Can A193 Grade B7M bolts be hot-dip galvanized?
A193 Grade B7M is a grade variation of the common A193 Grade B7 used in high-temperature, high-pressure applications. Nowhere in the A193 specification does it discuss whether or not A193B7M fasteners can be galvanized. In theory, there is nothing preventing someone from galvanizing the material, however, the application in which these bolts are intended to be used makes a solid point against galvanizing. As stated above, the A193 specification is designed for fasteners used in high-temperature, high-pressure situations. The intended use for A193B7M fasteners is for situations of even greater temperatures than A193B7 can withstand. Seeing as the intended temperatures these fasteners are being used in would well exceed the melting point of a galvanized coating, it is arguable that the fasteners would never need to be galvanized because the coating would be rendered useless once installed.
Another argument against galvanizing B7M bolts is in regard to the corresponding nut. A194 Grade 2HM nuts are typically used with A193 Grade B8M bolts. The B7M and 2HM specifications prohibit machining after heat-treatment. This is not an issue for the bolt, but it is for the corresponding nut.
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.”