The first thing you need to do is figure out which welding code or specification applies to your product(s).
Sometimes this information is included in contract documents, as dictated by the Customer, i.e. "All welding is to be performed within the requirements detailed in the latest edition of AWS D1.3: Structural Welding Code - Sheet Steel."
If your not required to do this by customer contract, but want to for liability protection, then you get to pick your own code or specification to follow.
If you don't know which spec to follow, reply with info on your product like material, thickness range, welding process, etc. and we'll try to help you figure it out.
Be aware though that this forum will be closed in a couple of weeks or less, so better be quick...

Tim

Kyle,

Hope you are still watching this.

In my area (stress this phrase in your mind because all cities and the Building Authorities in them have a different take on applying IBC, Fire Codes, and others) you would need to have a meeting with the Fire Marshall and City Building Authority to see what your area requires.  In mine, yes, you MUST be certified to do duct work in a building such as restaurants, Retirement Homes, schools, etc. 

Generally you get certified and have WPS's to D1.3 though the applicable code for production will usually be D9.1. 

When the duct is complete it will be examined by light and/or smoke.  These tests are detailed if various places, I will try to find some and post them up for you. 

Generally it is the Fire Marshall who ends up busting those not properly qualified and sends them to me to get paperwork in order and then I also do the testing and Special Inspection on the duct they already finished.  Most won't make them take it out and start over unless there is good reason: very terrible workmanship and/or attitude that the authority decides to make an example of to make the point of who is in control. 

He Is In Control, Have a Great Day,  Brent