Dear all,
I have many 56" diameter, 5/8" thick carbon steel ducts to be welded together(girth seam). Does it make any difference of using 60 degree weld bevel preparation instead of commonly seen 75 degree ?

The welding proceedure usually dictates the geometry of the joint design.

But, if for some reason, you're not required to follow any specific weld specification, I would do the 60 degree bevel, just to decrease the weld time, by way of reducing amount of filler.

5/8 thick walls are very large - compared to the "ususal" thin walled Sch 40 and even Sch 80. Your "pipes" are also very large diameter, so the extra welding you would need (as noted above) for even one less pass on the final is a massive difference in schedule and budget. And filler material. 8<)

The conventional thick-wall weld prep also allows a double bevel (a 1/16 square land, a 37.5 bevel (to yield the 75 degree total prep on both sides) in the middle of the joint, and a tighter 45 or 50 degree bevel towards the outside wall. But, this requires a way to cut the double bevel quickly without thousands of inches of hand-grinding. Some pipe-prep attachments can provide this. They clamp a portable grinder/mill to the end of the pipe, then the tech runs the pipe prep machine around the joint to make the prep.

On a practical/technical basis, you only need a weld prep to provide access to the bottom of the root for the welder, and to "clean up" the original metal for the weld so you have a good fit with clean, weldablew metal exposed. .