Can A320 Grade L7M bolts be hot-dip galvanized?
The A320 Grade L7M is a variation of the A320 Grade L7 specification used in low-temperature, low-pressure applications. Nowhere in the A230 specification does it discuss whether or not A320L7M fasteners can be galvanized. In theory, there is nothing preventing someone from galvanizing the material, however, the L7M designation comes from heat treating being the last procedure performed on the fasteners without any additional machining.
The biggest argument against galvanizing L7M bolts is in regard to the corresponding nut. A194 Grade 7M nuts are typically used with A320 Grade L8M bolts. The L7M and 7M specifications prohibit any form of machining after heat-treatment has taken place. This is not an issue for the bolt, but it is for the corresponding nut since the threads would have to be machined after galvanizing.
The A194 specification reads as follows:
“For Grade 2HM and 7M nuts, a final stress relief shall be done at or above the minimum tempering temperature after all forming, machining, and tapping operations.”