There are techniques specific to welding to cast iron, including preheating the pieces in an oven.However, some just "go for it" and make out fine... I personally think on smaller pieces the weld does the preheating for you, if you move slow.
Do they make a turbo to downpipe flange for the VNT15 off A4 TDI's?
can you weld to cast?
Said Flange:http://i192.photobucket.com/albums/z236/dieselkraftstoff/SSPX0016.jpg?t=1191857488PM,IM,EM, eieio me if you want it