Bolts dimensions change after galvanizing

[demsha]

Hello All,

We have a problem with galvanized step bolts stripping from the nut. When we complained this situation to the manufacturer, they come up with an excuse to say that these bolts could have reduced dimentions (diameter) because of hot dip galvanizing or picking process. The lab. has done basic tests and found after removing the galvanized layer, the bolt diameter has reduced slightly compare to oroginal diameter of the bolt (before galvanizing). Can anyone tell me what is the scientific reason behind the phenomina?

Thanks in advance

 

[SnTMan]

http://galvanizeit.org/resources/technical-faq-dr-galv/fasteners/194

Regards,

Mike

 

[HoudiniMD]

AISC/RCSC Section 2 (Para 2.1 Parge 16.2-5) is very specific on galvanized bolt because of this phenomena. It is a common complaint in the field. The manufacturer is required to test each lot of bolt assembly (bolt and nut) after galvanizing to verify that the galvanizing process has not reduced bolt tension ability
From AISC/RCSC
"This testing must show that the galvanized or Zn/AQl coated nut with the lubricant provided may be rotated from the snug-tight condition well in excess of the rotation required for pretensioned installation without stripping."

Was a Certificate of Conformance (C of C) provided for each lot showing the testing had been performed? Even if it was and you are stripping bolt during normal tensioning I would go back to supplier and have them replace lots affected based on AISC/RCSC.

Just as background on this. Bolts/nuts to be galvanized are machined slightly under size to accommodate addition of coating thickness to bring bolt/nut into the required fit tolerance that would prevent the stripping you are experiencing. That is why the Code requires the testing to verify the variables in the galvanizing process do not degrade the tension capability of the bolt/nut assembly.