4. In 2017, Grade 4 nuts were removed from the A194 specification.

Difference in Chemical Requirements 1

Grade Carbon % Manganese % Chromium % Molybdenum %
4 .40-.50 .70-.90 .20-.30
7 .37-.49 .65-1.10 .75-1.20 .15-.25
  1. Per ASTM A194/A194M-13
Written

6 comments

    The SA-194 in ASME code Section II 2019 , still put the nuts gr 4.
    Do you know why in the ASME Code SA-194 specification did not remove the nuts gr 4 as ASTM did in 2017.

    @Gonzalo- Apologies, but we do not have any insight as to why the ASME committee chose to ignore the removal by ASTM of grade 4 nuts.

    As shown on the cover page of SA-194/SA-194M SPEC, Identical with ASTM Spec A194/A14M-12). That is the 2012 edition prior to the elimination of Grade 4.

    Thank you Nate for the impressive introduction.
    My question is the A194Grades 4&7 are mandatory heat treated? what recommended hardness when application is heat exchanger and last thing is who produce these hexagonal rods other than AK Steel?which exists no more.
    Thank you again Nate
    .

    @Essam- Yes, grade 4 and 7 nuts do require a mandatory heat treat. The hardness range is 248-327 HBN regardless of application. We do not know who, if anyone is producing hexagonal bar that meets this standard.

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