There are numerous accepted methods of making a welded joint for grease exhaust ductwork. The possibilities will depend upon the code in your jurisdiction but really should include things like flanged, telescoping, Bell style or overlapping joints. CE & UKCA marked Ducts should be made from non-combustible supplies like steel, https://charlieshvjw.goabroadblog.com/35994145/5-simple-techniques-for-fire-resisting-ductwork-insulation