Grade 5 bolts not marked if galvanized


I went to a local building supply company – one that tends to have a better selection than the big-box places – seeking some Grade 5 carriage bolts.

They pointed me to bins of galvanized carriage bolts. I checked and did not see the 3 radial lines that normally mark Grade 5. They said "galvanized bolts do not have the marking, but rest assured they are Grade 5"; in fact, I believe they implied that galvanized are always Grade 5 (or higher).

I tend to think they are lying or misinformed, but thought I would seek clarification.

Best Answer

Highly unlikely that a Grade 5 bolt (90.000 psi yield, depending on size) would be galvanized. If electro-galvanizing were used there is a high risk of hydrogen cracking at that hardness level. If it were hot dip galvanized, the required temperature of about 800 F would temper the steel to a lower strength. This tempering affect can be fixed using alloy like chrome/moly, but now you have a pretty expensive bolt; not one available in a hardware store.

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