Grade 3 vs Grade 4 nuts

[kevlar49]

Are there any metallurgical reasons why I shouldn't use ASTM A194 Grade 3 nuts (what we bought) instead of Grade 4 (what is currently in place) on some external bolts for heat exchangers in diesel hydrotreater service provided they meet the code requirements for the service?

 

[IRstuff]

Seems to me that a more germane question might be that if "grade 3" bolts "meet the code requirements for the service" why were grade 4 bolts ordered?

There must have been some difference to warrant the grade 4, so what's changed now, that you can specify grade 3?

TTFN

FAQ731-376
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[metengr]

There is a significant difference between Grade 3 and Grade 4 nuts.

Grade 3 - 5% Cr- 0.5% Mo, more for higher temperature applications

Grade 4 - is a carbon-molybdenum (0.25%) steel.

The major difference will be increased corrosion performance by the higher chromium steel.