Welding joint

I'm not the right person to answer this, but I can see why no one has tried to.  

There is not enough info in the question to even start to guess let alone calculate. 

What material are the pipe and flange you welding - uPVC, aluminium, cast iron, steel  other ?

What is the pipe for, air, fuel water, other ?

How big is it,  metres diameter, tens of centimetres diameter, few millimetres diameter ?

Sidewall and flange thickness ?

If you can provide a bit of background there is far more chance of someone knowing.

Mike.