Hi Steve
If you are working with C/Steel, then I can only imagine that the purging is required to make sure that there are no longer any flammable (explosive) gasses present in the pipe. As such, N2 should not be any problem, because C/Steel is normally welded without any backing gas. It is usually welded with air in contact with the root of the weld. As air is 80% N2, changing this to 100% N2 will really not make a difference. You will only reduce the surface oxidation slightly, which can only be a good thing.
Regards
Niekie Jooste
Fabristruct Solutions