What is the difference between A194 grade 7 and grade 7L nuts?

A194 grade 7 nuts are commonly used nuts for both high temperature and low temperature applications, and is commonly paired with bolts made to either A193 or A320. When used in conjunction with A320 bolts in a low temperature application, or if specially requested, they are charpy tested to assure they will perform well in a low temperature environment. When that charpy test passes, the nuts are marked with an “L” to show that they have been tested. The same is true with A194 grade 4 and 4L nuts, although Grade 4 and 4L nuts are being withdrawn from the A194 standard by the ASTM committee due to lack of use and material availability.

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14 comments

    Can Gr 7 bolts be used with Gr 7L nuts, or the other way around, i.e. Gr 7L bolts used with Gr 7 nuts, or must both the bolt and the nut be either 7 or 7L?
    Which component is used to give to the material the impact resistance?

    @Eli- A320 L7 bolts are required to have a charpy test performed, whereas for A194 gr.7 nuts, the charpy is a supplemental requirement. When the test has been performed, they are then marked and sold as A194 gr.7L nuts. ASTM recommends that charpy’d nuts be used with A320 L7 bolts, so A194 gr.7L nuts should be used. If you are using gr.7 nuts on other grades of bolt (like A193 B7 or B16), then the charpy test is not required and a standard grade 7 is acceptable.
    As for what is added to give the bolts and nuts their impact resistance, we are not certain, but the -150F/-100C charpy can be a difficult one to achieve.

    @Brent- In regards to A193 B7M and A320 L7M for bolts, or A194 gr.2HM and gr.7M for nuts, the “M” designates a modified grade. The ‘M’ grades are slightly lower in strength than their non ‘M’ counterparts, and differ in that they are required to be 100% hardness tested. Additionally they are tempered at a higher temperature and have prohibitions on machining or cutting after the heat treat process is completed. The ‘M’ grades are for specialized applications and we are not asked for them very often.

    @ thanks Dane, However understand the Gr 7L is not listed in the ASTM A194 standard.
    Other hand Gr 7 is done with impact testing by default as per standard A703.

    Best regards
    Anjani

    Are these bolts suitable to be used with stainless steel flanges? i.e. not cause galvanic corrosion?

    @Sean- The largest variable in galvanic corrosion is the atmosphere. Metals will react differently in the presence of different catalysts i.e water, salt water, acids, etc. A metallurgist would need to be consulted in order to derive a complete answer for your question.

    @Cade- The difference is chemistry. Mechanically they are identical, but the 4L is made from slightly different steel than the 7L.

    @Andi- Normally, grade 7L nuts would be an acceptable alternative for grade 4. The ASTM committee has actually removed grade 4 from A194, so grade 7 will be the norm going forward.

    @Mojtaba- Our torque tables are calculated based on the bolt yield strength. A320 L43 has the same yield as A193 B7, so you could use that table for your bolt.

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